sctp: prefer struct_size over open coded arithmetic

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1][2].

As the "ids" variable is a pointer to "struct sctp_assoc_ids" and this
structure ends in a flexible array:

struct sctp_assoc_ids {
	[...]
	sctp_assoc_t	gaids_assoc_id[];
};

the preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the calculation "size + size * count" in
the kmalloc() function.

Also, refactor the code adding the "ids_size" variable to avoid sizing
twice.

This way, the code is more readable and safer.

This code was detected with the help of Coccinelle, and audited and
modified manually.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: KSPP/linux#160 [2]
Signed-off-by: Erick Archer <erick.archer@outlook.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/PAXPR02MB724871DB78375AB06B5171C88B152@PAXPR02MB7248.eurprd02.prod.outlook.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Erick Archer

net/sctp/socket.c

@@ -7119,6 +7119,7 @@ static int sctp_getsockopt_assoc_ids(struct sock *sk, int len,
 	struct sctp_sock *sp = sctp_sk(sk);
 	struct sctp_association *asoc;
 	struct sctp_assoc_ids *ids;
+	size_t ids_size;
 	u32 num = 0;
 
 	if (sctp_style(sk, TCP))
@@ -7131,11 +7132,11 @@ static int sctp_getsockopt_assoc_ids(struct sock *sk, int len,
 		num++;
 	}
 
-	if (len < sizeof(struct sctp_assoc_ids) + sizeof(sctp_assoc_t) * num)
+	ids_size = struct_size(ids, gaids_assoc_id, num);
+	if (len < ids_size)
 		return -EINVAL;
 
-	len = sizeof(struct sctp_assoc_ids) + sizeof(sctp_assoc_t) * num;
-
+	len = ids_size;
 	ids = kmalloc(len, GFP_USER | __GFP_NOWARN);
 	if (unlikely(!ids))
 		return -ENOMEM;