9p: fix QEMU crash when renaming files

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 <ppandit@redhat.com>
Reported-by: zhibin hu <noirfate@gmail.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
---
 hw/9pfs/9p.c |    3 +++
 1 file changed, 3 insertions(+)

+-- 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 <ppandit@redhat.com>
| Reported-by: zhibin hu <noirfate@gmail.com>
| Signed-off-by: Greg Kurz <groug@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 <pjp@fedoraproject.org>

Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
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