[PATCH net-next v2] sctp: sctp_sock_filter(): avoid list_entry() on possibly empty list

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Use list_is_first() to check whether tsp->asoc matches the first
element of ep->asocs, as the list is not guaranteed to have an entry.

Fixes: 8f840e47f190 ("sctp: add the sctp_diag.c file")
Signed-off-by: Pietro Borrello <borrello@xxxxxxxxxxxxxxxx>
Changes in v2:
- Use list_is_first()
- Link to v1: https://lore.kernel.org/r/20230208-sctp-filter-v1-1-84ae70d90091@xxxxxxxxxxxxxxxx

The list_entry on an empty list creates a type confused pointer.
While using it is undefined behavior, in this case it seems there
is no big risk, as the `tsp->asoc != assoc` check will almost
certainly fail on the type confused pointer.
We report this bug also since it may hide further problems since
the code seems to assume a non-empty `ep->asocs`.

We were able to trigger sctp_sock_filter() using syzkaller, and
cause a panic inserting `BUG_ON(list_empty(&ep->asocs))`, so the
list may actually be empty.
But we were not able to minimize our testcase and understand how
sctp_sock_filter may end up with an empty asocs list.
We suspect a race condition between a connecting sctp socket
and the diag query.

We attach the stacktrace when triggering the injected

[  217.044169][T18237] kernel BUG at net/sctp/diag.c:364!
[  217.044845][T18237] invalid opcode: 0000 [#1] PREEMPT SMP KASAN
[  217.045681][T18237] CPU: 0 PID: 18237 Comm: syz-executor Not
tainted 6.1.0-00003-g190ee984c3e0-dirty #72
[  217.046934][T18237] Hardware name: QEMU Standard PC (i440FX +
PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
[  217.048241][T18237] RIP: 0010:sctp_sock_filter+0x1ce/0x1d0
[  217.060554][T18237] Call Trace:
[  217.061003][T18237]  <TASK>
[  217.061409][T18237]  sctp_transport_traverse_process+0x17d/0x470
[  217.062212][T18237]  ? sctp_ep_dump+0x620/0x620
[  217.062835][T18237]  ? sctp_sock_filter+0x1d0/0x1d0
[  217.063524][T18237]  ? sctp_transport_lookup_process+0x280/0x280
[  217.064330][T18237]  ? sctp_diag_get_info+0x260/0x2c0
[  217.065026][T18237]  ? sctp_for_each_endpoint+0x16f/0x200
[  217.065762][T18237]  ? sctp_diag_get_info+0x2c0/0x2c0
[  217.066435][T18237]  ? sctp_for_each_endpoint+0x1c0/0x200
[  217.067155][T18237]  sctp_diag_dump+0x2ea/0x480
[  217.093117][T18237]  do_writev+0x22d/0x460
 net/sctp/diag.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/net/sctp/diag.c b/net/sctp/diag.c
index a557009e9832..c3d6b92dd386 100644
--- a/net/sctp/diag.c
+++ b/net/sctp/diag.c
@@ -343,11 +343,9 @@ static int sctp_sock_filter(struct sctp_endpoint *ep, struct sctp_transport *tsp
 	struct sctp_comm_param *commp = p;
 	struct sock *sk = ep->base.sk;
 	const struct inet_diag_req_v2 *r = commp->r;
-	struct sctp_association *assoc =
-		list_entry(ep->asocs.next, struct sctp_association, asocs);
 	/* find the ep only once through the transports by this condition */
-	if (tsp->asoc != assoc)
+	if (!list_is_first(&tsp->asoc->asocs, &ep->asocs))
 		return 0;
 	if (r->sdiag_family != AF_UNSPEC && sk->sk_family != r->sdiag_family)

base-commit: 4ec5183ec48656cec489c49f989c508b68b518e3
change-id: 20230208-sctp-filter-73453e659360

Best regards,
Pietro Borrello <borrello@xxxxxxxxxxxxxxxx>

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