Re: [PATCH] sctp: avoid NULL pointer dereference in sctp_sf_violation

[Xin Long]

On Tue, Nov 2, 2021 at 4:27 PM Alexey Khoroshilov <khoroshilov@xxxxxxxxx> wrote:
>
> Some callers (e.g. sctp_sf_violation_chunk) passes NULL to
> asoc argument of sctp_sf_violation. So, it should check it
> before calling sctp_vtag_verify().
>
> Probably it could be exploited by a malicious SCTP packet
> to cause NULL pointer dereference.
I don't think asoc can be NULL in here, did you see any call trace
caused by it?

If this was found by a tool, please remove the unnecessary call from
sctp_sf_violation_chunk() instead:

@@ -4893,9 +4893,6 @@ static enum sctp_disposition sctp_sf_violation_chunk(
 {
        static const char err_str[] = "The following chunk violates protocol:";

-       if (!asoc)
-               return sctp_sf_violation(net, ep, asoc, type, arg, commands);
-

Thanks.

>
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>
> Signed-off-by: Alexey Khoroshilov <khoroshilov@xxxxxxxxx>
> Fixes: aa0f697e4528 ("sctp: add vtag check in sctp_sf_violation")
> ---
>  net/sctp/sm_statefuns.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/sctp/sm_statefuns.c b/net/sctp/sm_statefuns.c
> index fb3da4d8f4a3..77f3cd6c516e 100644
> --- a/net/sctp/sm_statefuns.c
> +++ b/net/sctp/sm_statefuns.c
> @@ -4669,7 +4669,7 @@ enum sctp_disposition sctp_sf_violation(struct net *net,
>  {
>         struct sctp_chunk *chunk = arg;
>
> -       if (!sctp_vtag_verify(chunk, asoc))
> +       if (asoc && !sctp_vtag_verify(chunk, asoc))
>                 return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);
>
>         /* Make sure that the chunk has a valid length. */
> --
> 2.7.4
>