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

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

On Thu, Feb 9, 2023 at 7:13 AM Pietro Borrello
<borrello@diag.uniroma1.it> wrote:
>
> 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@diag.uniroma1.it>
> ---
> Changes in v2:
> - Use list_is_first()
> - Link to v1: https://lore.kernel.org/r/20230208-sctp-filter-v1-1-84ae70d90091@diag.uniroma1.it
> ---
>
> 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@diag.uniroma1.it>
>
Acked-by: Xin Long <lucien.xin@gmail.com>