[PATCH v2] net: sctp: fix KMSAN uninit-value in sctp_inq_pop

Fix an issue detected by syzbot:

KMSAN reported an uninitialized-value access in sctp_inq_pop
BUG: KMSAN: uninit-value in sctp_inq_pop

The issue is actually caused by skb trimming via sk_filter() in sctp_rcv().
In the reproducer, skb->len becomes 1 after sk_filter(), which bypassed the
original check:

        if (skb->len < sizeof(struct sctphdr) + sizeof(struct sctp_chunkhdr) +
                       skb_transport_offset(skb))
To handle this safely, a new check should be performed after sk_filter().

Reported-by: syzbot+d101e12bccd4095460e7@syzkaller.appspotmail.com
Tested-by: syzbot+d101e12bccd4095460e7@syzkaller.appspotmail.com
Fixes: https://syzkaller.appspot.com/bug?extid=d101e12bccd4095460e7
Suggested-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Ranganath V N <redacted>
---
KMSAN reported an uninitialized-value access in sctp_inq_pop
---
Changes in v2:
- changes in commit message as per the code changes.
- fixed as per the suggestion.
- Link to v1: https://lore.kernel.org/r/20251023-kmsan_fix-v1-1-d08c18db8877@gmail.com (local)
---
 net/sctp/input.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sctp/input.c b/net/sctp/input.c
index 7e99894778d4..e119e460ccde 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c

@@ -190,7 +190,7 @@ int sctp_rcv(struct sk_buff *skb)
 		goto discard_release;
 	nf_reset_ct(skb);
 
-	if (sk_filter(sk, skb))
+	if (sk_filter(sk, skb) || skb->len < sizeof(struct sctp_chunkhdr))
 		goto discard_release;
 
 	/* Create an SCTP packet structure. */

---
base-commit: 43e9ad0c55a369ecc84a4788d06a8a6bfa634f1c
change-id: 20251023-kmsan_fix-78d527b9960b

Best regards,
-- 
Ranganath V N [off-list ref]