[PATCH v2 net] sctp: avoid NULL dereference when chunk data buffer is missing

[Alexey Simakov <bigalex934@gmail.com>]

chunk->skb pointer is dereferenced in the if-block where it's supposed
to be NULL only.

chunk->skb can only be NULL if chunk->head_skb is not. Check for frag_list
instead and do it just before replacing chunk->skb. We're sure that
otherwise chunk->skb is non-NULL because of outer if() condition.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 90017accff61 ("sctp: Add GSO support")
Signed-off-by: Alexey Simakov <bigalex934@gmail.com>
---

v2 - change the condition in if-block and update
its location for preventing potential memory leak
per Marcelo Ricardo Leitner's suggestion.

link to v1: https://lore.kernel.org/lkml/20251015184510.6547-1-bigalex934@gmail.com/

 net/sctp/inqueue.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/net/sctp/inqueue.c b/net/sctp/inqueue.c
index 5c1652181805..f5a7d5a38755 100644
--- a/net/sctp/inqueue.c
+++ b/net/sctp/inqueue.c
@@ -169,13 +169,14 @@ struct sctp_chunk *sctp_inq_pop(struct sctp_inq *queue)
 				chunk->head_skb = chunk->skb;
 
 			/* skbs with "cover letter" */
-			if (chunk->head_skb && chunk->skb->data_len == chunk->skb->len)
+			if (chunk->head_skb && chunk->skb->data_len == chunk->skb->len) {
+				if (WARN_ON(!skb_shinfo(chunk->skb)->frag_list)) {
+					__SCTP_INC_STATS(dev_net(chunk->skb->dev),
+							 SCTP_MIB_IN_PKT_DISCARDS);
+					sctp_chunk_free(chunk);
+					goto next_chunk;
+				}
 				chunk->skb = skb_shinfo(chunk->skb)->frag_list;
-
-			if (WARN_ON(!chunk->skb)) {
-				__SCTP_INC_STATS(dev_net(chunk->skb->dev), SCTP_MIB_IN_PKT_DISCARDS);
-				sctp_chunk_free(chunk);
-				goto next_chunk;
 			}
 		}
 
-- 
2.34.1