Re: [PATCH] 9pfs: fix missing rename lock in v9fs_co_readdir_many (CVE-2026-48004)

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Wednesday, 20 May 2026 20:26:09 CEST sin99xx wrote:
> Yes, please go ahead and add my Signed-off-by tag. Also, if possible, could
> you use this email instead?
> Signed-off-by: sin99xx sin99xx@proton.me

Please confirm by replying with that proton email address then I will replace 
your email address on this patch.

CC-ing qemu-stable, as this patch should be applied on stable branches, too.

> On Wed, May 20, 2026 at 2:22 PM Christian Schoenebeck <
> 
> qemu_oss@crudebyte.com> wrote:
> > On Wednesday, 20 May 2026 19:11:25 CEST Christian Schoenebeck wrote:
> > > From: sin99xx <sinxx198@gmail.com>
> > > 
> > > v9fs_co_readdir_many() dispatches do_readdir_many() to a worker thread
> > > that reads V9fsFidState's path.data without holding a rename lock.
> > > 
> > > A concurrent rename request, e.g. of its parent dir, causes the FID's
> > > absolute path to be altered by freeing the old path string and
> > > assigning a new one. This causes a heap-use-after-free race condition
> > > while do_readdir_many() is still accessing the old object.
> > > 
> > > This allows a DoS by an unprivileged guest user.
> > > 
> > > Fix this by wrapping the worker thread dispatch block within a pair of
> > > v9fs_path_read_lock() and v9fs_path_unlock() calls, like it's done at
> > > other places.
> > > 
> > > Fixes: 2149675b195f ("9pfs: add new function v9fs_co_readdir_many()")
> > > Fixes: CVE-2026-48004
> > > Reported-by: sin99xx <sinxx198@gmail.com>
> > > [Christian Schoenebeck: add commit log message]
> > > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > 
> > sin99xx, I forgot, may I add your Signed-off-by tag?
> > 
> >   Signed-off-by: sin99xx <sinxx198@gmail.com>
> > 
> > This is required [1] for making you the patch author:
> > 
> > "Your patches must include a Signed-off-by: line. This is a hard
> > requirement
> > because it’s how you say “I’m legally okay to contribute this and happy
> > for it
> > to go into QEMU”. For full guidance, read the Code provenance
> > documentation."
> > 
> > [1] https://www.qemu.org/docs/master/devel/submitting-a-patch.html
> > 
> > > ---
> > > 
> > >  hw/9pfs/codir.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c
> > > index bce7dd96e9..5568399343 100644
> > > --- a/hw/9pfs/codir.c
> > > +++ b/hw/9pfs/codir.c
> > > @@ -220,13 +220,16 @@ int coroutine_fn v9fs_co_readdir_many(V9fsPDU
> > > *pdu,
> > > V9fsFidState *fidp, bool dostat)
> > > 
> > >  {
> > >  
> > >      int err = 0;
> > > 
> > > +    V9fsState *s = pdu->s;
> > > 
> > >      if (v9fs_request_cancelled(pdu)) {
> > >      
> > >          return -EINTR;
> > >      
> > >      }
> > > 
> > > +    v9fs_path_read_lock(s);
> > > 
> > >      v9fs_co_run_in_worker({
> > >      
> > >          err = do_readdir_many(pdu, fidp, entries, offset, maxsize,
> > 
> > dostat);
> > 
> > > });
> > > +    v9fs_path_unlock(s);
> > > 
> > >      return err;
> > >  
> > >  }