crypto: af_alg - Work around empty control messages without MSG_MORE

The iwd daemon uses libell which sets up the skcipher operation with
two separate control messages.  As the first control message is sent
without MSG_MORE, it is interpreted as an empty request.

While libell should be fixed to use MSG_MORE where appropriate, this
patch works around the bug in the kernel so that existing binaries
continue to work.

We will print a warning however.

A separate issue is that the new kernel code no longer allows the
control message to be sent twice within the same request.  This
restriction is obviously incompatible with what iwd was doing (first
setting an IV and then sending the real control message).  This
patch changes the kernel so that this is explicitly allowed.
Reported-by: Caleb Jorden <caljorden@hotmail.com>
Fixes: f3c802a1 ("crypto: algif_aead - Only wake up when...")
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/af_alg.c

@@ -16,6 +16,7 @@
 #include <linux/module.h>
 #include <linux/net.h>
 #include <linux/rwsem.h>
+#include <linux/sched.h>
 #include <linux/sched/signal.h>
 #include <linux/security.h>
 
@@ -845,10 +846,16 @@ int af_alg_sendmsg(struct socket *sock, struct msghdr *msg, size_t size,
 	}
 
 	lock_sock(sk);
-	if (ctx->init && (init || !ctx->more)) {
+	if (ctx->init && !ctx->more) {
+		if (ctx->used) {
 			err = -EINVAL;
 			goto unlock;
 		}
+
+		pr_info_once(
+			"%s sent an empty control message without MSG_MORE.\n",
+			current->comm);
+	}
 	ctx->init = true;
 
 	if (init) {