crypto: algif_skcipher - Load TX SG list after waiting

commit 4f0414e5 upstream.

We need to load the TX SG list in sendmsg(2) after waiting for
incoming data, not before.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

crypto/algif_skcipher.c

@@ -441,13 +441,6 @@ static int skcipher_recvmsg(struct kiocb *unused, struct socket *sock,
 		char __user *from = iov->iov_base;
 
 		while (seglen) {
-			sgl = list_first_entry(&ctx->tsgl,
-					       struct skcipher_sg_list, list);
-			sg = sgl->sg;
-
-			while (!sg->length)
-				sg++;
-
 			used = ctx->used;
 			if (!used) {
 				err = skcipher_wait_for_data(sk, flags);
@@ -469,6 +462,13 @@ static int skcipher_recvmsg(struct kiocb *unused, struct socket *sock,
 			if (!used)
 				goto free;
 
+			sgl = list_first_entry(&ctx->tsgl,
+					       struct skcipher_sg_list, list);
+			sg = sgl->sg;
+
+			while (!sg->length)
+				sg++;
+
 			ablkcipher_request_set_crypt(&ctx->req, sg,
 						     ctx->rsgl.sg, used,
 						     ctx->iv);