Re: [RESEND 2/2] sctp: hold cached endpoints to prevent possible UAF

[Xin Long <lucien.xin@gmail.com>]

On Wed, Dec 15, 2021 at 8:48 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Tue, 14 Dec 2021 21:57:32 +0000 Lee Jones wrote:
> > The cause of the resultant dump_stack() reported below is a
> > dereference of a freed pointer to 'struct sctp_endpoint' in
> > sctp_sock_dump().
> >
> > This race condition occurs when a transport is cached into its
> > associated hash table followed by an endpoint/sock migration to a new
> > association in sctp_assoc_migrate() prior to their subsequent use in
> > sctp_diag_dump() which uses sctp_for_each_transport() to walk the hash
> > table calling into sctp_sock_dump() where the dereference occurs.
in sctp_sock_dump():
        struct sock *sk = ep->base.sk;
        ... <--[1]
        lock_sock(sk);

Do you mean in [1], the sk is peeled off and gets freed elsewhere?
if that's true, it's still late to do sock_hold(sk) in your this patch.

I talked with Marcelo about this before, if the possible UAF in [1] exists,
the problem also exists in the main RX path sctp_rcv().

> >
> >   BUG: KASAN: use-after-free in sctp_sock_dump+0xa8/0x438 [sctp_diag]
> >   Call trace:
> >    dump_backtrace+0x0/0x2dc
> >    show_stack+0x20/0x2c
> >    dump_stack+0x120/0x144
> >    print_address_description+0x80/0x2f4
> >    __kasan_report+0x174/0x194
> >    kasan_report+0x10/0x18
> >    __asan_load8+0x84/0x8c
> >    sctp_sock_dump+0xa8/0x438 [sctp_diag]
> >    sctp_for_each_transport+0x1e0/0x26c [sctp]
> >    sctp_diag_dump+0x180/0x1f0 [sctp_diag]
> >    inet_diag_dump+0x12c/0x168
> >    netlink_dump+0x24c/0x5b8
> >    __netlink_dump_start+0x274/0x2a8
> >    inet_diag_handler_cmd+0x224/0x274
> >    sock_diag_rcv_msg+0x21c/0x230
> >    netlink_rcv_skb+0xe0/0x1bc
> >    sock_diag_rcv+0x34/0x48
> >    netlink_unicast+0x3b4/0x430
> >    netlink_sendmsg+0x4f0/0x574
> >    sock_write_iter+0x18c/0x1f0
> >    do_iter_readv_writev+0x230/0x2a8
> >    do_iter_write+0xc8/0x2b4
> >    vfs_writev+0xf8/0x184
> >    do_writev+0xb0/0x1a8
> >    __arm64_sys_writev+0x4c/0x5c
> >    el0_svc_common+0x118/0x250
> >    el0_svc_handler+0x3c/0x9c
> >    el0_svc+0x8/0xc
> >
> > To prevent this from happening we need to take a references to the
> > to-be-used/dereferenced 'struct sock' and 'struct sctp_endpoint's
> > until such a time when we know it can be safely released.
> >
> > When KASAN is not enabled, a similar, but slightly different NULL
> > pointer derefernce crash occurs later along the thread of execution in
> > inet_sctp_diag_fill() this time.
Are you able to reproduce this issue?

What I'm thinking is to fix it by freeing sk in call_rcu() by
sock_set_flag(sock->sk, SOCK_RCU_FREE),
and add rcu_read_lock() in sctp_sock_dump().

Thanks.

>
> Are you able to identify where the bug was introduced? Fixes tag would
> be good to have here.
>
> You should squash the two patches together.
>
> > diff --git a/net/sctp/diag.c b/net/sctp/diag.c
> > index 760b367644c12..2029b240b6f24 100644
> > --- a/net/sctp/diag.c
> > +++ b/net/sctp/diag.c
> > @@ -301,6 +301,8 @@ static int sctp_sock_dump(struct sctp_transport *tsp, void *p)
> >       struct sctp_association *assoc;
> >       int err = 0;
> >
> > +     sctp_endpoint_hold(ep);
> > +     sock_hold(sk);
> >       lock_sock(sk);
> >       list_for_each_entry(assoc, &ep->asocs, asocs) {
> >               if (cb->args[4] < cb->args[1])
> > @@ -341,6 +343,8 @@ static int sctp_sock_dump(struct sctp_transport *tsp, void *p)
> >       cb->args[4] = 0;
> >  release:
> >       release_sock(sk);
> > +     sock_put(sk);
> > +     sctp_endpoint_put(ep);
> >       return err;
> >  }
> >
>