Revert 79618 - Landing for Julien Tinnes, jln@google.com:

---
chroot to /proc instead of /tmp. This gets rid of a lot of unnecessary
complexity and fixes a race condition.

(Original idea from Markus)

The chroot helper will chroot to /proc/self/fdinfo (or /proc/self/fd). This is
pretty safe because access to this directory is protected by the ptrace() check
in the kernel and the helper is privileged.

Moreover, as soon as the helper _exit() and becomes a zombie, the directory
will be empty. Zygote should wait() for us to make everything deterministric.

We also export SBX_HELPER_PID so that Zygote can specifically wait for the
helper. 
---

BUG=76542
R=markus,agl
Review URL: http://codereview.chromium.org/6683056

TBR=cevans@chromium.org
Review URL: http://codereview.chromium.org/6675053

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@79867 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 124 insertions, 51 deletions

diff --git a/sandbox/linux/suid/sandbox.c b/sandbox/linux/suid/sandbox.c
index 1cf95284..d42474a 100644
--- a/sandbox/linux/suid/sandbox.c
+++ b/sandbox/linux/suid/sandbox.c
@@ -37,8 +37,29 @@
 #define CLONE_NEWNET 0x40000000
 #endif
 
+#if !defined(BTRFS_SUPER_MAGIC)
+#define BTRFS_SUPER_MAGIC 0x9123683E
+#endif
+#if !defined(EXT2_SUPER_MAGIC)
+#define EXT2_SUPER_MAGIC 0xEF53
+#endif
+#if !defined(EXT3_SUPER_MAGIC)
+#define EXT3_SUPER_MAGIC 0xEF53
+#endif
+#if !defined(EXT4_SUPER_MAGIC)
+#define EXT4_SUPER_MAGIC 0xEF53
+#endif
+#if !defined(REISERFS_SUPER_MAGIC)
+#define REISERFS_SUPER_MAGIC 0x52654973
+#endif
+#if !defined(TMPFS_MAGIC)
+#define TMPFS_MAGIC 0x01021994
+#endif
+#if !defined(XFS_SUPER_MAGIC)
+#define XFS_SUPER_MAGIC 0x58465342
+#endif
+
 static const char kSandboxDescriptorEnvironmentVarName[] = "SBX_D";
-static const char kSandboxHelperPidEnvironmentVarName[] = "SBX_HELPER_PID";
 
 // These are the magic byte values which the sandboxed process uses to request
 // that it be chrooted.
@@ -58,35 +79,84 @@ static void FatalError(const char *msg, ...) {
   exit(1);
 }
 
-// We will chroot() to the helper's /proc/self directory. Anything there will
-// not exist anymore if we make sure to wait() for the helper.
-//
-// /proc/self/fdinfo or /proc/self/fd are especially safe and will be empty
-// even if the helper survives as a zombie.
-//
-// There is very little reason to use fdinfo/ instead of fd/ but we are
-// paranoid. fdinfo/ only exists since 2.6.22 so we allow fallback to fd/
-#define SAFE_DIR "/proc/self/fdinfo"
-#define SAFE_DIR2 "/proc/self/fd"
-
-static bool SpawnChrootHelper() {
+static int CloneChrootHelperProcess() {
   int sv[2];
   if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) == -1) {
     perror("socketpair");
-    return false;
+    return -1;
   }
 
-  char *safedir = NULL;
-  struct stat sdir_stat;
-  if (!stat(SAFE_DIR, &sdir_stat) && S_ISDIR(sdir_stat.st_mode))
-    safedir = SAFE_DIR;
-  else
-    if (!stat(SAFE_DIR2, &sdir_stat) && S_ISDIR(sdir_stat.st_mode))
-      safedir = SAFE_DIR2;
-    else {
-      fprintf(stderr, "Could not find %s\n", SAFE_DIR2);
-      return false;
+  // Some people mount /tmp on a non-POSIX filesystem (e.g. NFS). This
+  // breaks all sorts of assumption in our code. So, if we don't recognize the
+  // filesystem, we will try to use an alternative location for our temp
+  // directory.
+  char tempDirectoryTemplate[80] = "/tmp/chrome-sandbox-chroot-XXXXXX";
+  struct statfs sfs;
+  if (!statfs("/tmp", &sfs) &&
+      (unsigned long)sfs.f_type != BTRFS_SUPER_MAGIC &&
+      (unsigned long)sfs.f_type != EXT2_SUPER_MAGIC &&
+      (unsigned long)sfs.f_type != EXT3_SUPER_MAGIC &&
+      (unsigned long)sfs.f_type != EXT4_SUPER_MAGIC &&
+      (unsigned long)sfs.f_type != REISERFS_SUPER_MAGIC &&
+      (unsigned long)sfs.f_type != TMPFS_MAGIC &&
+      (unsigned long)sfs.f_type != XFS_SUPER_MAGIC) {
+    // If /dev/shm exists, it is supposed to be a tmpfs filesystem. While we
+    // are not actually using it for shared memory, moving our temp directory
+    // into a known tmpfs filesystem is preferable over using a potentially
+    // unreliable non-POSIX filesystem.
+    if (!statfs("/dev/shm", &sfs) && sfs.f_type == TMPFS_MAGIC) {
+      *tempDirectoryTemplate = '\000';
+      strncat(tempDirectoryTemplate, "/dev/shm/chrome-sandbox-chroot-XXXXXX",
+              sizeof(tempDirectoryTemplate) - 1);
+    } else {
+      // Neither /tmp is a well-known POSIX filesystem, nor /dev/shm is a
+      // tmpfs. After all, we now use /tmp as the location of our temp
+      // directory, but we quite likely fail the moment we try to access it
+      // through chroot_dir_fd. If so, we will print a verbose error message
+      // (see below)
     }
+  }
+
+  // We create a temp directory for our chroot. Nobody should ever write into
+  // it, so it's root:root mode 000.
+  const char* temp_dir = mkdtemp(tempDirectoryTemplate);
+  if (!temp_dir) {
+    perror("Failed to create temp directory for chroot");
+    return -1;
+  }
+
+  const int chroot_dir_fd = open(temp_dir, O_DIRECTORY | O_RDONLY);
+  if (chroot_dir_fd < 0) {
+    rmdir(temp_dir);
+    perror("Failed to open chroot temp directory");
+    return -1;
+  }
+
+  if (rmdir(temp_dir)) {
+    perror("rmdir");
+    return -1;
+  }
+
+  char proc_self_fd_str[128];
+  int printed = snprintf(proc_self_fd_str, sizeof(proc_self_fd_str),
+                         "/proc/self/fd/%d", chroot_dir_fd);
+  if (printed < 0 || printed >= (int)sizeof(proc_self_fd_str)) {
+    fprintf(stderr, "Error in snprintf");
+    return -1;
+  }
+
+  if (fchown(chroot_dir_fd, 0 /* root */, 0 /* root */)) {
+    fprintf(stderr, "Could not set up sandbox work directory. Maybe, /tmp is "
+                    "a non-POSIX filesystem and /dev/shm doesn't exist "
+                    "either. Consider mounting a \"tmpfs\" on /tmp.\n");
+    return -1;
+  }
+
+  if (fchmod(chroot_dir_fd, 0000 /* no-access */)) {
+    perror("fchmod");
+    return -1;
+  }
+
 
   const pid_t pid = syscall(
       __NR_clone, CLONE_FS | SIGCHLD, 0, 0, 0);
@@ -95,7 +165,7 @@ static bool SpawnChrootHelper() {
     perror("clone");
     close(sv[0]);
     close(sv[1]);
-    return false;
+    return -1;
   }
 
   if (pid == 0) {
@@ -126,12 +196,18 @@ static bool SpawnChrootHelper() {
     if (msg != kMsgChrootMe)
       FatalError("Unknown message from sandboxed process");
 
-    // sanity check
-    if (chdir(safedir))
-      FatalError("Cannot chdir into /proc/ directory");
+    if (fchdir(chroot_dir_fd))
+      FatalError("Cannot chdir into chroot temp directory");
+
+    struct stat st;
+    if (fstat(chroot_dir_fd, &st))
+      FatalError("stat");
+
+    if (st.st_uid || st.st_gid || st.st_mode & 0777)
+      FatalError("Bad permissions on chroot temp directory");
 
-    if (chroot(safedir))
-      FatalError("Cannot chroot into /proc/ directory");
+    if (chroot(proc_self_fd_str))
+      FatalError("Cannot chroot into temp directory");
 
     if (chdir("/"))
       FatalError("Cannot chdir to / after chroot");
@@ -145,11 +221,13 @@ static bool SpawnChrootHelper() {
       FatalError("Writing reply");
 
     _exit(0);
-    // We now become a zombie. /proc/self/fd(info) is now an empty dir and we
-    // are chrooted there.
-    // Our (unprivileged) parent should not even be able to open "." or "/"
-    // since they would need to pass the ptrace() check. If our parent wait()
-    // for us, our root directory will completely disappear.
+  }
+
+  if (close(chroot_dir_fd)) {
+    close(sv[0]);
+    close(sv[1]);
+    perror("close(chroot_dir_fd)");
+    return false;
   }
 
   if (close(sv[0])) {
@@ -158,10 +236,19 @@ static bool SpawnChrootHelper() {
     return false;
   }
 
+  return sv[1];
+}
+
+static bool SpawnChrootHelper() {
+  const int chroot_signal_fd = CloneChrootHelperProcess();
+
+  if (chroot_signal_fd == -1)
+    return false;
+
   // In the parent process, we install an environment variable containing the
   // number of the file descriptor.
   char desc_str[64];
-  int printed = snprintf(desc_str, sizeof(desc_str), "%u", sv[1]);
+  int printed = snprintf(desc_str, sizeof(desc_str), "%d", chroot_signal_fd);
   if (printed < 0 || printed >= (int)sizeof(desc_str)) {
     fprintf(stderr, "Failed to snprintf\n");
     return false;
@@ -169,21 +256,7 @@ static bool SpawnChrootHelper() {
 
   if (setenv(kSandboxDescriptorEnvironmentVarName, desc_str, 1)) {
     perror("setenv");
-    close(sv[1]);
-    return false;
-  }
-
-  // We also install an environment variable containing the pid of the child
-  char helper_pid_str[64];
-  printed = snprintf(helper_pid_str, sizeof(helper_pid_str), "%u", pid);
-  if (printed < 0 || printed >= (int)sizeof(helper_pid_str)) {
-    fprintf(stderr, "Failed to snprintf\n");
-    return false;
-  }
-
-  if (setenv(kSandboxHelperPidEnvironmentVarName, helper_pid_str, 1)) {
-    perror("setenv");
-    close(sv[1]);
+    close(chroot_signal_fd);
     return false;
   }