On Wed, Oct 2, 2019 at 9:04 AM Marcelo Ricardo Leitner
<marcelo.leitner@xxxxxxxxx> wrote:
>
> On Mon, Sep 30, 2019 at 09:10:18PM +0800, Xin Long wrote:
> > This patch is to fix a NULL-ptr deref crash in selinux_sctp_bind_connect:
> >
> > [...] kasan: GPF could be caused by NULL-ptr deref or user memory access
> > [...] RIP: 0010:selinux_sctp_bind_connect+0x16a/0x230
> > [...] Call Trace:
> > [...] security_sctp_bind_connect+0x58/0x90
> > [...] sctp_process_asconf+0xa52/0xfd0 [sctp]
> > [...] sctp_sf_do_asconf+0x782/0x980 [sctp]
> > [...] sctp_do_sm+0x139/0x520 [sctp]
> > [...] sctp_assoc_bh_rcv+0x284/0x5c0 [sctp]
> > [...] sctp_backlog_rcv+0x45f/0x880 [sctp]
> > [...] __release_sock+0x120/0x370
> > [...] release_sock+0x4f/0x180
> > [...] sctp_accept+0x3f9/0x5a0 [sctp]
> > [...] inet_accept+0xe7/0x6f0
> >
> > It was caused by that the 'newsk' sk_socket was not set before going to
> > security sctp hook when doing accept() on a tcp-type socket:
> >
> > inet_accept()->
> > sctp_accept():
> > lock_sock():
> > lock listening 'sk'
> > do_softirq():
> > sctp_rcv(): <-- [1]
> > asconf chunk arrived and
> > enqueued in 'sk' backlog
> > sctp_sock_migrate():
> > set asoc's sk to 'newsk'
> > release_sock():
> > sctp_backlog_rcv():
> > lock 'newsk'
> > sctp_process_asconf() <-- [2]
> > unlock 'newsk'
> > sock_graft():
> > set sk_socket <-- [3]
> >
> > As it shows, at [1] the asconf chunk would be put into the listening 'sk'
> > backlog, as accept() was holding its sock lock. Then at [2] asconf would
> > get processed with 'newsk' as asoc's sk had been set to 'newsk'. However,
> > 'newsk' sk_socket is not set until [3], while selinux_sctp_bind_connect()
> > would deref it, then kernel crashed.
>
> Note that sctp will migrate such incoming chunks from sk to newsk in
> sctp_rcv() if they arrived after the mass-migration performed at
> sctp_sock_migrate().
>
> That said, did you explore changing inet_accept() so that
> sk1->sk_prot->accept() would return sk2 still/already locked?
> That would be enough to block [2] from happening as then it would be
> queued on newsk backlog this time and avoid nearly duplicating
> inet_accept(). (too bad for this chunk, hit 2 backlogs..)
We don't have to bother inet_accept() for it. I had this one below,
and I was just thinking the locks order doesn't look nice. Do you
think this is more acceptable?
@@ -4963,15 +4963,19 @@ static struct sock *sctp_accept(struct sock
*sk, int flags, int *err, bool kern)
* asoc to the newsk.
*/
error = sctp_sock_migrate(sk, newsk, asoc, SCTP_SOCKET_TCP);
- if (error) {
- sk_common_release(newsk);
- newsk = NULL;
+ if (!error) {
+ lock_sock_nested(newsk, SINGLE_DEPTH_NESTING);
+ release_sock(sk);
+ release_sock(newsk);
+ *err = error;
+
+ return newsk;
}
out:
release_sock(sk);
*err = error;
- return newsk;
+ return NULL;
}
>
> AFAICT TCP code would be fine with such change. Didn't check other
> protocols.
>
