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

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

[Ranganath V N]

Fix an issue detected by syzbot:

KMSAN reported an uninitialized-value access in sctp_inq_pop
while parsing an SCTP chunk header received frma a locally transmitted packet.

BUG: KMSAN: uninit-value in sctp_inq_pop

skb allocated in sctp_packet_transmit() contain uninitialized bytes.
sctp transmit path writes only the necessary header and chunk data,
the receive path read from uinitialized parts of the skb, triggering KMSAN.

Fix this by explicitly zeroing the skb payload area after allocation
and reservation, ensuring all future reads from this region are fully
initialized.

Reported-by: syzbot+d101e12bccd4095460e7@syzkaller.appspotmail.com
Tested-by: syzbot+d101e12bccd4095460e7@syzkaller.appspotmail.com
Fixes: https://syzkaller.appspot.com/bug?extid=d101e12bccd4095460e7
Signed-off-by: Ranganath V N <vnranganath.20@gmail.com>
---
KMSAN reported an uninitialized-value access in sctp_inq_pop
while parsing an SCTP chunk header received frma a locally transmitted packet.
---
 net/sctp/output.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/sctp/output.c b/net/sctp/output.c
index 23e96305cad7..e76413741faf 100644
--- a/net/sctp/output.c
+++ b/net/sctp/output.c
@@ -602,6 +602,8 @@ int sctp_packet_transmit(struct sctp_packet *packet, gfp_t gfp)
 	skb_reserve(head, packet->overhead + MAX_HEADER);
 	skb_set_owner_w(head, sk);
 
+	memset(head->data, 0, skb_tailroom(head));
+
 	/* set sctp header */
 	sh = skb_push(head, sizeof(struct sctphdr));
 	skb_reset_transport_header(head);

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

Best regards,
-- 
Ranganath V N <vnranganath.20@gmail.com>

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

[Xin Long]

On Thu, Oct 23, 2025 at 5:52 AM Ranganath V N <vnranganath.20@gmail.com> wrote:
>
> Fix an issue detected by syzbot:
>
> KMSAN reported an uninitialized-value access in sctp_inq_pop
Hi, Ranganath,

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))

(TBH, I didn't expect it would allow BPF to trim skb in sk_filter().)

To handle this safely, a new check should be performed after sk_filter() like:

+       if (sk_filter(sk, skb) || skb->len < sizeof(struct sctp_chunkhdr))
                goto discard_release;

Could you please proceed with this change in sctp_rcv()?

Thanks.

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

[Ranganath V N]

Hi Xin,

Thank you for the feedback and response to the patch.
I would like to know that above analysis is valid or not.
And do you want me to test this suggestion with the syzbot?

regards,
Ranganath

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

[Xin Long]

On Thu, Oct 23, 2025 at 1:38 PM Ranganath V N <vnranganath.20@gmail.com> wrote:
>
> Hi Xin,
>
> Thank you for the feedback and response to the patch.
> I would like to know that above analysis is valid or not.
> And do you want me to test this suggestion with the syzbot?
>
Yes, if it's possible.