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

[Greg Kurz <groug@kaod.org>]

On Fri, 6 Dec 2024 12:20:29 +0100
Christian Schoenebeck <qemu_oss@crudebyte.com> 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>
> ---

Reviewed-by: Greg Kurz <groug@kaod.org>

>  hw/9pfs/9p-util.h | 27 +++++++++++++++++----------
>  1 file changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
> index 51c94b0116..95ee4da9bd 100644
> --- a/hw/9pfs/9p-util.h
> +++ b/hw/9pfs/9p-util.h
> @@ -177,20 +177,27 @@ again:
>          return -1;
>      }
>  
> -    if (close_if_special_file(fd) < 0) {
> -        return -1;
> -    }
> -
> -    serrno = errno;
> -    /* O_NONBLOCK was only needed to open the file. Let's drop it. We don't
> -     * do that with O_PATH since fcntl(F_SETFL) isn't supported, and openat()
> -     * ignored it anyway.
> -     */
> +    /* Only if O_PATH is not set ... */
>      if (!(flags & O_PATH_9P_UTIL)) {
> +        /*
> +         * Prevent I/O on special files (device files, etc.) on host side,
> +         * however it is safe and required to allow opening them with O_PATH,
> +         * as this is limited to (required) path based operations only.
> +         */
> +        if (close_if_special_file(fd) < 0) {
> +            return -1;
> +        }
> +
> +        serrno = errno;
> +        /*
> +         * O_NONBLOCK was only needed to open the file. Let's drop it. We don't
> +         * do that with O_PATH since fcntl(F_SETFL) isn't supported, and
> +         * openat() ignored it anyway.
> +         */
>          ret = fcntl(fd, F_SETFL, flags);
>          assert(!ret);
> +        errno = serrno;
>      }
> -    errno = serrno;
>      return fd;
>  }
>  



-- 
Greg