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

[Hangyu Hua]

Yes.

And there are two ways to release ax25, ax25_release and time expiry. I 
tested that ax25_release will not be invoked before ax25_connect is done 
by closing fd from user space. I think the reason is that __sys_connect 
use fdget() to protect fd. But i can't test if a function like 
ax25_std_heartbeat_expiry will release ax25 between sk_to_ax25(sk) and 
lock_sock(sk).

So i think it's better to protect sk_to_ax25(sk) by a lock. Beacause 
functions like ax25_release use sk_to_ax25 after a lock.


On 2022/1/12 下午5:59, Eric Dumazet wrote:
> 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) {