Re: [PATCH] cifs: fix use-after-free of server info on cifs_ses_add_channel error

[henry martin]

Hi Jeon,

You're right, thanks for catching that.  chan points into
ses->chans[], so once chan_count is decremented under chan_lock a
concurrent add_channel can reuse the slot and overwrite chan->server
before the unlocked put reads it.

v2 caches the pointer in a local under chan_lock and uses the cached
pointer for both cifs_chan_clear_need_reconnect() and the final
cifs_put_tcp_session(), so the reference we drop is always the one
this error path took.

Regards,
Henry

Namjae Jeon <linkinjeon@xxxxxxxxxx> 于2026年10月9日周五 21:32写道：
>
> On Fri, Oct 9, 2026 at 5:03 PM Henry Martin <bsdhenrymartin@xxxxxxxxx> wrote:
> >
> > The error path of cifs_ses_add_channel() calls cifs_put_tcp_session()
> > before cifs_chan_clear_need_reconnect().  The latter reaches
> > cifs_ses_get_chan_index(), which dereferences server->terminate; if
> > the put was the final reference, TCP_Server_Info is already freed and
> > this is a use-after-free.  Same put-then-use pattern as the recently
> > fixed sibling in commit 717e0a25036b6.
> >
> > Reorder: clear the reconnect bit and drop chan_count under chan_lock
> > first, release the server reference last.
> >
> > This vulnerability was discovered by Tencent CodeBuddy Security.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: ee1d21794e55a ("cifs: handle when server stops supporting multichannel")
> > Signed-off-by: Henry Martin <bsdhenrymartin@xxxxxxxxx>
> > ---
> >  fs/smb/client/sess.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
> > index e095f41b5882..650c0f855a29 100644
> > --- a/fs/smb/client/sess.c
> > +++ b/fs/smb/client/sess.c
> > @@ -626,12 +626,16 @@ cifs_ses_add_channel(struct cifs_ses *ses,
> >
> >  out:
> >         if (rc && chan->server) {
> > -               cifs_put_tcp_session(chan->server, 0);
> > -
> >                 spin_lock(&ses->chan_lock);
> >
> > -               /* we rely on all bits beyond chan_count to be clear */
> > +               /* Clear the reconnect bitmask BEFORE dropping the server
> > +                * reference: cifs_ses_get_chan_index() dereferences
> > +                * server->terminate, and cifs_put_tcp_session() may be the
> > +                * final put that frees chan->server (use-after-free).
> > +                */
> >                 cifs_chan_clear_need_reconnect(ses, chan->server);
> > +
> > +               /* we rely on all bits beyond chan_count to be clear */
> >                 ses->chan_count--;
> >                 /*
> >                  * chan_count should never reach 0 as at least the primary
> > @@ -639,6 +643,8 @@ cifs_ses_add_channel(struct cifs_ses *ses,
> >                  */
> >                 WARN_ON(ses->chan_count < 1);
> >                 spin_unlock(&ses->chan_lock);
> > +
> > +               cifs_put_tcp_session(chan->server, 0);
> As soon as the lock is released, a new channel can be added in the
> slot indicated by the decremented chan_count. Then the final put will
> act on the new channel. Please save the server pointer under chan_lock
> and use that for cifs_put_tcp_session().