NaCl SDK doc: Recover sandbox internal doc for x86-64

native_client_sdk/src/doc/reference/sandbox_internals/x86-64-sandbox.rst

Index: native_client_sdk/src/doc/reference/sandbox_internals/x86-64-sandbox.rst
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+.. _x86-64-sandbox:
+
+================================
+NaCl SFI model on x86-64 systems
+================================
+
+.. contents::
+   :local:
+   :backlinks: none
+   :depth: 2
+
+Summary
+=======
+
+This document addresses the details of the Software Fault Isolation
+(SFI) model for executable code that can be run in Native Client on an
+x86-64 system. An overview of this model can be found in the paper:
+`Adapting Software Fault Isolation to Contemporary CPU Architectures
+<https://research.google.com/pubs/archive/35649.pdf>`_.
+The primary focus of the SFI model is a Windows x86-64 system but the
+same techniques can be applied to run identical x86-64 binaries on
+other x86-64 systems such as Linux, Mac, FreeBSD, etc, so the
+description of the SFI model tries to abstract away system
+dependencies when possible.
+
+Please note: throughout this document we use the AT&T notation for
+assembler syntax, in which the target operand appears last, e.g. ``mov
+src, dst``.
+
+Binary Format
+=============
+
+The format of Native Client executable binaries is identical to the
+x86-64 ELF binary format (`[0]
+<http://en.wikipedia.org/wiki/Executable_and_Linkable_Format>`_, `[1]
+<http://www.sco.com/developers/devspecs/gabi41.pdf>`_, `[2]
+<http://www.sco.com/developers/gabi/latest/contents.html>`_, `[3]
+<http://downloads.openwatcom.org/ftp/devel/docs/elf-64-gen.pdf>`_) for
+Linux or BSD with a few extra requirements. The additional rules that
+a Native Client ELF binary must follow are:
+
+* The ELF magic OS ABI field must be 123.
+* The ELF magic OS ABI VERSION field must be 5.
+* The ELF e_flags field must be 0x200000 (32-byte alignment).
+* There must be exactly one PT_LOAD text segment. It must begin at
+  0x20000 (128 kB) and be marked RX (no W). The contents of the text
+  segment must follow :ref:`Text Segment Rules <x86-64-text-segment-rules>`.
+* There can be at most one PT_LOAD data segment marked R.
+* There can be at most one PT_LOAD data segment marked RW.
+* There can be at most one PT_GNU_STACK segment. It must be marked RW.
+* All segments must end before limit address (4 GiB).
+
+Runtime Invariants
+==================
+
+To ensure fault isolation at runtime, the system must maintain a
+number of runtime *invariants* across the lifetime of the running
+program. Both the *Validator* and the *Service Runtime* are
+responsible for maintaining the invariants. See the paper for the
+rationale for the invariants:
+
+* ``RIP`` always points to valid instruction boundary (the validator must
+  ensure this with direct jumps and direct calls).
+* ``R15`` (aka ``RBASE`` and ``RZP``) is never modified by code (the
+  validator must ensure this). Low 32 bits of ``RZP`` are all zero
+  (loader must ensure this).
+* ``RIP``, ``RBP`` and ``RSP`` are always in the **safe zone**: between
+  ``R15`` and ``R15+4GiB``.
+
+ * Exception: ``RSP`` and ``RBP`` are allowed to be in the range of
+   ``0..4GiB`` inside *pseudo-instructions*: ``naclrestbp``,
+   ``naclrestsp``, ``naclspadj``, ``naclasp``, ``naclssp``.
+
+* 84GiB are allocated for NaCl module (i.e. **untrusted region**):
+
+ * ``R15-40GiB..R15`` and ``R15+4GIB..R15+44GiB`` are buffer zones with
+   PROT_NONE flags.
+ * The 4GB *safe zone* has pages with either PROT_WRITE or PROT_EXEC
+   but must not have PROT_WRITE+PROT_EXEC pages.
+ * All executable code in PROT_EXEC pages is validatable and
+   guaranteed to obey the invariant.
+
+* Trampoline/springboard code is mapped to a non-writable region in
+  the *untrusted 84GB region*; each trampoline/springboard is 32-byte
+  aligned and fits within a single *bundle*.
+* The OS must not put any internal structures/code into the untrusted
+  region at any time (not using OS dynamic linker, etc)
+
+.. _x86-64-text-segment-rules:
+
+Text Segment Rules
+==================
+
+* The validation process must ensure that the text segment complies
+  with the following rules. The validation process must complete
+  successfully strictly before executing any instruction of the
+  untrusted code.
+* The following instructions are illegal and must be rejected by the
+  validator (the list is not exhaustive as the validator uses a
+  whiteist, not a blacklist; this means there is a large but finite
+  list of instructions the validator allows, not a small list of
+  instructions the validator rejects):
+
+ * any privileged instructions
+ * ``mov`` to/from segment registers
+ * ``int``
+ * ``pusha``/``popa`` (not dangerous but not needed for GCC)
+
+* There must be space for at least 32 bytes after the text segment and
+  before the next segment in ELF (towards higher addresses) that ends
+  strictly at a 64K boundary (a minimum page size for untrusted
+  code). This space will be padded with HLT instructions as part of
+  the validation process, along with the optional 64K page.
+* Neither instructions nor *pseudo-instructions* are permitted to span
+  a 32-byte boundary.
+* The ELF entry address must be 32-byte aligned.
+* Direct ``CALL``/``JUMP`` targets:
+
+ * must point to a valid instruction boundary
+ * must not point into a *pseudo-instruction*
+ * must not point between a *restricted register* (see below for
+   definition) producer instruction and its corresponding restricted
+   register consumer instruction.
+
+* ``CALL`` instructions must be 5 bytes before a 32-byte boundary, so
+  that the return address will be 32-byte aligned.
+* Indirect call targets must be 32-byte aligned. Instead of indirect
+  ``CALL``/``JMP`` x, use ``nacljmp`` and ``naclcall`` (see below for
+  definitions of these *pseudo-instructions*)
+* All instructions that **read** or **write** from/to memory must use
+  one of the four registers ``RZP``, ``RIP``, ``RBP`` or ``RSP`` as a
+  base, restricted (see below) register index (multiplied by 0, 1, 2,
+  4 or 8) and constant displacement (optional).
+
+ * Exception to this rule: string instructions are allowed if used in
+   following sequences (the sequences should not cross *bundle*
+   boundaries; segment overrides are disallowed):
+
+   .. naclcode::
+    :prettyprint: 0
+
+     mov %edi, %edi
+     lea (%rZP,%rdi),%rdi
+     [rep] stos  ; other string instructions can be used here
+
+   Note: this is identical to the *pseudo-instruction*: ``[rep] stos
+   %?ax, %nacl:(%rdi),%rZP``
+
+* An operand of a command is said to be a **restricted register** iff
+  it is a register that is the target of a 32-bit move in the
+  immediately-preceding command in the same *bundle* (consider the
+  previous command as additional sandboxing prefix):
+
+ .. naclcode::
+   :prettyprint: 0
+
+    ; any 32-bit register can be used here; the first operand is
+    ; unrestricted but often is the same register
+    mov ..., %eXX
+
+* Instructions capable of changing ``%RBP`` and ``%RSP`` are
+  forbidden, except the instruction sequences in the whitelist below,
+  which must not cross *bundle* boundaries:
+
+ .. naclcode::
+   :prettyprint: 0
+
+    mov %rbp, %rsp
+    mov %rsp, %rbp
+    mov ..., %ebp
+    ; restoration of %RBP from memory, register or stack - keeps the
+    ; invariant intact
+    add %rZP, %rbp
+    mov ..., %esp
+    ; restoration of %RSP from memory, register or stack - keeps the
+    ; invariant intact
+    add %rZP, %rsp
+    lea xxx(%rbp), %esp
+    add %rZP, %rsp  ; restoration of %RSP from %RBP with adjust
+    sub ..., %esp
+    add %rZP, %rsp  ; stack space allocation
+    add ..., %esp
+    add %rZP, %rsp  ; stack space deallocation
+    and $XX, %rsp  ; alignment; XX must be between -128 and -1
+    pushq ...
+    popq ...  ; except pop %RSP, pop %RBP
+
+List of Pseudo-instructions
+===========================
+
+Pseudo-instructions were introduced to let the compiler maintain the
+invariants without needing to know the code alignment rules. The
+assembler guarantees 32-bit alignment for all *pseudo-instructions* in
+the table below. In addition, to the pseudo-instructions, one
+pseudo-operand prefix is introduced: ``%nacl``. Presence of the
+``%nacl`` operand prefix ensures that:
+
+* The instruction ``"%mov %eXX, %eXX"`` is added immediately before the
+  actual command using prefix ``%nacl`` (where ``%eXX`` is a 32-bit
+  part of the index register of the actual command, for example: in
+  operand ``%nacl:(,%r11)``,  the notation ``%eXX`` is referring to
+  ``%r11d``)
+* The resulting sequence of two instructions does not cross the
+  *bundle* boundary.
+
+For example, the instruction:
+
+.. naclcode::
+  :prettyprint: 0
+
+  mov %eax,%nacl:(%r15,%rdi,2)
+
+is translated by the assembler to:
+
+.. naclcode::
+  :prettyprint: 0
+
+  mov %edi,%edi
+  mov %eax,(%r15,%rdi,2)
+
+The complete list of introduced *pseudo-instructions* is as follows:
+
+.. TODO(hamaji): Use rst's table instead of the raw HTML below.
+
+.. raw:: html
+
+ <table border=1>
+ <tbody>
+ <tr>
+ <td>Pseudo-instruction</td>
+ <td>Is translated to<br/>
+ </td>
+ </tr>
+ <tr>
+ <td>[rep] cmps %nacl:(%rsi),%nacl:(%rdi),%rZP<br/>
+ <i>(sandboxed cmps)</i><br/>
+ </td>
+ <td>mov %esi,%esi<br/>
+ lea (%rZP,%rsi,1),%rsi<br/>
+ mov %edi,%edi<br/>
+ lea (%rZP,%rdi,1),%rdi<br/>
+ [rep] cmps (%rsi),(%rdi)<i><br/>
+ </i>
+ </td>
+ </tr>
+ <tr>
+ <td>[rep] movs %nacl:(%rsi),%nacl:(%rdi),%rZP<br/>
+ <i>(sandboxed movs)</i><br/>
+ </td>
+ <td>mov %esi,%esi<br/>
+ lea (%rZP,%rsi,1),%rsi<br/>
+ mov %edi,%edi<br/>
+ lea (%rZP,%rdi,1),%rdi<br/>
+ [rep] movs (%rsi),(%rdi)<i><br/>
+ </i>
+ </td>
+ </tr>
+ <tr>
+ <td>naclasp ...,%rZP<br/>
+ <i>(sandboxed stack increment)</i></td>
+ <td>add ...,%esp<br/>
+ add %rZP,%rsp</td>
+ </tr>
+ <tr>
+ <td>naclcall %eXX,%rZP<br/>
+ <i>(sandboxed indirect call)</i></td>
+ <td>and $-32, %eXX<br/>
+ add %rZP, %rXX<br/>
+ call *%rXX<br/>
+ <i>Note: the assembler ensures all calls (including
+ naclcall) will end at the bundle boundary.</i></td>
+ </tr>
+ <tr>
+ <td>nacljmp %eXX,%rZP<br/>
+ <i>(sandboxed indirect jump)</i></td>
+ <td>and $-32,%eXX<br/>
+ add %rZP,%rXX<br/>
+ jmp *%rXX<br/>
+ </td>
+ </tr>
+ <tr>
+ <td>naclrestbp ...,%rZP<br/>
+ <i>(sandboxed %ebp/rbp restore)</i></td>
+ <td>mov ...,%ebp<br/>
+ add %rZP,%rbp</td>
+ </tr>
+ <tr>
+ <td>naclrestsp ...,%rZP
+ <i>(sandboxed %esp/rsp restore)</i></td>
+ <td>mov ...,%esp<br/>
+ add %rZP,%rsp</td>
+ </tr>
+ <tr>
+ <td>naclrestsp_noflags ...,%rZP
+ <i>(sandboxed %esp/rsp restore)</i></td>
+ <td>mov ...,%esp<br/>
+ lea (%rsp,%rZP,1),%rsp</td>
+ </tr>
+ <tr>
+ <td>naclspadj $N,%rZP<br/>
+ <i>(sandboxed %esp/rsp restore from %rbp; incudes $N offset)</i></td>
+ <td>lea N(%rbp),%esp<br/>
+ add %rZP,%rsp</td>
+ </tr>
+ <tr>
+ <td>naclssp ...,%rZP<br/>
+ <i>(sandboxed stack decrement)</i></td>
+ <td>sub ...,%esp<br/>
+ add %rZP,%rsp</td>
+ </tr>
+ <tr>
+ <td>[rep] scas %nacl:(%rdi),%?ax,%rZP<br/>
+ <i>(sandboxed stos)</i></td>
+ <td>mov %edi,%edi<br/>
+ lea (%rZP,%rdi,1),%rdi<br/>
+ [rep] scas (%rdi),%?ax<br/>
+ </td>
+ </tr>
+ <tr>
+ <td>[rep] stos %?ax,%nacl:(%rdi),%rZP<br/>
+ <i>(sandboxed stos)</i></td>
+ <td>mov %edi,%edi<br/>
+ lea (%rZP,%rdi,1),%rdi<br/>
+ [rep] stos %?ax,(%rdi)<br/>
+ </td>
+ </tr>
+ </tbody>
+ </table>