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Unified Diff: sandbox/linux/bpf_dsl/policy_compiler.cc

Issue 670183003: Update from chromium 62675d9fb31fb8cedc40f68e78e8445a74f362e7 (Closed) Base URL: git@github.com:domokit/mojo.git@master
Patch Set: Created 6 years, 2 months ago
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Index: sandbox/linux/bpf_dsl/policy_compiler.cc
diff --git a/sandbox/linux/bpf_dsl/policy_compiler.cc b/sandbox/linux/bpf_dsl/policy_compiler.cc
new file mode 100644
index 0000000000000000000000000000000000000000..0eb85ca6736bb3662dbf520b13d7723e7c2cc3d8
--- /dev/null
+++ b/sandbox/linux/bpf_dsl/policy_compiler.cc
@@ -0,0 +1,523 @@
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "sandbox/linux/bpf_dsl/policy_compiler.h"
+
+#include <errno.h>
+#include <linux/filter.h>
+#include <sys/syscall.h>
+
+#include <limits>
+
+#include "base/logging.h"
+#include "base/macros.h"
+#include "sandbox/linux/bpf_dsl/bpf_dsl.h"
+#include "sandbox/linux/bpf_dsl/bpf_dsl_impl.h"
+#include "sandbox/linux/seccomp-bpf/codegen.h"
+#include "sandbox/linux/seccomp-bpf/die.h"
+#include "sandbox/linux/seccomp-bpf/errorcode.h"
+#include "sandbox/linux/seccomp-bpf/instruction.h"
+#include "sandbox/linux/seccomp-bpf/linux_seccomp.h"
+#include "sandbox/linux/seccomp-bpf/syscall.h"
+#include "sandbox/linux/seccomp-bpf/syscall_iterator.h"
+
+namespace sandbox {
+namespace bpf_dsl {
+
+namespace {
+
+#if defined(__i386__) || defined(__x86_64__)
+const bool kIsIntel = true;
+#else
+const bool kIsIntel = false;
+#endif
+#if defined(__x86_64__) && defined(__ILP32__)
+const bool kIsX32 = true;
+#else
+const bool kIsX32 = false;
+#endif
+
+const int kSyscallsRequiredForUnsafeTraps[] = {
+ __NR_rt_sigprocmask,
+ __NR_rt_sigreturn,
+#if defined(__NR_sigprocmask)
+ __NR_sigprocmask,
+#endif
+#if defined(__NR_sigreturn)
+ __NR_sigreturn,
+#endif
+};
+
+bool HasExactlyOneBit(uint64_t x) {
+ // Common trick; e.g., see http://stackoverflow.com/a/108329.
+ return x != 0 && (x & (x - 1)) == 0;
+}
+
+bool IsDenied(const ErrorCode& code) {
+ return (code.err() & SECCOMP_RET_ACTION) == SECCOMP_RET_TRAP ||
+ (code.err() >= (SECCOMP_RET_ERRNO + ErrorCode::ERR_MIN_ERRNO) &&
+ code.err() <= (SECCOMP_RET_ERRNO + ErrorCode::ERR_MAX_ERRNO));
+}
+
+// A Trap() handler that returns an "errno" value. The value is encoded
+// in the "aux" parameter.
+intptr_t ReturnErrno(const struct arch_seccomp_data&, void* aux) {
+ // TrapFnc functions report error by following the native kernel convention
+ // of returning an exit code in the range of -1..-4096. They do not try to
+ // set errno themselves. The glibc wrapper that triggered the SIGSYS will
+ // ultimately do so for us.
+ int err = reinterpret_cast<intptr_t>(aux) & SECCOMP_RET_DATA;
+ return -err;
+}
+
+intptr_t BPFFailure(const struct arch_seccomp_data&, void* aux) {
+ SANDBOX_DIE(static_cast<char*>(aux));
+}
+
+bool HasUnsafeTraps(const SandboxBPFDSLPolicy* policy) {
+ for (uint32_t sysnum : SyscallSet::All()) {
+ if (SyscallSet::IsValid(sysnum) &&
+ policy->EvaluateSyscall(sysnum)->HasUnsafeTraps()) {
+ return true;
+ }
+ }
+ return policy->InvalidSyscall()->HasUnsafeTraps();
+}
+
+} // namespace
+
+struct PolicyCompiler::Range {
+ Range(uint32_t f, const ErrorCode& e) : from(f), err(e) {}
+ uint32_t from;
+ ErrorCode err;
+};
+
+PolicyCompiler::PolicyCompiler(const SandboxBPFDSLPolicy* policy,
+ TrapRegistry* registry)
+ : policy_(policy),
+ registry_(registry),
+ conds_(),
+ gen_(),
+ has_unsafe_traps_(HasUnsafeTraps(policy_)) {
+}
+
+PolicyCompiler::~PolicyCompiler() {
+}
+
+scoped_ptr<CodeGen::Program> PolicyCompiler::Compile() {
+ if (!IsDenied(policy_->InvalidSyscall()->Compile(this))) {
+ SANDBOX_DIE("Policies should deny invalid system calls.");
+ }
+
+ // If our BPF program has unsafe traps, enable support for them.
+ if (has_unsafe_traps_) {
+ // As support for unsafe jumps essentially defeats all the security
+ // measures that the sandbox provides, we print a big warning message --
+ // and of course, we make sure to only ever enable this feature if it
+ // is actually requested by the sandbox policy.
+ if (Syscall::Call(-1) == -1 && errno == ENOSYS) {
+ SANDBOX_DIE(
+ "Support for UnsafeTrap() has not yet been ported to this "
+ "architecture");
+ }
+
+ for (int sysnum : kSyscallsRequiredForUnsafeTraps) {
+ if (!policy_->EvaluateSyscall(sysnum)->Compile(this)
+ .Equals(ErrorCode(ErrorCode::ERR_ALLOWED))) {
+ SANDBOX_DIE(
+ "Policies that use UnsafeTrap() must unconditionally allow all "
+ "required system calls");
+ }
+ }
+
+ if (!registry_->EnableUnsafeTraps()) {
+ // We should never be able to get here, as UnsafeTrap() should never
+ // actually return a valid ErrorCode object unless the user set the
+ // CHROME_SANDBOX_DEBUGGING environment variable; and therefore,
+ // "has_unsafe_traps" would always be false. But better double-check
+ // than enabling dangerous code.
+ SANDBOX_DIE("We'd rather die than enable unsafe traps");
+ }
+ }
+
+ // Assemble the BPF filter program.
+ scoped_ptr<CodeGen::Program> program(new CodeGen::Program());
+ gen_.Compile(AssemblePolicy(), program.get());
+ return program.Pass();
+}
+
+Instruction* PolicyCompiler::AssemblePolicy() {
+ // A compiled policy consists of three logical parts:
+ // 1. Check that the "arch" field matches the expected architecture.
+ // 2. If the policy involves unsafe traps, check if the syscall was
+ // invoked by Syscall::Call, and then allow it unconditionally.
+ // 3. Check the system call number and jump to the appropriate compiled
+ // system call policy number.
+ return CheckArch(MaybeAddEscapeHatch(DispatchSyscall()));
+}
+
+Instruction* PolicyCompiler::CheckArch(Instruction* passed) {
+ // If the architecture doesn't match SECCOMP_ARCH, disallow the
+ // system call.
+ return gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS,
+ SECCOMP_ARCH_IDX,
+ gen_.MakeInstruction(
+ BPF_JMP + BPF_JEQ + BPF_K,
+ SECCOMP_ARCH,
+ passed,
+ RetExpression(Kill("Invalid audit architecture in BPF filter"))));
+}
+
+Instruction* PolicyCompiler::MaybeAddEscapeHatch(Instruction* rest) {
+ // If no unsafe traps, then simply return |rest|.
+ if (!has_unsafe_traps_) {
+ return rest;
+ }
+
+ // Allow system calls, if they originate from our magic return address
+ // (which we can query by calling Syscall::Call(-1)).
+ uint64_t syscall_entry_point =
+ static_cast<uint64_t>(static_cast<uintptr_t>(Syscall::Call(-1)));
+ uint32_t low = static_cast<uint32_t>(syscall_entry_point);
+ uint32_t hi = static_cast<uint32_t>(syscall_entry_point >> 32);
+
+ // BPF cannot do native 64-bit comparisons, so we have to compare
+ // both 32-bit halves of the instruction pointer. If they match what
+ // we expect, we return ERR_ALLOWED. If either or both don't match,
+ // we continue evalutating the rest of the sandbox policy.
+ //
+ // For simplicity, we check the full 64-bit instruction pointer even
+ // on 32-bit architectures.
+ return gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS,
+ SECCOMP_IP_LSB_IDX,
+ gen_.MakeInstruction(
+ BPF_JMP + BPF_JEQ + BPF_K,
+ low,
+ gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS,
+ SECCOMP_IP_MSB_IDX,
+ gen_.MakeInstruction(
+ BPF_JMP + BPF_JEQ + BPF_K,
+ hi,
+ RetExpression(ErrorCode(ErrorCode::ERR_ALLOWED)),
+ rest)),
+ rest));
+}
+
+Instruction* PolicyCompiler::DispatchSyscall() {
+ // Evaluate all possible system calls and group their ErrorCodes into
+ // ranges of identical codes.
+ Ranges ranges;
+ FindRanges(&ranges);
+
+ // Compile the system call ranges to an optimized BPF jumptable
+ Instruction* jumptable = AssembleJumpTable(ranges.begin(), ranges.end());
+
+ // Grab the system call number, so that we can check it and then
+ // execute the jump table.
+ return gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS, SECCOMP_NR_IDX, CheckSyscallNumber(jumptable));
+}
+
+Instruction* PolicyCompiler::CheckSyscallNumber(Instruction* passed) {
+ if (kIsIntel) {
+ // On Intel architectures, verify that system call numbers are in the
+ // expected number range.
+ Instruction* invalidX32 =
+ RetExpression(Kill("Illegal mixing of system call ABIs"));
+ if (kIsX32) {
+ // The newer x32 API always sets bit 30.
+ return gen_.MakeInstruction(
+ BPF_JMP + BPF_JSET + BPF_K, 0x40000000, passed, invalidX32);
+ } else {
+ // The older i386 and x86-64 APIs clear bit 30 on all system calls.
+ return gen_.MakeInstruction(
+ BPF_JMP + BPF_JSET + BPF_K, 0x40000000, invalidX32, passed);
+ }
+ }
+
+ // TODO(mdempsky): Similar validation for other architectures?
+ return passed;
+}
+
+void PolicyCompiler::FindRanges(Ranges* ranges) {
+ // Please note that "struct seccomp_data" defines system calls as a signed
+ // int32_t, but BPF instructions always operate on unsigned quantities. We
+ // deal with this disparity by enumerating from MIN_SYSCALL to MAX_SYSCALL,
+ // and then verifying that the rest of the number range (both positive and
+ // negative) all return the same ErrorCode.
+ const ErrorCode invalid_err = policy_->InvalidSyscall()->Compile(this);
+ uint32_t old_sysnum = 0;
+ ErrorCode old_err = SyscallSet::IsValid(old_sysnum)
+ ? policy_->EvaluateSyscall(old_sysnum)->Compile(this)
+ : invalid_err;
+
+ for (uint32_t sysnum : SyscallSet::All()) {
+ ErrorCode err =
+ SyscallSet::IsValid(sysnum)
+ ? policy_->EvaluateSyscall(static_cast<int>(sysnum))->Compile(this)
+ : invalid_err;
+ if (!err.Equals(old_err)) {
+ ranges->push_back(Range(old_sysnum, old_err));
+ old_sysnum = sysnum;
+ old_err = err;
+ }
+ }
+ ranges->push_back(Range(old_sysnum, old_err));
+}
+
+Instruction* PolicyCompiler::AssembleJumpTable(Ranges::const_iterator start,
+ Ranges::const_iterator stop) {
+ // We convert the list of system call ranges into jump table that performs
+ // a binary search over the ranges.
+ // As a sanity check, we need to have at least one distinct ranges for us
+ // to be able to build a jump table.
+ if (stop - start <= 0) {
+ SANDBOX_DIE("Invalid set of system call ranges");
+ } else if (stop - start == 1) {
+ // If we have narrowed things down to a single range object, we can
+ // return from the BPF filter program.
+ return RetExpression(start->err);
+ }
+
+ // Pick the range object that is located at the mid point of our list.
+ // We compare our system call number against the lowest valid system call
+ // number in this range object. If our number is lower, it is outside of
+ // this range object. If it is greater or equal, it might be inside.
+ Ranges::const_iterator mid = start + (stop - start) / 2;
+
+ // Sub-divide the list of ranges and continue recursively.
+ Instruction* jf = AssembleJumpTable(start, mid);
+ Instruction* jt = AssembleJumpTable(mid, stop);
+ return gen_.MakeInstruction(BPF_JMP + BPF_JGE + BPF_K, mid->from, jt, jf);
+}
+
+Instruction* PolicyCompiler::RetExpression(const ErrorCode& err) {
+ switch (err.error_type()) {
+ case ErrorCode::ET_COND:
+ return CondExpression(err);
+ case ErrorCode::ET_SIMPLE:
+ case ErrorCode::ET_TRAP:
+ return gen_.MakeInstruction(BPF_RET + BPF_K, err.err());
+ default:
+ SANDBOX_DIE("ErrorCode is not suitable for returning from a BPF program");
+ }
+}
+
+Instruction* PolicyCompiler::CondExpression(const ErrorCode& cond) {
+ // Sanity check that |cond| makes sense.
+ if (cond.argno_ < 0 || cond.argno_ >= 6) {
+ SANDBOX_DIE("sandbox_bpf: invalid argument number");
+ }
+ if (cond.width_ != ErrorCode::TP_32BIT &&
+ cond.width_ != ErrorCode::TP_64BIT) {
+ SANDBOX_DIE("sandbox_bpf: invalid argument width");
+ }
+ if (cond.mask_ == 0) {
+ SANDBOX_DIE("sandbox_bpf: zero mask is invalid");
+ }
+ if ((cond.value_ & cond.mask_) != cond.value_) {
+ SANDBOX_DIE("sandbox_bpf: value contains masked out bits");
+ }
+ if (cond.width_ == ErrorCode::TP_32BIT &&
+ ((cond.mask_ >> 32) != 0 || (cond.value_ >> 32) != 0)) {
+ SANDBOX_DIE("sandbox_bpf: test exceeds argument size");
+ }
+ // TODO(mdempsky): Reject TP_64BIT on 32-bit platforms. For now we allow it
+ // because some SandboxBPF unit tests exercise it.
+
+ Instruction* passed = RetExpression(*cond.passed_);
+ Instruction* failed = RetExpression(*cond.failed_);
+
+ // We want to emit code to check "(arg & mask) == value" where arg, mask, and
+ // value are 64-bit values, but the BPF machine is only 32-bit. We implement
+ // this by independently testing the upper and lower 32-bits and continuing to
+ // |passed| if both evaluate true, or to |failed| if either evaluate false.
+ return CondExpressionHalf(cond,
+ UpperHalf,
+ CondExpressionHalf(cond, LowerHalf, passed, failed),
+ failed);
+}
+
+Instruction* PolicyCompiler::CondExpressionHalf(const ErrorCode& cond,
+ ArgHalf half,
+ Instruction* passed,
+ Instruction* failed) {
+ if (cond.width_ == ErrorCode::TP_32BIT && half == UpperHalf) {
+ // Special logic for sanity checking the upper 32-bits of 32-bit system
+ // call arguments.
+
+ // TODO(mdempsky): Compile Unexpected64bitArgument() just per program.
+ Instruction* invalid_64bit = RetExpression(Unexpected64bitArgument());
+
+ const uint32_t upper = SECCOMP_ARG_MSB_IDX(cond.argno_);
+ const uint32_t lower = SECCOMP_ARG_LSB_IDX(cond.argno_);
+
+ if (sizeof(void*) == 4) {
+ // On 32-bit platforms, the upper 32-bits should always be 0:
+ // LDW [upper]
+ // JEQ 0, passed, invalid
+ return gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS,
+ upper,
+ gen_.MakeInstruction(
+ BPF_JMP + BPF_JEQ + BPF_K, 0, passed, invalid_64bit));
+ }
+
+ // On 64-bit platforms, the upper 32-bits may be 0 or ~0; but we only allow
+ // ~0 if the sign bit of the lower 32-bits is set too:
+ // LDW [upper]
+ // JEQ 0, passed, (next)
+ // JEQ ~0, (next), invalid
+ // LDW [lower]
+ // JSET (1<<31), passed, invalid
+ //
+ // TODO(mdempsky): The JSET instruction could perhaps jump to passed->next
+ // instead, as the first instruction of passed should be "LDW [lower]".
+ return gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS,
+ upper,
+ gen_.MakeInstruction(
+ BPF_JMP + BPF_JEQ + BPF_K,
+ 0,
+ passed,
+ gen_.MakeInstruction(
+ BPF_JMP + BPF_JEQ + BPF_K,
+ std::numeric_limits<uint32_t>::max(),
+ gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS,
+ lower,
+ gen_.MakeInstruction(BPF_JMP + BPF_JSET + BPF_K,
+ 1U << 31,
+ passed,
+ invalid_64bit)),
+ invalid_64bit)));
+ }
+
+ const uint32_t idx = (half == UpperHalf) ? SECCOMP_ARG_MSB_IDX(cond.argno_)
+ : SECCOMP_ARG_LSB_IDX(cond.argno_);
+ const uint32_t mask = (half == UpperHalf) ? cond.mask_ >> 32 : cond.mask_;
+ const uint32_t value = (half == UpperHalf) ? cond.value_ >> 32 : cond.value_;
+
+ // Emit a suitable instruction sequence for (arg & mask) == value.
+
+ // For (arg & 0) == 0, just return passed.
+ if (mask == 0) {
+ CHECK_EQ(0U, value);
+ return passed;
+ }
+
+ // For (arg & ~0) == value, emit:
+ // LDW [idx]
+ // JEQ value, passed, failed
+ if (mask == std::numeric_limits<uint32_t>::max()) {
+ return gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS,
+ idx,
+ gen_.MakeInstruction(BPF_JMP + BPF_JEQ + BPF_K, value, passed, failed));
+ }
+
+ // For (arg & mask) == 0, emit:
+ // LDW [idx]
+ // JSET mask, failed, passed
+ // (Note: failed and passed are intentionally swapped.)
+ if (value == 0) {
+ return gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS,
+ idx,
+ gen_.MakeInstruction(BPF_JMP + BPF_JSET + BPF_K, mask, failed, passed));
+ }
+
+ // For (arg & x) == x where x is a single-bit value, emit:
+ // LDW [idx]
+ // JSET mask, passed, failed
+ if (mask == value && HasExactlyOneBit(mask)) {
+ return gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS,
+ idx,
+ gen_.MakeInstruction(BPF_JMP + BPF_JSET + BPF_K, mask, passed, failed));
+ }
+
+ // Generic fallback:
+ // LDW [idx]
+ // AND mask
+ // JEQ value, passed, failed
+ return gen_.MakeInstruction(
+ BPF_LD + BPF_W + BPF_ABS,
+ idx,
+ gen_.MakeInstruction(
+ BPF_ALU + BPF_AND + BPF_K,
+ mask,
+ gen_.MakeInstruction(
+ BPF_JMP + BPF_JEQ + BPF_K, value, passed, failed)));
+}
+
+ErrorCode PolicyCompiler::Unexpected64bitArgument() {
+ return Kill("Unexpected 64bit argument detected");
+}
+
+ErrorCode PolicyCompiler::Error(int err) {
+ if (has_unsafe_traps_) {
+ // When inside an UnsafeTrap() callback, we want to allow all system calls.
+ // This means, we must conditionally disable the sandbox -- and that's not
+ // something that kernel-side BPF filters can do, as they cannot inspect
+ // any state other than the syscall arguments.
+ // But if we redirect all error handlers to user-space, then we can easily
+ // make this decision.
+ // The performance penalty for this extra round-trip to user-space is not
+ // actually that bad, as we only ever pay it for denied system calls; and a
+ // typical program has very few of these.
+ return Trap(ReturnErrno, reinterpret_cast<void*>(err));
+ }
+
+ return ErrorCode(err);
+}
+
+ErrorCode PolicyCompiler::MakeTrap(TrapRegistry::TrapFnc fnc,
+ const void* aux,
+ bool safe) {
+ uint16_t trap_id = registry_->Add(fnc, aux, safe);
+ return ErrorCode(trap_id, fnc, aux, safe);
+}
+
+ErrorCode PolicyCompiler::Trap(TrapRegistry::TrapFnc fnc, const void* aux) {
+ return MakeTrap(fnc, aux, true /* Safe Trap */);
+}
+
+ErrorCode PolicyCompiler::UnsafeTrap(TrapRegistry::TrapFnc fnc,
+ const void* aux) {
+ return MakeTrap(fnc, aux, false /* Unsafe Trap */);
+}
+
+bool PolicyCompiler::IsRequiredForUnsafeTrap(int sysno) {
+ for (size_t i = 0; i < arraysize(kSyscallsRequiredForUnsafeTraps); ++i) {
+ if (sysno == kSyscallsRequiredForUnsafeTraps[i]) {
+ return true;
+ }
+ }
+ return false;
+}
+
+ErrorCode PolicyCompiler::CondMaskedEqual(int argno,
+ ErrorCode::ArgType width,
+ uint64_t mask,
+ uint64_t value,
+ const ErrorCode& passed,
+ const ErrorCode& failed) {
+ return ErrorCode(argno,
+ width,
+ mask,
+ value,
+ &*conds_.insert(passed).first,
+ &*conds_.insert(failed).first);
+}
+
+ErrorCode PolicyCompiler::Kill(const char* msg) {
+ return Trap(BPFFailure, const_cast<char*>(msg));
+}
+
+} // namespace bpf_dsl
+} // namespace sandbox
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