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

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Message-ID: <20251017071550.q7qg2a5e7xu6yvlr@home-server>
Date: Fri, 17 Oct 2025 10:15:50 +0300
From: Alexey Simakov <bigalex934@...il.com>
To: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc: Xin Long <lucien.xin@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, linux-sctp@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	lvc-project@...uxtesting.org
Subject: Re: [PATCH net] sctp: avoid NULL dereference when chunk data buffer
 is missing

On Wed, Oct 15, 2025 at 04:50:07PM -0300, Marcelo Ricardo Leitner wrote:
> 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@...il.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;
> >  			}
I'm not sure, that correctly understand the new location of updated check.
There a few assumtions below.
> > -- 
> > 2.34.1
> > 
		/* Is the queue empty?  */
		entry = sctp_list_dequeue(&queue->in_chunk_list);
		if (!entry)
			return NULL;

		chunk = list_entry(entry, struct sctp_chunk, list);

		if (skb_is_gso(chunk->skb) && skb_is_gso_sctp(chunk->skb)) {
			/* 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)
Adding this check here will not fix problem, since chunk->skb always true here because it dereferencing in
checks above.
				chunk->skb = skb_shinfo(chunk->skb)->frag_list;
Adding here could make sense, chunk->skb changed => do something if it became null.

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