sctp: validate stream count in sctp_process_strreset_inreq()

sctp: validate stream count in sctp_process_strreset_inreq()

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 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
request size, leading to an undersized skb allocation and 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 validates the generated reset request size.
However, for an incoming-only reset, it accounts for the smaller IN
request even though the peer must generate an OUT request with the same
stream list. Such a request cannot be completed successfully by the
peer.

Reject peer IN requests whose corresponding OUT request would exceed
SCTP_MAX_CHUNK_LEN. Also tighten the local check so it does not send an
IN request that would require an oversized OUT request from the peer.

Fixes: 7f9d68ac944e ("sctp: implement sender-side procedures for SSN Reset Request Parameter")
Reported-by: AutonomousCodeSecurity@microsoft.com
Closes: https://lore.kernel.org/all/20260707203215.2752-1-blbllhy@gmail.com/
Suggested-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20260710010718.20318-1-blbllhy@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>