Re: [PATCH net] sctp: avoid NULL dereference when chunk data buffer is missing

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Wed, Oct 15, 2025 at 09:45:10PM +0300, Alexey Simakov wrote:
> chunk->skb pointer is dereferenced in the if-block where it's supposed
> to be NULL only.

The issue is well spotted. More below.

> 
> Use the chunk header instead, which should be available at this point
> in execution.
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Fixes: 90017accff61 ("sctp: Add GSO support")
> Signed-off-by: Alexey Simakov <bigalex934@gmail.com>
> ---
>  net/sctp/inqueue.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sctp/inqueue.c b/net/sctp/inqueue.c
> index 5c1652181805..f1830c21953f 100644
> --- a/net/sctp/inqueue.c
> +++ b/net/sctp/inqueue.c
> @@ -173,7 +173,8 @@ struct sctp_chunk *sctp_inq_pop(struct sctp_inq *queue)

With more context here:

               if ((skb_shinfo(chunk->skb)->gso_type & SKB_GSO_SCTP) == SKB_GSO_SCTP) {
                       /* GSO-marked skbs but without frags, handle
                        * them normally
                        */

                       if (skb_shinfo(chunk->skb)->frag_list)
                               chunk->head_skb = chunk->skb;

                       /* skbs with "cover letter" */
                       if (chunk->head_skb && chunk->skb->data_len == chunk->skb->len)
		           ^^^^^^^^^^^^^^^^^^

chunk->head_skb would also not be guaranteed.

>  				chunk->skb = skb_shinfo(chunk->skb)->frag_list;

But chunk->skb can only be NULL if chunk->head_skb is not, then.

Thing is, we cannot replace chunk->skb here then, because otherwise
when freeing this chunk in sctp_chunk_free below it will not reference
chunk->head_skb and will cause a leak.

With that, the check below should be done just before replacing
chunk->skb right above, inside the if() block. We're sure that
otherwise chunk->skb is non-NULL because of outer if() condition.

Thanks,
Marcelo

>  
>  			if (WARN_ON(!chunk->skb)) {
> -				__SCTP_INC_STATS(dev_net(chunk->skb->dev), SCTP_MIB_IN_PKT_DISCARDS);
> +				__SCTP_INC_STATS(dev_net(chunk->head_skb->dev),
> +						 SCTP_MIB_IN_PKT_DISCARDS);
>  				sctp_chunk_free(chunk);
>  				goto next_chunk;
>  			}
> -- 
> 2.34.1
>