sctp: fix a potential overflow in sctp_ifwdtsn_skip

Currently, when traversing ifwdtsn skips with _sctp_walk_ifwdtsn, it only
checks the pos against the end of the chunk. However, the data left for
the last pos may be < sizeof(struct sctp_ifwdtsn_skip), and dereference
it as struct sctp_ifwdtsn_skip may cause coverflow.

This patch fixes it by checking the pos against "the end of the chunk -
sizeof(struct sctp_ifwdtsn_skip)" in sctp_ifwdtsn_skip, similar to
sctp_fwdtsn_skip.

Fixes: 0fc2ea922c ("sctp: implement validate_ftsn for sctp_stream_interleave")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/2a71bffcd80b4f2c61fac6d344bb2f11c8fd74f7.1681155810.git.lucien.xin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/sctp/stream_interleave.c

@@ -1154,7 +1154,8 @@ static void sctp_generate_iftsn(struct sctp_outq *q, __u32 ctsn)
 
 #define _sctp_walk_ifwdtsn(pos, chunk, end) \
 	for (pos = chunk->subh.ifwdtsn_hdr->skip; \
-	     (void *)pos < (void *)chunk->subh.ifwdtsn_hdr->skip + (end); pos++)
+	     (void *)pos <= (void *)chunk->subh.ifwdtsn_hdr->skip + (end) - \
+			    sizeof(struct sctp_ifwdtsn_skip); pos++)
 
 #define sctp_walk_ifwdtsn(pos, ch) \
 	_sctp_walk_ifwdtsn((pos), (ch), ntohs((ch)->chunk_hdr->length) - \