* Stefan Hajnoczi (stefa...@redhat.com) wrote:
> In order to prevent /proc/self/fd escapes a temporary directory is
> created where /proc/self/fd is bind-mounted. This doesn't work on
> read-only file systems.
>
> Avoid the temporary directory by bind-mounting /proc/self/fd over /proc.
> This does not affect other processes since we remounted / with MS_REC |
> MS_SLAVE. /proc must exist and virtiofsd does not use it so it's safe to
> do this.
>
> Path traversal can be tested with the following function:
>
> static void test_proc_fd_escape(struct lo_data *lo)
> {
> int fd;
> int level = 0;
> ino_t last_ino = 0;
>
> fd = lo->proc_self_fd;
> for (;;) {
> struct stat st;
>
> if (fstat(fd, &st) != 0) {
> perror("fstat");
> return;
> }
> if (last_ino && st.st_ino == last_ino) {
> fprintf(stderr, "inode number unchanged, stopping\n");
> return;
> }
> last_ino = st.st_ino;
>
> fprintf(stderr, "Level %d dev %lu ino %lu\n", level,
> (unsigned long)st.st_dev,
> (unsigned long)last_ino);
> fd = openat(fd, "..", O_PATH | O_DIRECTORY | O_NOFOLLOW);
> level++;
> }
> }
>
> Before and after this patch only Level 0 is displayed. Without
> /proc/self/fd bind-mount protection it is possible to traverse parent
> directories.
>
> Fixes: 397ae982f4df4 ("virtiofsd: jail lo->proc_self_fd")
> Cc: Miklos Szeredi <mszer...@redhat.com>
> Cc: Jens Freimann <jfreim...@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
Yes, getting rid of the tmpdir altogether seems better.
Reviewed-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
> ---
> tools/virtiofsd/passthrough_ll.c | 34 +++++++++++---------------------
> 1 file changed, 11 insertions(+), 23 deletions(-)
>
> diff --git a/tools/virtiofsd/passthrough_ll.c
> b/tools/virtiofsd/passthrough_ll.c
> index 0b229ebd57..6ae7ffcdd7 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -2393,8 +2393,6 @@ static void setup_wait_parent_capabilities(void)
> static void setup_namespaces(struct lo_data *lo, struct fuse_session *se)
> {
> pid_t child;
> - char template[] = "virtiofsd-XXXXXX";
> - char *tmpdir;
>
> /*
> * Create a new pid namespace for *child* processes. We'll have to
> @@ -2458,33 +2456,23 @@ static void setup_namespaces(struct lo_data *lo,
> struct fuse_session *se)
> exit(1);
> }
>
> - tmpdir = mkdtemp(template);
> - if (!tmpdir) {
> - fuse_log(FUSE_LOG_ERR, "tmpdir(%s): %m\n", template);
> + /*
> + * We only need /proc/self/fd. Prevent ".." from accessing parent
> + * directories of /proc/self/fd by bind-mounting it over /proc. Since /
> was
> + * previously remounted with MS_REC | MS_SLAVE this mount change only
> + * affects our process.
> + */
> + if (mount("/proc/self/fd", "/proc", NULL, MS_BIND, NULL) < 0) {
> + fuse_log(FUSE_LOG_ERR, "mount(/proc/self/fd, MS_BIND): %m\n");
> exit(1);
> }
>
> - if (mount("/proc/self/fd", tmpdir, NULL, MS_BIND, NULL) < 0) {
> - fuse_log(FUSE_LOG_ERR, "mount(/proc/self/fd, %s, MS_BIND): %m\n",
> - tmpdir);
> - exit(1);
> - }
> -
> - /* Now we can get our /proc/self/fd directory file descriptor */
> - lo->proc_self_fd = open(tmpdir, O_PATH);
> + /* Get the /proc (actually /proc/self/fd, see above) file descriptor */
> + lo->proc_self_fd = open("/proc", O_PATH);
> if (lo->proc_self_fd == -1) {
> - fuse_log(FUSE_LOG_ERR, "open(%s, O_PATH): %m\n", tmpdir);
> + fuse_log(FUSE_LOG_ERR, "open(/proc, O_PATH): %m\n");
> exit(1);
> }
> -
> - if (umount2(tmpdir, MNT_DETACH) < 0) {
> - fuse_log(FUSE_LOG_ERR, "umount2(%s, MNT_DETACH): %m\n", tmpdir);
> - exit(1);
> - }
> -
> - if (rmdir(tmpdir) < 0) {
> - fuse_log(FUSE_LOG_ERR, "rmdir(%s): %m\n", tmpdir);
> - }
> }
>
> /*
> --
> 2.26.2
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
