[PATCH net] sctp: account for chunk padding in sctp_init_cause()

[Masafumi Suzuki <shengshilingmu97@gmail.com>]

sctp_init_cause() accepts a chunk when skb_tailroom() can hold the
cause header and the payload. sctp_addto_chunk() then writes 0-3
bytes of padding before that header so the chunk stays 4-byte
aligned. The check does not include the padding, and the following
skb_put() calls skb_over_panic() from softirq.

Count the padding sctp_addto_chunk() is about to write. The cause
length on the wire stays the header plus the payload.

Fixes: 8914f4bace3f ("sctp: add sctp_make_op_error_limited and reuse inner functions")
Cc: stable@vger.kernel.org
Signed-off-by: Masafumi Suzuki <shengshilingmu97@gmail.com>
---
 net/sctp/sm_make_chunk.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
index 84a4c97d0f75..8933caadcdb6 100644
--- a/net/sctp/sm_make_chunk.c
+++ b/net/sctp/sm_make_chunk.c
@@ -133,20 +133,27 @@ static const struct sctp_paramhdr prsctp_param = {
  * cause codes will be embedded inside an abort chunk.
  */
 int sctp_init_cause(struct sctp_chunk *chunk, __be16 cause_code,
 		    size_t paylen)
 {
 	struct sctp_errhdr err;
+	int chunklen, padlen;
 	__u16 len;
 
 	/* Cause code constants are now defined in network order.  */
 	err.cause = cause_code;
 	len = sizeof(err) + paylen;
 	err.length = htons(len);
 
-	if (skb_tailroom(chunk->skb) < len)
+	/* sctp_addto_chunk() pads the chunk up to a 4-byte boundary before
+	 * appending the cause, so account for that padding here as well.
+	 */
+	chunklen = ntohs(chunk->chunk_hdr->length);
+	padlen = SCTP_PAD4(chunklen) - chunklen;
+
+	if (skb_tailroom(chunk->skb) < padlen + len)
 		return -ENOSPC;
 
 	chunk->subh.err_hdr = sctp_addto_chunk(chunk, sizeof(err), &err);
 
 	return 0;
 }
-- 
2.47.0