crypto: af_alg - wait for data at beginning of recvmsg

The wait for data is a non-atomic operation that can sleep and therefore
potentially release the socket lock. The release of the socket lock
allows another thread to modify the context data structure. The waiting
operation for new data therefore must be called at the beginning of
recvmsg. This prevents a race condition where checks of the members of
the context data structure are performed by recvmsg while there is a
potential for modification of these values.

Fixes: e870456d8e ("crypto: algif_skcipher - overhaul memory management")
Fixes: d887c52d6a ("crypto: algif_aead - overhaul memory management")
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: <stable@vger.kernel.org> # v4.14+
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/af_alg.c

@@ -1137,12 +1137,6 @@ int af_alg_get_rsgl(struct sock *sk, struct msghdr *msg, int flags,
 		if (!af_alg_readable(sk))
 			break;
 
-		if (!ctx->used) {
-			err = af_alg_wait_for_data(sk, flags);
-			if (err)
-				return err;
-		}
-
 		seglen = min_t(size_t, (maxsize - len),
 			       msg_data_left(msg));
 

crypto/algif_aead.c

@@ -111,6 +111,12 @@ static int _aead_recvmsg(struct socket *sock, struct msghdr *msg,
 	size_t usedpages = 0;		/* [in]  RX bufs to be used from user */
 	size_t processed = 0;		/* [in]  TX bufs to be consumed */
 
+	if (!ctx->used) {
+		err = af_alg_wait_for_data(sk, flags);
+		if (err)
+			return err;
+	}
+
 	/*
 	 * Data length provided by caller via sendmsg/sendpage that has not
 	 * yet been processed.

crypto/algif_skcipher.c

@@ -72,6 +72,12 @@ static int _skcipher_recvmsg(struct socket *sock, struct msghdr *msg,
 	int err = 0;
 	size_t len = 0;
 
+	if (!ctx->used) {
+		err = af_alg_wait_for_data(sk, flags);
+		if (err)
+			return err;
+	}
+
 	/* Allocate cipher request for current operation. */
 	areq = af_alg_alloc_areq(sk, sizeof(struct af_alg_async_req) +
 				     crypto_skcipher_reqsize(tfm));