blob: 003708d2c898fd3324a6737d9b8304c537fd243c [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/trap.h"
#include <errno.h>
#include <signal.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <sys/syscall.h>
#include <algorithm>
#include <limits>
#include <tuple>
#include "base/compiler_specific.h"
#include "base/logging.h"
#include "build/build_config.h"
#include "sandbox/linux/bpf_dsl/seccomp_macros.h"
#include "sandbox/linux/seccomp-bpf/die.h"
#include "sandbox/linux/seccomp-bpf/syscall.h"
#include "sandbox/linux/services/syscall_wrappers.h"
#include "sandbox/linux/system_headers/linux_seccomp.h"
#include "sandbox/linux/system_headers/linux_signal.h"
namespace {
struct arch_sigsys {
void* ip;
int nr;
unsigned int arch;
const int kCapacityIncrement = 20;
// Unsafe traps can only be turned on, if the user explicitly allowed them
// by setting the CHROME_SANDBOX_DEBUGGING environment variable.
const char kSandboxDebuggingEnv[] = "CHROME_SANDBOX_DEBUGGING";
// We need to tell whether we are performing a "normal" callback, or
// whether we were called recursively from within a UnsafeTrap() callback.
// This is a little tricky to do, because we need to somehow get access to
// per-thread data from within a signal context. Normal TLS storage is not
// safely accessible at this time. We could roll our own, but that involves
// a lot of complexity. Instead, we co-opt one bit in the signal mask.
// If BUS is blocked, we assume that we have been called recursively.
// There is a possibility for collision with other code that needs to do
// this, but in practice the risks are low.
// If SIGBUS turns out to be a problem, we could instead co-opt one of the
// realtime signals. There are plenty of them. Unfortunately, there is no
// way to mark a signal as allocated. So, the potential for collision is
// possibly even worse.
bool GetIsInSigHandler(const ucontext_t* ctx) {
// Note: on Android, sigismember does not take a pointer to const.
return sigismember(const_cast<sigset_t*>(&ctx->uc_sigmask), LINUX_SIGBUS);
void SetIsInSigHandler() {
sigset_t mask;
if (sigemptyset(&mask) || sigaddset(&mask, LINUX_SIGBUS) ||
sandbox::sys_sigprocmask(LINUX_SIG_BLOCK, &mask, NULL)) {
SANDBOX_DIE("Failed to block SIGBUS");
bool IsDefaultSignalAction(const struct sigaction& sa) {
if (sa.sa_flags & SA_SIGINFO || sa.sa_handler != SIG_DFL) {
return false;
return true;
} // namespace
namespace sandbox {
: trap_array_(NULL),
has_unsafe_traps_(false) {
// Set new SIGSYS handler
struct sigaction sa = {};
// In some toolchain, sa_sigaction is not declared in struct sigaction.
// So, here cast the pointer to the sa_handler's type. This works because
// |sa_handler| and |sa_sigaction| shares the same memory.
sa.sa_handler = reinterpret_cast<void (*)(int)>(SigSysAction);
struct sigaction old_sa = {};
if (sys_sigaction(LINUX_SIGSYS, &sa, &old_sa) < 0) {
SANDBOX_DIE("Failed to configure SIGSYS handler");
if (!IsDefaultSignalAction(old_sa)) {
static const char kExistingSIGSYSMsg[] =
"Existing signal handler when trying to install SIGSYS. SIGSYS needs "
"to be reserved for seccomp-bpf.";
DLOG(FATAL) << kExistingSIGSYSMsg;
LOG(ERROR) << kExistingSIGSYSMsg;
// Unmask SIGSYS
sigset_t mask;
if (sigemptyset(&mask) || sigaddset(&mask, LINUX_SIGSYS) ||
sys_sigprocmask(LINUX_SIG_UNBLOCK, &mask, NULL)) {
SANDBOX_DIE("Failed to configure SIGSYS handler");
bpf_dsl::TrapRegistry* Trap::Registry() {
// Note: This class is not thread safe. It is the caller's responsibility
// to avoid race conditions. Normally, this is a non-issue as the sandbox
// can only be initialized if there are no other threads present.
// Also, this is not a normal singleton. Once created, the global trap
// object must never be destroyed again.
if (!global_trap_) {
global_trap_ = new Trap();
if (!global_trap_) {
SANDBOX_DIE("Failed to allocate global trap handler");
return global_trap_;
void Trap::SigSysAction(int nr, LinuxSigInfo* info, void* void_context) {
if (info) {
MSAN_UNPOISON(info, sizeof(*info));
// Obtain the signal context. This, most notably, gives us access to
// all CPU registers at the time of the signal.
ucontext_t* ctx = reinterpret_cast<ucontext_t*>(void_context);
if (ctx) {
MSAN_UNPOISON(ctx, sizeof(*ctx));
if (!global_trap_) {
"This can't happen. Found no global singleton instance "
"for Trap() handling.");
global_trap_->SigSys(nr, info, ctx);
void Trap::SigSys(int nr, LinuxSigInfo* info, ucontext_t* ctx) {
// Signal handlers should always preserve "errno". Otherwise, we could
// trigger really subtle bugs.
const int old_errno = errno;
// Various sanity checks to make sure we actually received a signal
// triggered by a BPF filter. If something else triggered SIGSYS
// (e.g. kill()), there is really nothing we can do with this signal.
if (nr != LINUX_SIGSYS || info->si_code != SYS_SECCOMP || !ctx ||
info->si_errno <= 0 ||
static_cast<size_t>(info->si_errno) > trap_array_size_) {
// ATI drivers seem to send SIGSYS, so this cannot be FATAL.
// See
// TODO(jln): add a DCHECK or move back to FATAL.
RAW_LOG(ERROR, "Unexpected SIGSYS received.");
errno = old_errno;
// Obtain the siginfo information that is specific to SIGSYS. Unfortunately,
// most versions of glibc don't include this information in siginfo_t. So,
// we need to explicitly copy it into a arch_sigsys structure.
struct arch_sigsys sigsys;
memcpy(&sigsys, &info->_sifields, sizeof(sigsys));
#if defined(__mips__)
// When indirect syscall (syscall(__NR_foo, ...)) is made on Mips, the
// number in register SECCOMP_SYSCALL(ctx) is always __NR_syscall and the
// real number of a syscall (__NR_foo) is in SECCOMP_PARM1(ctx)
bool sigsys_nr_is_bad = != static_cast<int>(SECCOMP_SYSCALL(ctx)) && != static_cast<int>(SECCOMP_PARM1(ctx));
bool sigsys_nr_is_bad = != static_cast<int>(SECCOMP_SYSCALL(ctx));
// Some more sanity checks.
if (sigsys.ip != reinterpret_cast<void*>(SECCOMP_IP(ctx)) ||
sigsys_nr_is_bad || sigsys.arch != SECCOMP_ARCH) {
// TODO(markus):
// SANDBOX_DIE() can call LOG(FATAL). This is not normally async-signal
// safe and can lead to bugs. We should eventually implement a different
// logging and reporting mechanism that is safe to be called from
// the sigSys() handler.
RAW_SANDBOX_DIE("Sanity checks are failing after receiving SIGSYS.");
intptr_t rc;
if (has_unsafe_traps_ && GetIsInSigHandler(ctx)) {
errno = old_errno;
if ( == __NR_clone) {
RAW_SANDBOX_DIE("Cannot call clone() from an UnsafeTrap() handler.");
#if defined(__mips__)
// Mips supports up to eight arguments for syscall.
// However, seccomp bpf can filter only up to six arguments, so using eight
// arguments has sense only when using UnsafeTrap() handler.
rc = Syscall::Call(SECCOMP_SYSCALL(ctx),
rc = Syscall::Call(SECCOMP_SYSCALL(ctx),
#endif // defined(__mips__)
} else {
const TrapKey& trap = trap_array_[info->si_errno - 1];
if (! {
// Copy the seccomp-specific data into a arch_seccomp_data structure. This
// is what we are showing to TrapFnc callbacks that the system call
// evaluator registered with the sandbox.
struct arch_seccomp_data data = {
// Now call the TrapFnc callback associated with this particular instance
rc = trap.fnc(data, const_cast<void*>(trap.aux));
// Update the CPU register that stores the return code of the system call
// that we just handled, and restore "errno" to the value that it had
// before entering the signal handler.
Syscall::PutValueInUcontext(rc, ctx);
errno = old_errno;
bool Trap::TrapKey::operator<(const TrapKey& o) const {
return std::tie(fnc, aux, safe) < std::tie(o.fnc, o.aux,;
uint16_t Trap::Add(TrapFnc fnc, const void* aux, bool safe) {
if (!safe && !SandboxDebuggingAllowedByUser()) {
// Unless the user set the CHROME_SANDBOX_DEBUGGING environment variable,
// we never return an ErrorCode that is marked as "unsafe". This also
// means, the BPF compiler will never emit code that allow unsafe system
// calls to by-pass the filter (because they use the magic return address
// from Syscall::Call(-1)).
// This SANDBOX_DIE() can optionally be removed. It won't break security,
// but it might make error messages from the BPF compiler a little harder
// to understand. Removing the SANDBOX_DIE() allows callers to easily check
// whether unsafe traps are supported (by checking whether the returned
// ErrorCode is ET_INVALID).
"Cannot use unsafe traps unless CHROME_SANDBOX_DEBUGGING "
"is enabled");
return 0;
// Each unique pair of TrapFnc and auxiliary data make up a distinct instance
TrapKey key(fnc, aux, safe);
// We return unique identifiers together with SECCOMP_RET_TRAP. This allows
// us to associate trap with the appropriate handler. The kernel allows us
// identifiers in the range from 0 to SECCOMP_RET_DATA (0xFFFF). We want to
// avoid 0, as it could be confused for a trap without any specific id.
// The nice thing about sequentially numbered identifiers is that we can also
// trivially look them up from our signal handler without making any system
// calls that might be async-signal-unsafe.
// In order to do so, we store all of our traps in a C-style trap_array_.
TrapIds::const_iterator iter = trap_ids_.find(key);
if (iter != trap_ids_.end()) {
// We have seen this pair before. Return the same id that we assigned
// earlier.
return iter->second;
// This is a new pair. Remember it and assign a new id.
if (trap_array_size_ >= SECCOMP_RET_DATA /* 0xFFFF */ ||
trap_array_size_ >= std::numeric_limits<uint16_t>::max()) {
// In practice, this is pretty much impossible to trigger, as there
// are other kernel limitations that restrict overall BPF program sizes.
SANDBOX_DIE("Too many SECCOMP_RET_TRAP callback instances");
// Our callers ensure that there are no other threads accessing trap_array_
// concurrently (typically this is done by ensuring that we are single-
// threaded while the sandbox is being set up). But we nonetheless are
// modifying a live data structure that could be accessed any time a
// system call is made; as system calls could be triggering SIGSYS.
// So, we have to be extra careful that we update trap_array_ atomically.
// In particular, this means we shouldn't be using realloc() to resize it.
// Instead, we allocate a new array, copy the values, and then switch the
// pointer. We only really care about the pointer being updated atomically
// and the data that is pointed to being valid, as these are the only
// values accessed from the signal handler. It is OK if trap_array_size_
// is inconsistent with the pointer, as it is monotonously increasing.
// Also, we only care about compiler barriers, as the signal handler is
// triggered synchronously from a system call. We don't have to protect
// against issues with the memory model or with completely asynchronous
// events.
if (trap_array_size_ >= trap_array_capacity_) {
trap_array_capacity_ += kCapacityIncrement;
TrapKey* old_trap_array = trap_array_;
TrapKey* new_trap_array = new TrapKey[trap_array_capacity_];
std::copy_n(old_trap_array, trap_array_size_, new_trap_array);
// Language specs are unclear on whether the compiler is allowed to move
// the "delete[]" above our preceding assignments and/or memory moves,
// iff the compiler believes that "delete[]" doesn't have any other
// global side-effects.
// We insert optimization barriers to prevent this from happening.
// The first barrier is probably not needed, but better be explicit in
// what we want to tell the compiler.
// The clang developer mailing list couldn't answer whether this is a
// legitimate worry; but they at least thought that the barrier is
// sufficient to prevent the (so far hypothetical) problem of re-ordering
// of instructions by the compiler.
// TODO(mdempsky): Try to clean this up using base/atomicops or C++11
// atomics; see
asm volatile("" : "=r"(new_trap_array) : "0"(new_trap_array) : "memory");
trap_array_ = new_trap_array;
asm volatile("" : "=r"(trap_array_) : "0"(trap_array_) : "memory");
delete[] old_trap_array;
uint16_t id = trap_array_size_ + 1;
trap_ids_[key] = id;
trap_array_[trap_array_size_] = key;
return id;
bool Trap::SandboxDebuggingAllowedByUser() {
const char* debug_flag = getenv(kSandboxDebuggingEnv);
return debug_flag && *debug_flag;
bool Trap::EnableUnsafeTraps() {
if (!has_unsafe_traps_) {
// Unsafe traps are a one-way fuse. Once enabled, they can never be turned
// off again.
// We only allow enabling unsafe traps, if the user explicitly set an
// appropriate environment variable. This prevents bugs that accidentally
// disable all sandboxing for all users.
if (SandboxDebuggingAllowedByUser()) {
// We only ever print this message once, when we enable unsafe traps the
// first time.
SANDBOX_INFO("WARNING! Disabling sandbox for debugging purposes");
has_unsafe_traps_ = true;
} else {
"Cannot disable sandbox and use unsafe traps unless "
"CHROME_SANDBOX_DEBUGGING is turned on first");
// Returns the, possibly updated, value of has_unsafe_traps_.
return has_unsafe_traps_;
Trap* Trap::global_trap_;
} // namespace sandbox