[PATCH net] sctp: validate stream count in sctp_process_strreset_inreq()

["Cen Zhang (Microsoft)" <blbllhy@gmail.com>]

When processing a RESET_IN_REQUEST from a peer, 
sctp_process_strreset_inreq() derives the stream count from the 
parameter length but does not check whether the resulting 
RESET_OUT_REQUEST response would exceed SCTP_MAX_CHUNK_LEN.

The OUT request header (sctp_strreset_outreq, 16 bytes) is 8 bytes larger
than the IN request header (sctp_strreset_inreq, 8 bytes). Generally, the 
IP payload is bounded to 65535 bytes, so the stream list cannot be
large enough to trigger the overflow. However, on interfaces with MTU >
65535 (e.g., loopback with IPv6 jumbograms), a stream list that fits 
within the incoming IN parameter can cause a __u16 overflow in
sctp_make_strreset_req() when computing the OUT response size, leading to
an undersized skb allocation, raising a kernel BUG:

  net/core/skbuff.c:207        skb_panic
  net/core/skbuff.c:2625       skb_put
  net/sctp/sm_make_chunk.c:1535 sctp_addto_chunk
  net/sctp/sm_make_chunk.c:3695 sctp_make_strreset_req
  net/sctp/stream.c:655        sctp_process_strreset_inreq

The local setsockopt path (sctp_send_reset_streams) already performs length
validation, but the network packet path does not. Fix by adding similar
length check before calling sctp_make_strreset_req().

Fixes: 7f9d68ac944e ("sctp: implement sender-side procedures for SSN Reset
Request Parameter")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
---
 net/sctp/stream.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/sctp/stream.c b/net/sctp/stream.c
index 5c2fdedea..ea3805712 100644
--- a/net/sctp/stream.c
+++ b/net/sctp/stream.c
@@ -639,6 +639,10 @@ struct sctp_chunk *sctp_process_strreset_inreq(
 
 	nums = (ntohs(param.p->length) - sizeof(*inreq)) / sizeof(__u16);
 	str_p = inreq->list_of_streams;
+	if (nums * sizeof(__u16) + sizeof(struct sctp_strreset_outreq)
+			> SCTP_MAX_CHUNK_LEN - sizeof(struct sctp_reconf_chunk)) {
+		goto out;
+	}
 	for (i = 0; i < nums; i++) {
 		if (ntohs(str_p[i]) >= stream->outcnt) {
 			result = SCTP_STRRESET_ERR_WRONG_SSN;
-- 
2.53.0