Update from chromium 62675d9fb31fb8cedc40f68e78e8445a74f362e7

sandbox/linux/seccomp-bpf/trap.cc

+// 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 <string.h>
+#include <sys/syscall.h>
+
+#include <algorithm>
+#include <limits>
+
+#include "base/logging.h"
+#include "build/build_config.h"
+#include "sandbox/linux/seccomp-bpf/die.h"
+#include "sandbox/linux/seccomp-bpf/linux_seccomp.h"
+#include "sandbox/linux/seccomp-bpf/syscall.h"
+
+// Android's signal.h doesn't define ucontext etc.
+#if defined(OS_ANDROID)
+#include "sandbox/linux/services/android_ucontext.h"
+#endif
+
+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), SIGBUS);
+}
+
+void SetIsInSigHandler() {
+  sigset_t mask;
+  if (sigemptyset(&mask) || sigaddset(&mask, SIGBUS) ||
+      sigprocmask(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::Trap()
+    : trap_array_(NULL),
+      trap_array_size_(0),
+      trap_array_capacity_(0),
+      has_unsafe_traps_(false) {
+  // Set new SIGSYS handler
+  struct sigaction sa = {};
+  sa.sa_sigaction = SigSysAction;
+  sa.sa_flags = SA_SIGINFO | SA_NODEFER;
+  struct sigaction old_sa;
+  if (sigaction(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, SIGSYS) ||
+      sigprocmask(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, siginfo_t* info, void* void_context) {
+  if (!global_trap_) {
+    RAW_SANDBOX_DIE(
+        "This can't happen. Found no global singleton instance "
+        "for Trap() handling.");
+  }
+  global_trap_->SigSys(nr, info, void_context);
+}
+
+void Trap::SigSys(int nr, siginfo_t* info, void* void_context) {
+  // 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 != SIGSYS || info->si_code != SYS_SECCOMP || !void_context ||
+      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 crbug.com/178166.
+    // TODO(jln): add a DCHECK or move back to FATAL.
+    RAW_LOG(ERROR, "Unexpected SIGSYS received.");
+    errno = old_errno;
+    return;
+  }
+
+  // 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);
+
+  // 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 = sigsys.nr != static_cast<int>(SECCOMP_SYSCALL(ctx)) &&
+                          sigsys.nr != static_cast<int>(SECCOMP_PARM1(ctx));
+#else
+  bool sigsys_nr_is_bad = sigsys.nr != static_cast<int>(SECCOMP_SYSCALL(ctx));
+#endif
+
+  // 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 (sigsys.nr == __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),
+                       SECCOMP_PARM1(ctx),
+                       SECCOMP_PARM2(ctx),
+                       SECCOMP_PARM3(ctx),
+                       SECCOMP_PARM4(ctx),
+                       SECCOMP_PARM5(ctx),
+                       SECCOMP_PARM6(ctx),
+                       SECCOMP_PARM7(ctx),
+                       SECCOMP_PARM8(ctx));
+#else
+    rc = Syscall::Call(SECCOMP_SYSCALL(ctx),
+                       SECCOMP_PARM1(ctx),
+                       SECCOMP_PARM2(ctx),
+                       SECCOMP_PARM3(ctx),
+                       SECCOMP_PARM4(ctx),
+                       SECCOMP_PARM5(ctx),
+                       SECCOMP_PARM6(ctx));
+#endif  // defined(__mips__)
+  } else {
+    const TrapKey& trap = trap_array_[info->si_errno - 1];
+    if (!trap.safe) {
+      SetIsInSigHandler();
+    }
+
+    // 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 = {
+        static_cast<int>(SECCOMP_SYSCALL(ctx)),
+        SECCOMP_ARCH,
+        reinterpret_cast<uint64_t>(sigsys.ip),
+        {static_cast<uint64_t>(SECCOMP_PARM1(ctx)),
+         static_cast<uint64_t>(SECCOMP_PARM2(ctx)),
+         static_cast<uint64_t>(SECCOMP_PARM3(ctx)),
+         static_cast<uint64_t>(SECCOMP_PARM4(ctx)),
+         static_cast<uint64_t>(SECCOMP_PARM5(ctx)),
+         static_cast<uint64_t>(SECCOMP_PARM6(ctx))}};
+
+    // Now call the TrapFnc callback associated with this particular instance
+    // of SECCOMP_RET_TRAP.
+    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;
+
+  return;
+}
+
+bool Trap::TrapKey::operator<(const TrapKey& o) const {
+  if (fnc != o.fnc) {
+    return fnc < o.fnc;
+  } else if (aux != o.aux) {
+    return aux < o.aux;
+  } else {
+    return safe < o.safe;
+  }
+}
+
+uint16_t Trap::MakeTrap(TrapFnc fnc, const void* aux, bool safe) {
+  return Registry()->Add(fnc, aux, safe);
+}
+
+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).
+    SANDBOX_DIE(
+        "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
+  // of a SECCOMP_RET_TRAP.
+  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 crbug.com/414363.
+    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;
+  trap_array_size_++;
+  return id;
+}
+
+bool Trap::SandboxDebuggingAllowedByUser() const {
+  const char* debug_flag = getenv(kSandboxDebuggingEnv);
+  return debug_flag && *debug_flag;
+}
+
+bool Trap::EnableUnsafeTrapsInSigSysHandler() {
+  return Registry()->EnableUnsafeTraps();
+}
+
+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 {
+      SANDBOX_INFO(
+          "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