sctp: fix auth_hmacs array size in struct sctp_cookie

The auth_hmacs array in struct sctp_cookie is supposed to store a complete
SCTP_AUTH_HMAC_ALGO parameter, which consists of a struct sctp_paramhdr
followed by N HMAC identifiers.

However, the array size was calculated using an extra 2 bytes instead of
sizeof(struct sctp_paramhdr), which is 4 bytes. When four HMAC identifiers
are configured, the HMAC-ALGO parameter stored in the endpoint is larger
than the auth_hmacs buffer in the cookie.

As a result, sctp_association_init() copies beyond the end of auth_hmacs
when initializing the association, corrupting the adjacent auth_chunks
field. This can lead to an invalid HMAC identifier being accepted and later
cause an out-of-bounds read in sctp_auth_get_hmac().

Fix the array size calculation by including the full SCTP parameter header
size.

Fixes: 1f48564 ("[SCTP]: Implement SCTP-AUTH internals")
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Xin Liu <dstsmallbird@foxmail.com>
Reported-by: Zihan Xi <xizh2024@lzu.edu.cn>
Reported-by: Ren Wei <enjou1224z@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/634a0de0d5de29532915e6d47c92a0cbc206e03f.1783707155.git.lucien.xin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: lxin

include/net/sctp/structs.h

@@ -312,7 +312,8 @@ struct sctp_cookie {
 
 	__u8 auth_random[sizeof(struct sctp_paramhdr) +
 			 SCTP_AUTH_RANDOM_LENGTH];
-	__u8 auth_hmacs[SCTP_AUTH_NUM_HMACS * sizeof(__u16) + 2];
+	__u8 auth_hmacs[sizeof(struct sctp_paramhdr) +
+			SCTP_AUTH_NUM_HMACS * sizeof(__u16)];
 	__u8 auth_chunks[sizeof(struct sctp_paramhdr) + SCTP_AUTH_MAX_CHUNKS];
 
 	/* This is a shim for my peer's INIT packet, followed by