[Qemu-devel] [PULL 0/1] 9p fixes for v3.1.0-rc3

[Qemu-devel] [PULL 0/1] 9p fixes for v3.1.0-rc3

[Greg Kurz]

The following changes since commit 5298f4d67a911dd9cefa4c4185eed242074d64c2:

  Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging (2018-11-23 08:54:52 +0000)

are available in the Git repository at:

  https://github.com/gkurz/qemu.git tags/for-upstream

for you to fetch changes up to 1d20398694a3b67a388d955b7a945ba4aa90a8a8:

  9p: fix QEMU crash when renaming files (2018-11-23 13:28:03 +0100)

----------------------------------------------------------------
Fixes a QEMU crash triggerable by guest userspace (CVE-2018-19489).

----------------------------------------------------------------
Greg Kurz (1):
      9p: fix QEMU crash when renaming files

 hw/9pfs/9p.c | 3 +++
 1 file changed, 3 insertions(+)
-- 
2.17.2

[Qemu-devel] [PULL 1/1] 9p: fix QEMU crash when renaming files

[Greg Kurz]

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.

Fixes: CVE-2018-19489
Cc: P J P <ppandit@redhat.com>
Reported-by: zhibin hu <noirfate@gmail.com>
Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>
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;
         }
-- 
2.17.2

Re: [Qemu-devel] [PULL 0/1] 9p fixes for v3.1.0-rc3

[Peter Maydell]

On Mon, 26 Nov 2018 at 09:57, Greg Kurz <groug@kaod.org> wrote:
>
> The following changes since commit 5298f4d67a911dd9cefa4c4185eed242074d64c2:
>
>   Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging (2018-11-23 08:54:52 +0000)
>
> are available in the Git repository at:
>
>   https://github.com/gkurz/qemu.git tags/for-upstream
>
> for you to fetch changes up to 1d20398694a3b67a388d955b7a945ba4aa90a8a8:
>
>   9p: fix QEMU crash when renaming files (2018-11-23 13:28:03 +0100)
>
> ----------------------------------------------------------------
> Fixes a QEMU crash triggerable by guest userspace (CVE-2018-19489).
>
> ----------------------------------------------------------------
> Greg Kurz (1):
>       9p: fix QEMU crash when renaming files
>
>  hw/9pfs/9p.c | 3 +++
>  1 file changed, 3 insertions(+)
> --

Applied, thanks.

-- PMM