blob: 72a79670d32132c9e6a1448dd32bf339e770f0e1 [file] [log] [blame]
// 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/seccomp-bpf/sandbox_bpf.h"
#include <errno.h>
#include <stdint.h>
#include <sys/prctl.h>
#include <sys/types.h>
#include <unistd.h>
#include "base/compiler_specific.h"
#include "base/files/scoped_file.h"
#include "base/logging.h"
#include "base/macros.h"
#include "base/posix/eintr_wrapper.h"
#include "sandbox/linux/bpf_dsl/bpf_dsl.h"
#include "sandbox/linux/bpf_dsl/codegen.h"
#include "sandbox/linux/bpf_dsl/policy.h"
#include "sandbox/linux/bpf_dsl/policy_compiler.h"
#include "sandbox/linux/bpf_dsl/seccomp_macros.h"
#include "sandbox/linux/bpf_dsl/syscall_set.h"
#include "sandbox/linux/seccomp-bpf/die.h"
#include "sandbox/linux/seccomp-bpf/syscall.h"
#include "sandbox/linux/seccomp-bpf/trap.h"
#include "sandbox/linux/services/proc_util.h"
#include "sandbox/linux/services/syscall_wrappers.h"
#include "sandbox/linux/services/thread_helpers.h"
#include "sandbox/linux/system_headers/linux_filter.h"
#include "sandbox/linux/system_headers/linux_seccomp.h"
#include "sandbox/linux/system_headers/linux_syscalls.h"
namespace sandbox {
namespace {
// Check if the kernel supports seccomp-filter (a.k.a. seccomp mode 2) via
// prctl().
bool KernelSupportsSeccompBPF() {
errno = 0;
const int rv = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, nullptr);
if (rv == -1 && EFAULT == errno) {
return true;
return false;
// LG introduced a buggy syscall, sys_set_media_ext, with the same number as
// seccomp. Return true if the current kernel has this buggy syscall.
// We want this to work with upcoming versions of seccomp, so we pass bogus
// flags that are unlikely to ever be used by the kernel. A normal kernel would
// return -EINVAL, but a buggy LG kernel would return 1.
bool KernelHasLGBug() {
#if defined(OS_ANDROID)
// sys_set_media will see this as NULL, which should be a safe (non-crashing)
// way to invoke it. A genuine seccomp syscall will see it as
const unsigned int operation = 0;
// Chosen by fair dice roll. Guaranteed to be random.
const unsigned int flags = 0xf7a46a5c;
const int rv = sys_seccomp(operation, flags, nullptr);
// A genuine kernel would return -EINVAL (which would set rv to -1 and errno
// to EINVAL), or at the very least return some kind of error (which would
// set rv to -1). Any other behavior indicates that whatever code received
// our syscall was not the real seccomp.
if (rv != -1) {
return true;
#endif // defined(OS_ANDROID)
return false;
// Check if the kernel supports seccomp-filter via the seccomp system call
// and the TSYNC feature to enable seccomp on all threads.
bool KernelSupportsSeccompTsync() {
if (KernelHasLGBug()) {
return false;
errno = 0;
const int rv =
if (rv == -1 && errno == EFAULT) {
return true;
DCHECK_EQ(-1, rv);
DCHECK(ENOSYS == errno || EINVAL == errno);
return false;
uint64_t EscapePC() {
intptr_t rv = Syscall::Call(-1);
if (rv == -1 && errno == ENOSYS) {
return 0;
return static_cast<uint64_t>(static_cast<uintptr_t>(rv));
intptr_t SandboxPanicTrap(const struct arch_seccomp_data&, void* aux) {
SANDBOX_DIE(static_cast<const char*>(aux));
bpf_dsl::ResultExpr SandboxPanic(const char* error) {
return bpf_dsl::Trap(SandboxPanicTrap, error);
} // namespace
SandboxBPF::SandboxBPF(std::unique_ptr<bpf_dsl::Policy> policy)
: proc_fd_(), sandbox_has_started_(false), policy_(std::move(policy)) {}
SandboxBPF::~SandboxBPF() {
// static
bool SandboxBPF::SupportsSeccompSandbox(SeccompLevel level) {
switch (level) {
case SeccompLevel::SINGLE_THREADED:
return KernelSupportsSeccompBPF();
case SeccompLevel::MULTI_THREADED:
return KernelSupportsSeccompTsync();
return false;
bool SandboxBPF::StartSandbox(SeccompLevel seccomp_level) {
CHECK(seccomp_level == SeccompLevel::SINGLE_THREADED ||
seccomp_level == SeccompLevel::MULTI_THREADED);
if (sandbox_has_started_) {
"Cannot repeatedly start sandbox. Create a separate Sandbox "
"object instead.");
return false;
if (!proc_fd_.is_valid()) {
const bool supports_tsync = KernelSupportsSeccompTsync();
if (seccomp_level == SeccompLevel::SINGLE_THREADED) {
// Wait for /proc/self/task/ to update if needed and assert the
// process is single threaded.
} else if (seccomp_level == SeccompLevel::MULTI_THREADED) {
if (!supports_tsync) {
SANDBOX_DIE("Cannot start sandbox; kernel does not support synchronizing "
"filters for a threadgroup");
return false;
// We no longer need access to any files in /proc. We want to do this
// before installing the filters, just in case that our policy denies
// close().
if (proc_fd_.is_valid()) {
// Install the filters.
InstallFilter(supports_tsync ||
seccomp_level == SeccompLevel::MULTI_THREADED);
return true;
void SandboxBPF::SetProcFd(base::ScopedFD proc_fd) {
// static
bool SandboxBPF::IsValidSyscallNumber(int sysnum) {
return SyscallSet::IsValid(sysnum);
// static
bool SandboxBPF::IsRequiredForUnsafeTrap(int sysno) {
return bpf_dsl::PolicyCompiler::IsRequiredForUnsafeTrap(sysno);
// static
intptr_t SandboxBPF::ForwardSyscall(const struct arch_seccomp_data& args) {
return Syscall::Call(, static_cast<intptr_t>(args.args[0]),
static_cast<intptr_t>(args.args[1]), static_cast<intptr_t>(args.args[2]),
static_cast<intptr_t>(args.args[3]), static_cast<intptr_t>(args.args[4]),
CodeGen::Program SandboxBPF::AssembleFilter() {
bpf_dsl::PolicyCompiler compiler(policy_.get(), Trap::Registry());
if (Trap::SandboxDebuggingAllowedByUser()) {
return compiler.Compile();
void SandboxBPF::InstallFilter(bool must_sync_threads) {
// We want to be very careful in not imposing any requirements on the
// policies that are set with SetSandboxPolicy(). This means, as soon as
// the sandbox is active, we shouldn't be relying on libraries that could
// be making system calls. This, for example, means we should avoid
// using the heap and we should avoid using STL functions.
// Temporarily copy the contents of the "program" vector into a
// stack-allocated array; and then explicitly destroy that object.
// This makes sure we don't ex- or implicitly call new/delete after we
// installed the BPF filter program in the kernel. Depending on the
// system memory allocator that is in effect, these operators can result
// in system calls to things like munmap() or brk().
CodeGen::Program program = AssembleFilter();
struct sock_filter bpf[program.size()];
const struct sock_fprog prog = {static_cast<unsigned short>(program.size()),
memcpy(bpf, &program[0], sizeof(bpf));
CodeGen::Program().swap(program); // vector swap trick
// Make an attempt to release memory that is no longer needed here, rather
// than in the destructor. Try to avoid as much as possible to presume of
// what will be possible to do in the new (sandboxed) execution environment.
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
SANDBOX_DIE("Kernel refuses to enable no-new-privs");
// Install BPF filter program. If the thread state indicates multi-threading
// support, then the kernel hass the seccomp system call. Otherwise, fall
// back on prctl, which requires the process to be single-threaded.
if (must_sync_threads) {
int rv =
if (rv) {
"Kernel refuses to turn on and synchronize threads for BPF filters");
} else {
SANDBOX_DIE("Kernel refuses to turn on BPF filters");
sandbox_has_started_ = true;
} // namespace sandbox