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

[Christian Schoenebeck]

On Tuesday, December 10, 2024 11:11:47 AM CET Peter Maydell wrote:
> On Tue, 10 Dec 2024 at 09:57, Christian Schoenebeck
> <qemu_oss@crudebyte.com> wrote:
> >
> > 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.
> 
> 9.2 is due to be released today, so this would need to be:
>  * a fix for a critical bug
>  * the bug must be a regression since 9.1
>  * with a clear justification attached for why it's important
>    enough to delay the release
> 
> Commit f6b0de53fb8 was in 9.1, which suggests that this isn't
> a regression since 9.1 ?

Hi Peter,

I just saw there was still one open milestone ticket for 9.2 and assumed the
release to be delayed by one week anyway.

I am not trying to convince you of a delay. So unless somebody objects, just
disregard the PR for now.

Thanks!

/Christian