gecko-dev/security/sandbox/Sandbox.cpp

/* -*- 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,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

#include <unistd.h>
#include <stdio.h>
#include <sys/ptrace.h>
#include <signal.h>

#include "mozilla/Util.h"
#if defined(ANDROID)
#include "android_ucontext.h"
#endif
#include "seccomp_filter.h"

#include "linux_seccomp.h"
#ifdef MOZ_LOGGING
#define FORCE_PR_LOG 1
#endif
#include "prlog.h"

namespace mozilla {
#ifdef PR_LOGGING
static PRLogModuleInfo* gSeccompSandboxLog;
#endif

struct sock_filter seccomp_filter[] = {
  VALIDATE_ARCHITECTURE,
  EXAMINE_SYSCALL,
  SECCOMP_WHITELIST,
#ifdef MOZ_CONTENT_SANDBOX_REPORTER
  TRAP_PROCESS,
#else
  KILL_PROCESS,
#endif
};

struct sock_fprog seccomp_prog = {
  len: (unsigned short)MOZ_ARRAY_LENGTH(seccomp_filter),
  filter: seccomp_filter,
};

/**
 * This is the SIGSYS handler function. It is used to report to the user
 * which system call has been denied by Seccomp.
 * This function also makes the process exit as denying the system call
 * will otherwise generally lead to unexpected behavior from the process,
 * since we don't know if all functions will handle such denials gracefully.
 *
 * @see InstallSyscallReporter() function.
 */
#ifdef MOZ_CONTENT_SANDBOX_REPORTER
static void
Reporter(int nr, siginfo_t *info, void *void_context)
{
  ucontext_t *ctx = static_cast<ucontext_t*>(void_context);
  unsigned int syscall, arg1;

  if (nr != SIGSYS) {
    return;
  }
  if (info->si_code != SYS_SECCOMP) {
    return;
  }
  if (!ctx) {
    return;
  }

  syscall = SECCOMP_SYSCALL(ctx);
  arg1 = SECCOMP_PARM1(ctx);

  PR_LOG(gSeccompSandboxLog, PR_LOG_ERROR, ("PID %u is missing syscall %u, arg1 %u\n", getpid(), syscall, arg1));

  _exit(127);
}

/**
 * The reporter is called when the process receives a SIGSYS signal.
 * The signal is sent by the kernel when Seccomp encounter a system call
 * that has not been allowed.
 * We register an action for that signal (calling the Reporter function).
 *
 * This function should not be used in production and thus generally be
 * called from debug code. In production, the process is directly killed.
 * For this reason, the function is ifdef'd, as there is no reason to
 * compile it while unused.
 *
 * @return 0 on success, -1 on failure.
 * @see Reporter() function.
 */
static int
InstallSyscallReporter(void)
{
  struct sigaction act;
  sigset_t mask;
  memset(&act, 0, sizeof(act));
  sigemptyset(&mask);
  sigaddset(&mask, SIGSYS);

  act.sa_sigaction = &Reporter;
  act.sa_flags = SA_SIGINFO | SA_NODEFER;
  if (sigaction(SIGSYS, &act, NULL) < 0) {
    return -1;
  }
  if (sigemptyset(&mask) ||
    sigaddset(&mask, SIGSYS) ||
    sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
      return -1;
  }
  return 0;
}
#endif

/**
 * This function installs the syscall filter, a.k.a. seccomp.
 * PR_SET_NO_NEW_PRIVS ensures that it is impossible to grant more
 * syscalls to the process beyond this point (even after fork()).
 * SECCOMP_MODE_FILTER is the "bpf" mode of seccomp which allows
 * to pass a bpf program (in our case, it contains a syscall
 * whitelist).
 *
 * @return 0 on success, 1 on failure.
 * @see sock_fprog (the seccomp_prog).
 */
static int
InstallSyscallFilter(void)
{
  if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
    return 1;
  }

  if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &seccomp_prog, 0, 0)) {
    return 1;
  }
  return 0;
}

/**
 * Starts the seccomp sandbox for this process.
 * Generally called just after SetCurrentProcessPrivileges.
 * Should be called only once, and before any potentially harmful content is loaded.
 *
 * Should normally make the process exit on failure.
*/
void
SetCurrentProcessSandbox(void)
{
#ifdef PR_LOGGING
  if (!gSeccompSandboxLog) {
    gSeccompSandboxLog = PR_NewLogModule("SeccompSandbox");
  }
  PR_ASSERT(gSeccompSandboxLog);
#endif

#ifdef MOZ_CONTENT_SANDBOX_REPORTER
  if (InstallSyscallReporter()) {
    PR_LOG(gSeccompSandboxLog, PR_LOG_ERROR, ("install_syscall_reporter() failed\n"));
    /* This is disabled so that we do not exit if seccomp-bpf is not available
     * This will be re-enabled when all B2G devices are required to support seccomp-bpf
     * See bug 880797 for reversal
     */

    /* _exit(127); */
  }

#endif

  if (InstallSyscallFilter()) {
    PR_LOG(gSeccompSandboxLog, PR_LOG_ERROR, ("install_syscall_filter() failed\n"));
    /* This is disabled so that we do not exit if seccomp-bpf is not available
     * This will be re-enabled when all B2G devices are required to support seccomp-bpf
     * See bug 880797 for reversal
     */

    /* _exit(127); */
  }

}
} // namespace mozilla