NaCl docs: add ARM 32-bit sandbox

native_client_sdk/src/doc/reference/sandbox_internals/arm-32-bit-sandbox.rst

+==================
+ARM 32-bit Sandbox
+==================
+
+Native Client for ARM is a method for running programs---even malicious

[Andy, 2014/02/07 19:41:13]

"method" --> "sandboxing technology"

[JF, 2014/02/07 21:15:14]

Done.

+ones---safely, on computers that use 32-bit ARM processors. It's an

[Andy, 2014/02/07 19:41:13]

"It's" --> "The ARM sandbox is"

[JF, 2014/02/07 21:15:14]

Done.

+extension of earlier work on Native Client for x86 processors. This

[Andy, 2014/02/07 19:41:13]

"This security" --> "Security"

[JF, 2014/02/07 21:15:14]

Done.

+security is provided with a low performance overhead of about 10% over
+regular ARM code, and as you'll see in this document the sandbox model
+is beautifully simple, meaning that the trusted codebase is much easier
+to validate.
+
+As an implementation detail, the Native Client 32-bit ARM sandbox is
+currently used by Portable Native Client to execute code on 32-bit ARM
+machines in a safe manner. The portable bitcode contained in a **pexe**
+is translated to a 32-bit ARM **nexe** before execution. This may change
+at a point in time: Portable Native Client doesn't necessarily need this
+sandbox to execute code on ARM. Note that the Portable Native Client
+compiler itself is also untrusted: it too runs in the ARM sandbox
+described in this document.
+
+On this page, we describe how Native Client works on 32-bit ARM. We
+assume no prior knowledge about the internals of Native Client, on x86
+or any other architecture, but we do assume some familiarity with
+assembly languages in general.
+
+.. contents::
+   :local:
+   :backlinks: none
+   :depth: 3
+
+An Introduction to the ARM Architecture
+=======================================
+
+In this section, we summarize the relevant parts of the ARM processor
+architecture.
+
+About ARM and ARMv7-A
+---------------------
+
+ARM is one of the older commercial "RISC" processor designs, dating back
+to the early 1980s. Today, it is used primarily in embedded systems:
+everything from toys, to home automation, to automobiles. However, its
+most visible use is in cellular phones, tablets and some
+laptops.
+
+Through the years, there have been many revisions of the ARM
+architecture, written as ARMv\ *X* for some version *X*. Native Client
+specifically targets the ARMv7-A architecture commonly used in high-end
+phones and smartbooks. This revision, defined in the mid-2000s, adds a
+number of useful instructions, and specifies some portions of the system
+that used to be left to individual chip manufacturers. Critically,
+ARMv7-A specifies the "eXecute Never" bit, or *XN*. This pagetable
+attribute lets us mark memory as non-executable. Our security relies on
+the presence of this feature.
+
+ARMv8 adds a new 64-bit instruction set architecture called A64, while
+also enhancing the 32-bit A32 ISA. For Native Client's purposes the A32
+ISA is equivalent to the ARMv7 ARM ISA, albeit with a few new
+instructions. This document only discussed the 32-bit A32 instruction
+set: A64 would require a different sandboxing model.
+
+ARM Programmer's Model
+----------------------
+
+While modern ARM chips support several instruction encodings, 32-bit
+Native Client on ARM focuses on a single one: a fixed-width encoding
+where every instruction is 32-bits wide called A32 (previously, and
+confusingly, called simply ARM). Thumb, Thumb2 (now confusingly called
+T32), Jazelle, ThumbEE and such aren't supported by Native Client. This
+dramatically simplifies some of our analyses, as we'll see later. Nearly
+every instruction can be conditionally executed based on the contents of
+a dedicated condition code register.
+
+ARM processors have 16 general-purpose registers used for integer and
+memory operations, written ``r0`` through ``r15``. Of these, two have
+special roles baked in to the hardware:
+
+* ``r14`` is the Link Register. The ARM *call* instruction
+  (*branch-with-link*) doesn't use the stack directly. Instead, it
+  stashes the return address in ``r14``. In other circumstances, ``r14``
+  can be (and is!) used as a general-purpose register. When ``r14`` is
+  playing its Link Register role, it's referred to as ``lr``.
+* ``r15`` is the Program Counter. While it can be read and written like
+  any other register, setting it to a new value will cause execution to
+  jump to a new address. Using it in some circumstances is also
+  undefined by the ARM architecture. Because of this, ``r15`` is never
+  used for anything else, and is referred to as ``pc``.
+
+Other registers are given roles by convention. The only important
+registers to Native Client are ``r9`` and ``r13``, which are used as the
+Thread Pointer location and Stack Pointer. When playing this role,
+they're referred to as ``tp`` and ``sp``.
+
+Like other RISC-inspired designs, ARM programs use explicit *load* and
+*store* instructions to access memory. All other instructions operate
+only on registers, or on registers and small constants called
+immediates. Because both instructions and data words are 32-bits, we
+can't simply embed a 32-bit number into an instruction. ARM programs use
+three methods to work around this, all of which Native Client exploits:
+
+1. Many instructions can encode a modified immediate, which is an 8-bit
+   number rotated right by an even number of bits.
+2. The ``movw`` and ``movt`` instructions can be used to set the top and
+   bottom 16-bits of a register, and can therefore encode any 32-bit
+   immediate.
+3. For values that can't be represented as modified immediates, ARM
+   programs use ``pc``-relative loads to load data from inside the
+   code---hidden in a place where it won't be executed such as "constant
+   pools", just past the final return of a function.
+
+We'll introduce more details of the ARM instruction set later, as we
+walk through the system.
+
+The Native Client Approach
+==========================
+
+Native Client runs an untrusted program, potentially from an unknown or
+malicious source, inside a sandbox created by a trusted runtime. The
+trusted runtime allows the untrusted program to "call-out" and perform
+certain actions, such as drawing graphics, but prevents it from
+accessing the operating system directly. This "call-out" facility,
+called a trampoline, looks like a standard function call to the
+untrusted program, but it allows control to escape from the sandbox in a
+controlled way.
+
+The untrusted program and trusted runtime inhabit the same process, or
+virtual address space, maintained by the operating system. To keep the
+trusted runtime behaving the way we expect, we must prevent the
+untrusted program from accessing and modifying its internals. Since they
+share a virtual address space, we can't rely on the operating system for
+this. Instead, we isolate the untrusted program from the trusted
+runtime.
+
+Unlike modern operating systems, we use a cooperative isolation
+method. Native Client can't run any off-the-shelf program compiled for
+an off-the-shelf operating system. The program must be compiled to
+comply with Native Client's rules. The details vary on each platform,
+but in general, the untrusted program:
+
+* Must not attempt to use certain forbidden instructions, such as system
+  calls.
+* Must not attempt to modify its own code without abiding by Native
+  Client's code modification rules.
+* Must not jump into the middle of an instruction group, or otherwise do
+  tricky things to cause instructions to be interpreted multiple ways.
+* Must use special, strictly-defined instruction sequences to perform
+  permitted but potentially dangerous actions. We call these sequences
+  pseudo-instructions.
+
+We can't simply take the program's word that it complies with these
+rules---we call it "untrusted" for a reason! Nor do we require it to be
+produced by a special compiler; in practice, we don't trust our
+compilers either. Instead, we apply a load-time validator that
+disassembles the program. The validator either proves that the program
+complies with our rules, or rejects it as unsafe. By keeping the rules
+simple, we keep the validator simple, small, and fast. We like to put
+our trust in small, simple things, and the validator is key to the
+system's security.
+
+.. Note::
+  :class: note
+
+  For the computationally-inclined, all our validators scale linearly in
+  the size of the program.
+
+NaCl/ARM: Pure Software Fault Isolation
+---------------------------------------
+
+In the original Native Client system for the x86, we used unusual
+hardware features of that processor (the segment registers) to isolate
+untrusted programs. This was simple and fast, but won't work on ARM,
+which has nothing equivalent. Instead, we use pure software fault
+isolation.
+
+We use a fixed address space layout: the untrusted program gets the
+lowest gigabyte, addresses ``0`` through ``0x3FFFFFFF``. The rest of the
+address space holds the trusted runtime and the operating system. We
+isolate the program by requiring every *load*, *store*, and *indirect
+branch* (to an address in a register) to use a pseudo-instruction. The
+pseudo-instructions ensure that the address stays within the
+sandbox. The *indirect branch* pseudo-instruction, in turn, ensures that
+such branches won't split up other pseudo-instructions.
+
+At either side of the sandbox, we place small (8KiB) guard
+regions. These are simply areas in the process's address space that are
+mapped without read, write, or execute permissions, so any attempt to
+access them for any reason---*load*, *store*, or *jump*---will cause a
+fault.
+
+Finally, we ban the use of certain instructions, notably direct system
+calls. This is to ensure that the untrusted program can be run on any
+operating system supported by Native Client, and to prevent access to
+certain system features that might be used to subvert the sandbox. As a
+side effect, it helps to prevent programs from exploiting buggy
+operating system APIs.
+
+Let's walk through the details, starting with the simplest part: *load*
+and *store*.
+
+*Load* and *Store*
+^^^^^^^^^^^^^^^^^^
+
+All access to memory must be through *load* and *store*
+pseudo-instructions. These are simply a native *load* or *store*
+instruction, preceded by a guard instruction.
+
+Each *load* or *store* pseudo-instruction is similar to the *load* shown
+below. We use abstract "placeholder" registers instead of specific
+numbered registers for the sake of discussion. ``rA`` is the register
+holding the address to load from. ``rD`` is the destination for the
+loaded data.
+
+.. naclcode::
+  :prettyprint: 0
+
+  bic    rA,  #0xC0000000
+  ldr    rD,  [rA]
+
+The first instruction, ``bic``, clears the top two bits of ``rA``. In
+this case, that means that the value in ``rA`` is forced to an address
+inside our sandbox, between ``0`` and ``0x3FFFFFFF``, inclusive.
+
+The second instruction, ``ldr``, uses the previously-sandboxed address
+to load a value. This address might not be the address that the program
+intended, and might cause an access to an unmapped memory location
+within the sandbox: ``bic`` forces the address to be valid, by clearing
+the top two bits. This is a no-op in a correct program.
+
+This illustrates a common property of all Native Client systems: we aim
+for safety, not correctness. A program using an invalid address in
+``rA`` here is simply broken, so we are free to do whatever we want to
+preserve safety. In this case the program might load an invalid (but
+safe) value, or cause a segmentation fault limited to the untrusted
+code.
+
+Now, if we allowed arbitrary branches within the program, a malicious
+program could set up carefully-crafted values in ``rA``, and then jump
+straight to the ``ldr``. This is why we validate that programs never
+split pseudo-instructions.
+
+Alternative Sandboxing
+""""""""""""""""""""""
+
+.. naclcode::
+  :prettyprint: 0
+
+  tst    rA,  #0xC0000000
+  ldreq  rD,  [rA]
+
+The first instruction, ``tst``, performs a bitwise-\ ``AND`` of ``rA``
+and the modified immediate literal, ``0xC0000000``. It sets the
+condition flags based on the result, but does not write the result to a
+register. In particular, it sets the ``Z`` condition flag if the result
+was zero---if the two values had no set bits in common. In this case,
+that means that the value in ``rA`` was an address inside our sandbox,
+between ``0`` and ``0x3FFFFFFF``, inclusive.
+
+The second instruction, ``ldreq``, is a conditional load if equal. As we
+mentioned before, nearly all ARM instructions can be made
+conditional. In assembly language, we simply stick the desired condition
+on the end of the instruction's mnemonic name. Here, the condition is
+``EQ``, which causes the instruction to execute only if the ``Z`` flag
+is set.
+
+Thus, when the pseudo-instruction executes, the ``tst`` sets ``Z`` if
+(and only if) the value in ``rA`` is an address within the bounds of the
+sandbox, and then the ``ldreq`` loads if (and only if) it was. If ``rA``
+held an invalid address, the *load* does not execute, and ``rD`` is
+unchanged.
+
+.. Note::
+  :class: note
+
+  The ``tst``-based sequence is faster than the ``bic``-based sequence
+  on modern ARM chips. It avoids a data dependency in the address
+  register. This is why we keep both around. The ``tst``-based sequence
+  unfortunately leaks information on some processors, and is therefore
+  forbidden on certain processors. This effectively means that it cannot
+  be used for regular Native Client **nexe** files, but can be used with
+  Portable Native Client because the target processor is known at
+  translation time from **pexe** to **nexe**.
+
+Addressing Modes
+""""""""""""""""
+
+ARM has an unusually rich set of addressing modes. We allow all but one:
+register-indexed, where two registers are added to determine the
+address.
+
+We permit simple *load* and *store*, as shown above. We also permit
+displacement, pre-index, and post-index memory operations:
+
+.. naclcode::
+  :prettyprint: 0
+
+  bic    rA,  #0xC0000000
+  ldr    rD,  [rA, #1234]    ; This is fine.
+  bic    rA,  #0xC0000000
+  ldr    rD,  [rA, #1234]!   ; Also fine.
+  bic    rA,  #0xC0000000
+  ldr    rD,  [rA], #1234    ; Looking good.
+
+In each case, we know ``rA`` points into the sandbox when the ``ldr``
+executes. We allow adding an immediate displacement to ``rA`` to
+determine the final address (as in the first two examples here) because
+the largest immediate displacement is ±4095 bytes, while our guard pages
+are 8192 bytes wide.
+
+We also allow ARM's more unusual *load* and *store* instructions, such
+as *load-multiple* and *store-multiple*, etc.
+
+Conditional *Load* and *Store*
+""""""""""""""""""""""""""""""
+
+There's one problem with the pseudo-instructions shown above: they are
+unconditional (assuming ``rA`` is valid). ARM compilers regularly use
+conditional *load* and *store*, so we should support this in Native
+Client. We do so by defining alternate, predictable
+pseudo-instructions. Here is a conditional *store*
+(*store-if-greater-than*) using this pseudo-instruction sequence:
+
+.. naclcode::
+  :prettyprint: 0
+
+  bicgt  rA,  #0xC0000000 
+  strgt  rX,  [rA, #123]
+
+The Stack Pointer, Thread Pointer, and Program Counter
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Stack Pointer
+"""""""""""""
+
+In C-like languages, the stack is used to store return addresses during
+function calls, as well as any local variables that won't fit in
+registers. This makes stack operations very common.
+
+Native Client does not require guard instructions on any *load* or
+*store* involving the stack pointer, ``sp``. This improves performance
+and reduces code size. However, ARM's stack pointer isn't special: it's
+just another register, called ``sp`` only by convention. To make it safe
+to use this register as a *load* or *store* address without guards, we
+add a rule: ``sp`` must always contain a valid address.
+
+We enforce this rule by restricting the sorts of operations that
+programs can use to alter ``sp``. Programs can alter ``sp`` by adding or
+subtracting an immediate, as a side-effect of a *load* or *store*:
+
+.. naclcode::
+  :prettyprint: 0
+
+  ldr  rX,  [sp],  #4!   ; Load from stack, then add 4 to sp.
+  str  rX,  [sp, #1234]! ; Add 1234 to sp, then store to stack.
+
+These are safe because, as we mentioned before, the largest immediate
+available in a *load* or *store* is ±4095. Even after adding or
+subtracting 4095, the stack pointer will still be within the sandbox or
+guard regions.
+
+Any other operation that alters ``sp`` must be followed by a guard
+instruction. The most common alterations, in practice, are addition and
+subtraction of arbitrary integers:
+
+.. naclcode::
+  :prettyprint: 0
+
+  add  sp,  rX
+  bic  sp,  #0xC0000000
+
+The ``bic`` is similar to the one we used for conditional *load* and
+*store*, and serves exactly the same purpose: after it completes, ``sp``
+is a valid address.
+
+.. Note::
+  :class: note
+
+  Clever assembly programmers and compilers may want to use this
+  "trusted" property of ``sp`` to emit more efficient code: in a hot
+  loop instead of using ``sp`` as a stack pointer it can be temporarily
+  used as an index pointer (e.g. to traverse an array). This avoids the
+  extra ``bic`` whenever the pointer is updated in the loop.
+
+Thread Pointer Loads
+""""""""""""""""""""
+
+The thread pointer and IRT thread pointer are stored in the trusted
+address space. All uses and definitions of ``r9`` from untrusted code
+are forbidden except as follows:
+
+.. naclcode::
+  :prettyprint: 0
+
+  ldr Rn, [r9]     ; Load user thread pointer.
+  ldr Rn, [r9, #4] ; Load IRT thread pointer.
+
+``pc``-relative Loads
+"""""""""""""""""""""
+
+By extension, we also allow *load* through the ``pc`` without a
+mask. The explanation is quite similar:
+
+* Our control-flow isolation rules mean that the ``pc`` will always
+  point into the sandbox.
+* The maximum immediate displacement that can be used in a
+  ``pc``-relative *load* is smaller than the width of the guard pages.
+
+We do not allow ``pc``-relative stores, because they look suspiciously
+like self-modifying code, or any addressing mode that would alter the
+``pc`` as a side effect of the *load*.
+
+*Indirect Branch*
+^^^^^^^^^^^^^^^^^
+
+There are two types of control flow on ARM: direct and indirect. Direct
+control flow instructions have an embedded target address or
+offset. Indirect control flow instructions take their destination
+address from a register. The ``b`` (branch) and ``bl``
+(*branch-with-link*) instructions are *direct branch* and *call*,
+respectively. The ``bx`` (*branch-exchange*) and ``blx``
+(*branch-with-link-exchange*) are the indirect equivalents.
+
+Because the program counter ``pc`` is simply another register, ARM also
+has many implicit indirect control flow instructions. Programs can
+operate on the ``pc`` using *add* or *load*, or even outlandish (and
+often specified as having unpredictable-behavior) things like multiply!
+In Native Client we ban all such instructions. Indirect control flow is
+exclusively through ``bx`` and ``blx``. Because all of ARM's control
+flow instructions are called *branch* instructions, we'll use the term
+*indirect branch* from here on, even though this includes things like
+*virtual call*, *return*, and the like.
+
+The Trouble with Indirection
+""""""""""""""""""""""""""""
+
+*Indirect branch* present two problems for Native Client:
+
+* We must ensure that they don't send execution outside the sandbox.
+* We must ensure that they don't break up the instructions inside a
+  pseudo-instruction, by landing on the second one.
+
+.. Note::
+  :class: note
+
+  On the x86 architectures we must also ensure that it doesn't land
+  inside an instruction. This is unnecessary on ARM, where all
+  instructions are 32-bit wide.
+
+Checking both of these for *direct branch* is easy: the validator just
+pulls the (fixed) target address out of the instruction and checks what
+it points to.
+
+The Native Client Solution: "Bundles"
+"""""""""""""""""""""""""""""""""""""
+
+For *indirect branch*, we can address the first problem by simply
+masking some high-order bits off the address, like we did for *load* and
+*store*. The second problem is more subtle. Detecting every possible
+route that every *indirect branch* might take is difficult. Instead, we
+take the approach pioneered by the original Native Client: we restrict
+the possible places that any *indirect branch* can land. On Native
+Client for ARM, *indirect branch* can target any address that has its
+bottom four bits clear---any address that's ``0 mod 16``. We call these
+16-byte chunks of code "bundles". The validator makes sure that no
+pseudo-instruction straddles a bundle boundary. Compilers must pad with`
+`nop``\ s to ensure that every pseudo-instruction fits entirely inside
+one bundle.
+
+Here is the *indirect branch* pseudo-instruction. As you can see, it
+clears the top two and bottom four bits of the address:
+
+.. naclcode::
+  :prettyprint: 0
+
+  bic  rA,  #0xC000000F
+  bx   rA
+
+This particular pseudo-instruction (a ``bic`` followed by a ``bx``) is
+used for computed jumps in switch tables and returning from functions,
+among other uses. Recall that, under ARM's modified immediate rules, we
+can fit the constant ``0xC000000F`` into the ``bic`` instruction's
+immediate field: ``0xC000000F`` is the 8-bit constant ``0xFC``, rotated
+right by 4 bits.
+
+The other useful variant is the *indirect branch-with-link*, which is
+the ARM equivalent to *call*:
+
+.. naclcode::
+  :prettyprint: 0
+
+  bic  rA,  #0xC000000F
+  blx  rA
+
+This is used for indirect function calls---commonly seen in C++ programs
+as virtual calls, but also for calling function pointers in C.
+
+Note that both *indirect branch* pseudo-instructions use ``bic``, rather
+than the ``tst`` instruction we allow for *load* and *store*. There are
+two reasons for this:
+
+1. Conditional *branch* is very common. Much more common than
+   conditional *load* and *store*. If we supported an alternative
+   ``tst``-based sequence for *branch*, it would be rare.
+2. There's no performance benefit to using ``tst`` here on modern ARM
+   chips. *Branch* consumes its operands later in the pipeline than
+   *load* and *store* (since they don't have to generate an address,
+   etc) so this sequence doesn't stall.
+
+.. Note::
+  :class: note
+
+  At this point astute readers are wondering what the ``x`` in ``bx``
+  and ``blx`` means. We told you it stood for "exchange", but exchange
+  to what? ARM, for all the reduced-ness of its instruction set, can
+  change execution mode from A32 (ARM) to T32 (Thumb) and back with
+  these *branch* instructions, called *interworking branch*. Recall that
+  A32 instructions are 32-bit wide, and T32 instructions are a mix of
+  both 16-bit or 32-bit wide. The destination address given to a
+  *branch* therefore cannot sensibly have its bottom bit set in either
+  instruction set: that would be an unaligned instruction in both cases,
+  and ARM simply doesn't support this. The bottom bit for the *indirect
+  branch* was therefore cleverly recycled by the ARM architecture to
+  mean "switch to T32 mode" when set!
+
+  As you've figured out by now, Native Client's sandbox won't be very
+  happy if A32 instructions were to be executed as T32 instructions: who
+  know what they correspond to?  A malicious person could craft valid
+  A32 code that's actually very naughty T32 code, somewhat like forming
+  a sentence that happens to be valid in English and French but with
+  completely different meanings, complimenting the reader in one
+  language and insulting them in the other.
+
+  You've figured out by now that the bundle alignment restrictions of
+  the Native Client sandbox already take care of making this travesty
+  impossible: by masking off the bottom 4 bits of the destination the
+  interworking nature of ARM's *indirect branch* is completely avoided.
+
+*Call* and *Return*
+"""""""""""""""""""
+
+On ARM, there is no *call* or *return* instruction. A *call* is simply a
+*branch* that just happen to load a return address into ``lr``, the link
+register. If the called function is a leaf (that is, if it calls no
+other functions before returning), it simply branches to the address
+stored in ``lr`` to *return* to its caller:
+
+.. naclcode::
+  :prettyprint: 0
+
+  bic  lr,  #0xC000000F
+  bx   lr
+
+If the function called other functions, however, it had to spill ``lr``
+onto the stack. On x86, this is done implicitly, but it is explicit on
+ARM:
+
+.. naclcode::
+  :prettyprint: 0
+
+  push { lr }
+  ; Some code here...
+  pop  { lr }
+  bic  lr,  #0xC000000F
+  bx   lr
+
+There are two things to note about this code.
+
+1. As we mentioned before, we don't allow arbitrary instructions to
+   write to the Program Counter, ``pc``. Thus, while a traditional ARM
+   program might have popped directly into ``pc`` to end the function,
+   we require a pop into a register, followed by a pseudo-instruction.
+2. Function returns really are just *indirect branch*, with the same
+   restrictions. This means that functions can only return to addresses
+   that are bundle-aligned: ``0 mod 16``.
+
+The implication here is that a *call*\ ---the *branch* that enters
+functions---must be placed at the end of the bundle, so that the return
+address they generate is ``0 mod 16``. Otherwise, when we clear the
+bottom four bits, the program would enter an infinite loop!  (Native
+Client doesn't try to prevent infinite loops, but the validator actually
+does check the alignment of calls. This is because, when we were writing
+the compiler, it was annoying to find out our calls were in the wrong
+place by having the program run forever!)
+
+.. Note::
+  :class: note
+
+  Properly balancing the CPU's *call*/*return* actually allows it to
+  perform much better by allowing it to speculatively execute the return
+  address' code. For more information on ARM's *call*/*return* stack see
+  ARM's technical reference manual.
+
+Literal Pools and Data Bundles
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In the section where we described the ARM architecture, we mentioned
+ARM's unusual immediate forms. To restate:
+
+* ARM instructions are fixed-length, 32-bits, so we can't have an
+  instruction that includes an arbitrary 32-bit constant.
+* Many ARM instructions can include a modified immediate constant, which
+  is flexible, but limited.
+* For any other value (particularly addresses), ARM programs explicitly
+  load constants from inside the code itself.
+
+.. Note::
+  :class: note
+
+  ARMv7 introduces some instructions, ``movw`` and ``movt``, that try to
+  address this by letting us directly load larger constants. Our
+  toolchain uses this capability in some cases.
+
+Here's a typical example of the use of a literal pool. ARM assemblers
+typically hide the details---this is the sort of code you'd see produced
+by a disassembler, but with more comments.
+
+.. naclcode::
+  :prettyprint: 0
+
+  ; C equivalent: "table[3] = 4"
+  ; 'table' is a static array of bytes.
+  ldr   r0,  [pc, #124]    ; Load the address of the 'table',
+                           ; "124" is the offset from here
+                           ; to the constant below.
+  add   r0,  #3            ; Add the immediate array index.
+  mov   r1,  #4            ; Get the constant '4' into a register.
+  bic   r0,  #0xC0000000   ; Mask our array address.
+  strb  r1,  [r0]          ; Store one byte.
+  ; ...
+  .word table              ; Constant referenced above.
+
+Because table is a static array, the compiler knew its address at
+compile-time---but the address didn't fit in a modified immediate. (Most
+don't).  So, instead of loading an immediate into ``r0`` with a ``mov``,
+we stashed the address in the code, generated its address using ``pc``,
+and loaded the constant. ARM compilers will typically group all the
+embedded data together into a literal pool. These typically live just
+past the end of functions, where they won't be executed.
+
+This is an important trick in ARM code, so it's important to support it
+in Native Client... but there's a potential flaw. If we let programs
+contain arbitrary data, mingled in with the code, couldn't they hide
+malicious instructions this way?
+
+The answer is no, because the validator disassembles the entire
+executable region of the program, without regard to whether the
+programmer said a certain chunk was code or data. But this brings the
+opposite problem: what if the program needs to contain a certain
+constant that just happens to encode a malicious instruction?  We want
+to allow this, but we have to be certain it will never be executed as
+code!
+
+Data Bundles to the Rescue
+""""""""""""""""""""""""""
+
+As we discussed in the last section, ARM code in Native Client is
+structured in 16-byte bundles. We allow literal pools by putting them in
+special bundles, called data bundles. Each data bundle can contain 12
+bytes of arbitrary data, and the program can have as many data bundles
+as it likes.
+
+Each data bundle starts with a breakpoint instruction, ``bkpt``. This
+way, if an *indirect branch* tries to enter the data bundle, the process
+will take a fault and the trusted runtime will intervene (by terminating
+the program). For example:
+
+.. naclcode::
+  :prettyprint: 0
+
+  bkpt #0x5BE0          ; Must be aligned 0 mod 16!
+  .word 0xDEADBEEF      ; Arbitrary constants are A-OK.
+  svc #30               ; Trying to make a syscall? OK!
+  str r0, [r1]          ; Unmasked stores are fine too.
+
+So, we have a way for programs to create an arbitrary, even dangerous,
+chunk of data within their code. We can prevent *indirect branch* from
+entering it. We can also prevent fall-through from the code just before
+it, by the ``bkpt``. But what about *direct branch* straight into the
+middle?
+
+The validator detects all data bundles (because this ``bkpt`` has a
+special encoding) and marks them as off-limits for *direct branch*. If
+it finds a *direct branch* into a data bundle, the entire program is
+rejected as unsafe. Because *direct branch* cannot be modified at
+runtime, the data bundles cannot be executed.
+
+.. Note::
+  :class: note
+
+  Clever readers may wonder: why use ``bkpt #0x5BE0``, that seems
+  awfully specific when you just need a special "roadblock" instruction!
+  Quite true, young Padawan! It happens that this odd ``bkpt``
+  instruction is encoded as ``0xE125BE70`` in A32, and in T32 the
+  ``bkpt`` instruction is encoded as ``0xBExx`` (where ``xx`` could be
+  any 8-bit immediate, say ``0x70``) and ``0xE125`` encodes the *branch*
+  instruction ``b.n #0x250``. The special roadblock instruction
+  therefore doubles as a roadblock in T32, if anything were to go so
+  awry that we tried to execute it as a T32 instruction! Much defense,
+  such depth, wow!
+
+Trampolines and Memory Layout
+-----------------------------
+
+So far, the rules we've described make for boring programs: they can't
+communicate with the outside world!
+
+* The program can't call an external library, or the operating system,
+  even to do something simple like draw some pixels on the screen.
+* It also can't read or write memory outside of its dedicated sandbox,
+  so communicating that way is right out.
+
+We fix this by allowing the untrusted program to call into the trusted
+runtime using a trampoline. A trampoline is simply a short stretch of
+code, placed by the trusted runtime at a known location within the
+sandbox, that is permitted to do things the untrusted program can't.
+
+Even though trampolines are inside the sandbox, the untrusted program
+can't modify them: the trusted runtime marks them read-only. It also
+can't do anything clever with the special instructions inside the
+trampoline---for example, call it at a slightly offset address to bypass
+some checks---because the validator only allows trampolines to be
+reached by *indirect branch* (or *branch-with-link*). We structure the
+trampolines carefully so that they're safe to enter at any ``0 mod 16``
+address.
+
+The validator can detect attempts to use the trampolines because they're
+loaded at a fixed location in memory. Let's look at the memory map of
+the Native Client sandbox.
+
+Memory Map
+^^^^^^^^^^
+
+The ARM sandbox is always at virtual address ``0``, and is exactly 1GiB
+in size. This includes the untrusted program's code and data, the
+trampolines, and a small guard region to detect null pointer
+dereferences. In practice, the untrusted program takes up a bit more
+room than this, because of the need for additional guard regions at
+either end of the sandbox.
+
++----------------+-------+-------------------+--------------------------------------------------------------------+
+| Address        | Size  | Name              | Purpose                                                            |
++================+=======+===================+====================================================================+
+| ``-0x2000``    |  8KiB | Bottom Guard      | Keeps negative-displacement *load* or *store* from escaping.       |
++----------------+-------+-------------------+--------------------------------------------------------------------+
+| ``0``          | 64KiB | Null Guard        | Catches null pointer dereferences, guards against kernel exploits. |
++----------------+-------+-------------------+--------------------------------------------------------------------+
+| ``0x10000``    | 64KiB | Trampolines       | Up to 2048 unique syscall entry points.                            |
++----------------+-------+-------------------+--------------------------------------------------------------------+
+| ``0x20000``    | ~1GiB | Untrusted Sandbox | Contains untrusted code, followed by its heap/stack/memory.        |
++----------------+-------+-------------------+--------------------------------------------------------------------+
+| ``0x40000000`` |  8KiB | Top Guard         | Keeps positive-displacement *load* or *store* from escaping.       |
++----------------+-------+-------------------+--------------------------------------------------------------------+
+
+Within the trampolines, the untrusted program can call any address
+that's ``0 mod 16``. However, only even slots are used, so useful
+trampolines are always ``0 mod 32``. If the program calls an odd slot,
+it will fault, and the trusted runtime will shut it down.
+
+.. Note::
+  :class: note
+
+  This is a bit of speculative flexibility. While the current bundle
+  size of Native Client on ARM is 16 bytes, we've considered the
+  possibility of optional 32-byte bundles, to enable certain compiler
+  improvements. While this option isn't available to untrusted programs
+  today, we're trying to keep the system "32-byte clean".
+
+Inside a Trampoline
+^^^^^^^^^^^^^^^^^^^
+
+When we introduced trampolines, we mentioned that they can do things
+that untrusted programs can't. To be more specific, trampolines can jump
+to locations outside the sandbox. On ARM, this is all they do. Here's a
+typical trampoline fragment on ARM:
+
+.. naclcode::
+  :prettyprint: 0
+
+  ; Even trampoline bundle:
+  push  { r0-r3 }     ; Save arguments that may be in registers.
+  push  { lr }        ; Save the untrusted return address,
+                      ; separate step because it must be on top.
+  ldr   r0,  [pc, #4] ; Load the destination address from
+                      ; the next bundle.
+  blx   r0            ; Go!
+  ; The odd trampoline that immediately follows:
+  bkpt 0x5be0         ; Prevent entry to this data bundle.
+  .word address_of_routine
+
+The only odd thing here is that we push the incoming value of ``lr``,
+and then use ``blx``---not ``bx``---to escape the sandbox. This is
+because, in practice, all trampolines jump to the same routine in the
+trusted runtime, called the syscall hook. It uses the return address
+produced by the final ``blx`` instruction to determine which trampoline
+was called.
+
+Loose Ends
+----------
+
+Forbidden Instructions
+^^^^^^^^^^^^^^^^^^^^^^
+
+To complete the sandbox, the validator ensures that the program does not
+try to use certain forbidden instructions.
+
+* We forbid instructions that directly interact with the operating
+  system by going around the trusted runtime. We prevent this to limit
+  the functionality of the untrusted program, and to ensure portability
+  across operating systems.
+* We forbid instructions that change the processor's execution mode to
+  Thumb, ThumbEE, or Jazelle. This would cause the code to be
+  interpreted differently than the validator's original 32-bit ARM
+  disassembly, so the validator results might be invalidated.
+* We forbid instructions that aren't available to user code (i.e. have
+  to be used by an operating system kernel). This is purely out of
+  paranoia, because the hardware should prevent the instructions from
+  working. Essentially, we consider it "suspicious" if a program
+  contains these instructions---it might be trying to exploit a hardware
+  bug.
+* We forbid instructions, or variants of instructions, that are
+  implementation-defined ("unpredictable") or deprecated in the ARMv7-A
+  architecture manual.
+* Finally, we forbid a small number of instructions, such as ``setend``,
+  purely out of paranoia. It's easier to loosen the validator's
+  restrictions than to tighten them, so we err on the side of rejecting
+  safe instructions.
+
+If an instruction can't be decoded at all within the ARMv7-A instruction
+set specification, it is forbidden.
+
+.. Note::
+  :class: note
+
+  Here is a list of instructions currently forbidden for security
+  reasons (that is, excluding deprecated or undefined instructions):
+
+  * ``BLX`` (immediate): always changes to Thumb mode.
+  * ``BXJ``: always changes to Jazelle mode.
+  * ``CPS``: not available to user code.
+  * ``LDM``, exception return version: not available to user code.
+  * ``LDM``, kernel version: not available to user code.
+  * ``LDR*T`` (unprivileged load operations): theoretically harmless,
+    but suspicious when found in user code. Use ``LDR`` instead.
+  * ``MSR``, kernel version: not available to user code.
+  * ``RFE``: not available to user code.
+  * ``SETEND``: theoretically harmless, but suspicious when found in
+    user code. May make some future validator extensions difficult.
+  * ``SMC``: not available to user code.
+  * ``SRS``: not available to user code.
+  * ``STM``, kernel version: not available to user code.
+  * ``STR*T`` (unprivileged store operations): theoretically harmless,
+    but suspicious when found in user code. Use ``STR`` instead.
+  * ``SVC``/``SWI``: allows direct operating system interaction.
+  * Any unassigned hint instruction: difficult to reason about, so
+    treated as suspicious.
+
+  More details are available in the `ARMv7 instruction table definition
+  <http://src.chromium.org/viewvc/native_client/trunk/src/native_client/src/trusted/validator_arm/armv7.table>`_.
+
+Coprocessors
+^^^^^^^^^^^^
+
+ARM has traditionally added new instruction set features through
+coprocessors. Coprocessors are accessed through a small set of
+instructions, and often have their own register files. Floating point
+and the NEON vector extensions are both implemented as coprocessors, as
+is the MMU.
+
+We're confident that the side-effects of coprocessors in slots 10 and 11
+(that is, floating point, NEON, etc.) are well-understood. These are in
+the coprocessor space reserved by ARM Ltd. for their own extensions
+(``CP8``--\ ``CP15``), and are unlikely to change significantly. So, we
+allow untrusted code to use coprocessors 10 and 11, and we mandate the
+presence of at least VFPv3 and NEON/AdvancedSIMD. Multiprocessor
+Extension, VFPv4, FP16 and other extensions are allowed but not
+required, and may fail on processors that do not support them, it is
+therefore the program's responsibility to validate their availability
+before executing them.
+
+We don't allow access to any other ARM-reserved coprocessor
+(``CP8``--\ ``CP9`` or ``CP12``--\ ``CP15``). It's possible that read
+access to ``CP15`` might be useful, and we might allow it in the
+future---but again, it's easier to loosen the restrictions than tighten
+them, so we ban it for now.
+
+We do not, and probably never will, allow access to the vendor-specific
+coprocessor space, ``CP0``--\ ``CP7``. We're simply not confident in our
+ability to model the operations on these coprocessors, given that
+vendors often leave them poorly-specified. Unfortunately this eliminates
+some legacy floating point and vector implementations, but these are
+superceded on ARMv7-A parts anyway.
+
+Validator Code
+^^^^^^^^^^^^^^
+
+By now you're itching to see the sandbox validator's code and dissect
+it. You'll have a disapointing read: at less that 500 lines of code
+`validator.cc
+<http://src.chromium.org/viewvc/native_client/trunk/src/native_client/src/trusted/validator_arm/validator.cc>`_
+is quite simple to understand and much shorter than this document. It's
+of course dependent on the `ARMv7 instruction table definition
+<http://src.chromium.org/viewvc/native_client/trunk/src/native_client/src/trusted/validator_arm/armv7.table>`_,
+which teaches it about the ARMv7 instruction set.