Re: [PATCH net] ax25: use after free in ax25_connect

[Eric Dumazet]

On 1/11/22 18:13, Hangyu Hua wrote:
> I try to use ax25_release to trigger this bug like this:
> ax25_release                 ax25_connect
> lock_sock(sk);
> -----------------------------sk = sock->sk;
> -----------------------------ax25 = sk_to_ax25(sk);
> ax25_destroy_socket(ax25);
> release_sock(sk);
> -----------------------------lock_sock(sk);
> -----------------------------use ax25 again
>
> But i failed beacause their have large speed difference. And i
> don't have a physical device to test other function in ax25.
> Anyway, i still think there will have a function to trigger this
> race condition like ax25_destroy_timer. Beacause Any ohter
> functions in ax25_proto_ops like ax25_bind protect ax25_sock by 
> lock_sock(sk).


For a given sk pointer, sk_to_ax25(sk) is always returning the same value,

regardless of sk lock being held or not.

ax25_sk(sk)->cb  is set only from ax25_create() or ax25_make_new()

ax25_connect can not be called until these operations have completed ?



> Thanks.
>
>
>
>
> On 2022/1/12 上午4:56, Eric Dumazet wrote:
> > On 1/10/22 20:20, Hangyu Hua wrote:
> > > sk_to_ax25(sk) needs to be called after lock_sock(sk) to avoid UAF
> > > caused by a race condition.
> >
> > Can you describe what race condition you have found exactly ?
> >
> > sk pointer can not change.
> >
> >
> > > Signed-off-by: Hangyu Hua <hbh25y@xxxxxxxxx>
> > > ---
> > >   net/ax25/af_ax25.c | 4 +++-
> > >   1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
> > > index cfca99e295b8..c5d62420a2a8 100644
> > > --- a/net/ax25/af_ax25.c
> > > +++ b/net/ax25/af_ax25.c
> > > @@ -1127,7 +1127,7 @@ static int __must_check ax25_connect(struct 
> > > socket *sock,
> > >       struct sockaddr *uaddr, int addr_len, int flags)
> > >   {
> > >       struct sock *sk = sock->sk;
> > > -    ax25_cb *ax25 = sk_to_ax25(sk), *ax25t;
> > > +    ax25_cb *ax25, *ax25t;
> > >       struct full_sockaddr_ax25 *fsa = (struct full_sockaddr_ax25 
> > > *)uaddr;
> > >       ax25_digi *digi = NULL;
> > >       int ct = 0, err = 0;
> > > @@ -1155,6 +1155,8 @@ static int __must_check ax25_connect(struct 
> > > socket *sock,
> > >       lock_sock(sk);
> > > +    ax25 = sk_to_ax25(sk);
> > > +
> > >       /* deal with restarts */
> > >       if (sock->state == SS_CONNECTING) {
> > >           switch (sk->sk_state) {