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authorStefan Hajnoczi <stefanha@redhat.com>2020-10-06 10:58:26 +0100
committerDr. David Alan Gilbert <dgilbert@redhat.com>2020-10-12 12:39:38 +0100
commitebf101955ce8f8d72fba103b5151115a4335de2c (patch)
treecf9bc2357edd2006c0934c01e76c868a51d7d99f
parentff3995e2f0e12770dfa73d9e95c0461024840b9a (diff)
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virtiofsd: avoid /proc/self/fd tempdir
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 <mszeredi@redhat.com> Cc: Jens Freimann <jfreimann@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Message-Id: <20201006095826.59813-1-stefanha@redhat.com> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Tested-by: Jens Freimann <jfreimann@redhat.com> Reviewed-by: Jens Freimann <jfreimann@redhat.com> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
-rw-r--r--tools/virtiofsd/passthrough_ll.c34
1 files changed, 11 insertions, 23 deletions
diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 477e6ee..ff53df4 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);
- 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);
+ /*
+ * 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);
}
- /* 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);
- }
}
/*