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
`BUG_ON(list_empty(&ep->asocs))`:
```
[ 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>
