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From ebf101955ce8f8d72fba103b5151115a4335de2c Mon Sep 17 00:00:00 2001
From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Tue, 6 Oct 2020 10:58:26 +0100
Subject: [PATCH] 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>
Upstream-Status: Backport
[https://github.com/qemu/qemu/commit/ebf101955ce8f8d72fba103b5151115a4335de2c]
CVE: CVE-2020-35517
Signed-off-by: Chee Yang Lee <chee.yang.lee@intel.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 477e6ee0b53..ff53df44510 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);
- }
}
/*
|
