Re: [PATCH] 9pfs: fix regression regarding CVE-2023-2861

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Friday, December 6, 2024 12:20:29 PM CET Christian Schoenebeck wrote:
> The released fix for this CVE:
> 
>   f6b0de53fb8 ("9pfs: prevent opening special files (CVE-2023-2861)")
> 
> caused a regression with security_model=passthrough. When handling a
> 'Tmknod' request there was a side effect that 'Tmknod' request could fail
> as 9p server was trying to adjust permissions:
> 
>   #6  close_if_special_file (fd=30) at ../hw/9pfs/9p-util.h:140
>   #7  openat_file (mode=<optimized out>, flags=2228224,
>       name=<optimized out>, dirfd=<optimized out>) at
>       ../hw/9pfs/9p-util.h:181
>   #8  fchmodat_nofollow (dirfd=dirfd@entry=31,
>       name=name@entry=0x5555577ea6e0 "mysocket", mode=493) at
>       ../hw/9pfs/9p-local.c:360
>   #9  local_set_cred_passthrough (credp=0x7ffbbc4ace10, name=0x5555577ea6e0
>       "mysocket", dirfd=31, fs_ctx=0x55555811f528) at
>       ../hw/9pfs/9p-local.c:457
>   #10 local_mknod (fs_ctx=0x55555811f528, dir_path=<optimized out>,
>       name=0x5555577ea6e0 "mysocket", credp=0x7ffbbc4ace10) at
>       ../hw/9pfs/9p-local.c:702
>   #11 v9fs_co_mknod (pdu=pdu@entry=0x555558121140,
>       fidp=fidp@entry=0x5555574c46c0, name=name@entry=0x7ffbbc4aced0,
>       uid=1000, gid=1000, dev=<optimized out>, mode=49645,
>       stbuf=0x7ffbbc4acef0) at ../hw/9pfs/cofs.c:205
>   #12 v9fs_mknod (opaque=0x555558121140) at ../hw/9pfs/9p.c:3711
> 
> That's because server was opening the special file to adjust permissions,
> however it was using O_PATH and it would have not returned the file
> descriptor to guest. So the call to close_if_special_file() on that branch
> was incorrect.
> 
> Let's lift the restriction introduced by f6b0de53fb8 such that it would
> allow to open special files on host if O_PATH flag is supplied, not only
> for 9p server's own operations as described above, but also for any client
> 'Topen' request.
> 
> It is safe to allow opening special files with O_PATH on host, because
> O_PATH only allows path based operations on the resulting file descriptor
> and prevents I/O such as read() and write() on that file descriptor.
> 
> Fixes: f6b0de53fb8 ("9pfs: prevent opening special files (CVE-2023-2861)")
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2337
> Reported-by: Dirk Herrendorfer <d.herrendoerfer@de.ibm.com>
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  hw/9pfs/9p-util.h | 27 +++++++++++++++++----------
>  1 file changed, 17 insertions(+), 10 deletions(-)

Queued on 9p.next:
https://github.com/cschoenebeck/qemu/commits/9p.next

Let's see if we can still land this in 9.2.

/Christian