9pfs: fix regression regarding CVE-2023-2861

9pfs: fix regression regarding CVE-2023-2861

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>
Tested-by: Dirk Herrendorfer <d.herrendoerfer@de.ibm.com>
Message-Id: <E1tJWbk-007BH4-OB@kylie.crudebyte.com>