+-- On Thu, 22 Nov 2018, Greg Kurz wrote --+
| When using the 9P2000.u version of the protocol, the following shell
| command line in the guest can cause QEMU to crash:
|
| while true; do rm -rf aa; mkdir -p a/b & touch a/b/c & mv a aa; done
|
| With 9P2000.u, file renaming is handled by the WSTAT command. The
| v9fs_wstat() function calls v9fs_complete_rename(), which calls
| v9fs_fix_path() for every fid whose path is affected by the change.
| The involved calls to v9fs_path_copy() may race with any other access
| to the fid path performed by some worker thread, causing a crash like
| shown below:
|
| Thread 12 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
| 0x0000555555a25da2 in local_open_nofollow (fs_ctx=0x555557d958b8, path=0x0,
| flags=65536, mode=0) at hw/9pfs/9p-local.c:59
| 59 while (*path && fd != -1) {
| (gdb) bt
| #0 0x0000555555a25da2 in local_open_nofollow (fs_ctx=0x555557d958b8,
| path=0x0, flags=65536, mode=0) at hw/9pfs/9p-local.c:59
| #1 0x0000555555a25e0c in local_opendir_nofollow (fs_ctx=0x555557d958b8,
| path=0x0) at hw/9pfs/9p-local.c:92
| #2 0x0000555555a261b8 in local_lstat (fs_ctx=0x555557d958b8,
| fs_path=0x555556b56858, stbuf=0x7fff84830ef0) at hw/9pfs/9p-local.c:185
| #3 0x0000555555a2b367 in v9fs_co_lstat (pdu=0x555557d97498,
| path=0x555556b56858, stbuf=0x7fff84830ef0) at hw/9pfs/cofile.c:53
| #4 0x0000555555a1e9e2 in v9fs_stat (opaque=0x555557d97498)
| at hw/9pfs/9p.c:1083
| #5 0x0000555555e060a2 in coroutine_trampoline (i0=-669165424, i1=32767)
| at util/coroutine-ucontext.c:116
| #6 0x00007fffef4f5600 in __start_context () at /lib64/libc.so.6
| #7 0x0000000000000000 in ()
| (gdb)
|
| The fix is to take the path write lock when calling v9fs_complete_rename(),
| like in v9fs_rename().
|
| Impact: DoS triggered by unprivileged guest users.
|
| Cc: P J P <ppan...@redhat.com>
| Reported-by: zhibin hu <noirf...@gmail.com>
| Signed-off-by: Greg Kurz <gr...@kaod.org>
| ---
| hw/9pfs/9p.c | 3 +++
| 1 file changed, 3 insertions(+)
|
| diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
| index 267a25533b77..bdf7919abfc5 100644
| --- a/hw/9pfs/9p.c
| +++ b/hw/9pfs/9p.c
| @@ -2855,6 +2855,7 @@ static void coroutine_fn v9fs_wstat(void *opaque)
| struct stat stbuf;
| V9fsFidState *fidp;
| V9fsPDU *pdu = opaque;
| + V9fsState *s = pdu->s;
|
| v9fs_stat_init(&v9stat);
| err = pdu_unmarshal(pdu, offset, "dwS", &fid, &unused, &v9stat);
| @@ -2920,7 +2921,9 @@ static void coroutine_fn v9fs_wstat(void *opaque)
| }
| }
| if (v9stat.name.size != 0) {
| + v9fs_path_write_lock(s);
| err = v9fs_complete_rename(pdu, fidp, -1, &v9stat.name);
| + v9fs_path_unlock(s);
| if (err < 0) {
| goto out;
| }
It works to fix the issue.
Reviewed-by: Prasad J Pandit <p...@fedoraproject.org>
Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
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