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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this file,
#include "SandboxInfo.h"
#include "SandboxLogging.h"
#include "LinuxSched.h"
#include <errno.h>
#include <stdlib.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/wait.h>
#include <unistd.h>
#include "base/posix/eintr_wrapper.h"
#include "mozilla/Assertions.h"
#include "mozilla/ArrayUtils.h"
#include "mozilla/SandboxSettings.h"
#include "sandbox/linux/system_headers/linux_seccomp.h"
#include "sandbox/linux/system_headers/linux_syscalls.h"
#ifdef MOZ_VALGRIND
# include <valgrind/valgrind.h>
#endif
// A note about assertions: in general, the worst thing this module
// should be able to do is disable sandboxing features, so release
// asserts or MOZ_CRASH should be avoided, even for seeming
// impossibilities like an unimplemented syscall returning success
//
// MOZ_DIAGNOSTIC_ASSERT (debug builds, plus Nightly/Aurora non-debug)
// is probably the best choice for conditions that shouldn't be able
// to fail without the help of bugs in the kernel or system libraries.
//
// Regardless of assertion type, whatever condition caused it to fail
// should generally also disable the corresponding feature on builds
// that omit the assertion.
namespace mozilla {
static bool HasSeccompBPF() {
// Allow simulating the absence of seccomp-bpf support, for testing.
if (getenv("MOZ_FAKE_NO_SANDBOX")) {
return false;
}
// Valgrind and the sandbox don't interact well, probably because Valgrind
// does various system calls which aren't allowed, even if Firefox itself
// is playing by the rules.
#if defined(MOZ_VALGRIND)
if (RUNNING_ON_VALGRIND) {
return false;
}
#endif
// Determine whether seccomp-bpf is supported by trying to
// enable it with an invalid pointer for the filter. This will
// fail with EFAULT if supported and EINVAL if not, without
// changing the process's state.
int rv = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, nullptr);
MOZ_DIAGNOSTIC_ASSERT(rv == -1,
"prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER,"
" nullptr) didn't fail");
MOZ_DIAGNOSTIC_ASSERT(errno == EFAULT || errno == EINVAL);
return rv == -1 && errno == EFAULT;
}
static bool HasSeccompTSync() {
// Similar to above, but for thread-sync mode. See also Chromium's
// sandbox::SandboxBPF::SupportsSeccompThreadFilterSynchronization
if (getenv("MOZ_FAKE_NO_SECCOMP_TSYNC")) {
return false;
}
int rv = syscall(__NR_seccomp, SECCOMP_SET_MODE_FILTER,
SECCOMP_FILTER_FLAG_TSYNC, nullptr);
MOZ_DIAGNOSTIC_ASSERT(rv == -1,
"seccomp(..., SECCOMP_FILTER_FLAG_TSYNC,"
" nullptr) didn't fail");
MOZ_DIAGNOSTIC_ASSERT(errno == EFAULT || errno == EINVAL || errno == ENOSYS);
return rv == -1 && errno == EFAULT;
}
static bool HasUserNamespaceSupport() {
// Note: the /proc/<pid>/ns/* files track setns(2) support, which in
// some cases (e.g., pid) significantly postdates kernel support for
// the namespace type, so in general this type of check could be a
// false negative. However, for user namespaces, any kernel new
// enough for the feature to be usable for us has setns support
// (v3.8), so this is okay.
//
// The non-user namespaces all default to "y" in init/Kconfig, but
// check them explicitly in case someone has a weird custom config.
static const char* const paths[] = {
"/proc/self/ns/user",
"/proc/self/ns/pid",
"/proc/self/ns/net",
"/proc/self/ns/ipc",
};
for (size_t i = 0; i < ArrayLength(paths); ++i) {
if (access(paths[i], F_OK) == -1) {
MOZ_ASSERT(errno == ENOENT);
return false;
}
}
return true;
}
static bool CanCreateUserNamespace() {
// Unfortunately, the only way to verify that this process can
// create a new user namespace is to actually create one; because
// this process's namespaces shouldn't be side-effected (yet), it's
// necessary to clone (and collect) a child process. See also
// Chromium's sandbox::Credentials::SupportsNewUserNS.
//
// This is somewhat more expensive than the other tests, so it's
// cached in the environment to prevent child processes from having
// to re-run the test.
//
// This is run at static initializer time, while single-threaded, so
// locking isn't needed to access the environment.
static const char kCacheEnvName[] = "MOZ_ASSUME_USER_NS";
const char* cached = getenv(kCacheEnvName);
if (cached) {
return cached[0] > '0';
}
// the new capabilities (in this case, cloning another namespace) to
// detect AppArmor policies that allow CLONE_NEWUSER but don't allow
// doing anything useful with it.
//
// unsharing NEWUSER and NEWPID (or NEWNET etc.) in one syscall
// being allowed, but further use of capabilities will be blocked
// afterwards. That may be a bug, but we need to handle it.
pid_t pid = syscall(__NR_clone, SIGCHLD | CLONE_NEWUSER, nullptr, nullptr,
nullptr, nullptr);
if (pid == 0) {
// The exact meaning of `unshare(CLONE_NEWPID)` is slightly
// counterintuitive but in this case it doesn't matter. This just
// needs to be some operation that attempts to use capabilities,
// to check if it's blocked by an LSM.
int rv = unshare(CLONE_NEWPID);
// Exit with status 0 on success, 1 on failure.
_exit(rv == 0 ? 0 : 1);
}
if (pid == -1) {
// Failure.
MOZ_ASSERT(errno == EINVAL || // unsupported
errno == EPERM || // root-only, or we're already chrooted
errno == EUSERS); // already at user namespace nesting limit
setenv(kCacheEnvName, "0", 1);
return false;
}
// Otherwise, in the parent and successful.
int wstatus;
bool waitpid_ok = HANDLE_EINTR(waitpid(pid, &wstatus, 0)) == pid;
MOZ_ASSERT(waitpid_ok);
if (!waitpid_ok) {
return false;
}
// Check for failures reported by the child process.
if (!WIFEXITED(wstatus) || WEXITSTATUS(wstatus) != 0) {
setenv(kCacheEnvName, "0", 1);
return false;
}
setenv(kCacheEnvName, "1", 1);
return true;
}
/* static */
const SandboxInfo SandboxInfo::sSingleton = SandboxInfo();
SandboxInfo::SandboxInfo() {
int flags = 0;
static_assert(sizeof(flags) >= sizeof(Flags), "enum Flags fits in an int");
if (HasSeccompBPF()) {
flags |= kHasSeccompBPF;
if (HasSeccompTSync()) {
flags |= kHasSeccompTSync;
}
}
if (HasUserNamespaceSupport()) {
flags |= kHasPrivilegedUserNamespaces;
if (CanCreateUserNamespace()) {
flags |= kHasUserNamespaces;
}
}
// We can't use mozilla::IsContentSandboxEnabled() here because a)
// libmozsandbox can't depend on libxul, and b) this is called in a static
// initializer before the prefences service is ready.
if (!getenv("MOZ_DISABLE_CONTENT_SANDBOX")) {
flags |= kEnabledForContent;
}
if (getenv("MOZ_PERMISSIVE_CONTENT_SANDBOX")) {
flags |= kPermissive;
}
if (!getenv("MOZ_DISABLE_GMP_SANDBOX")) {
flags |= kEnabledForMedia;
}
if (getenv("MOZ_SANDBOX_LOGGING")) {
flags |= kVerbose;
}
mFlags = static_cast<Flags>(flags);
}
} // namespace mozilla