[PATCH] crypto: fix potential NULL pointer dereference in skcipher_alloc_sgl()

[Jeff Liu <jeff.liu@oracle.com>]

From: Jie Liu <jeff.liu@oracle.com>

In skcipher_alloc_sgl(), there is a potential null pointer dereference
issue to retrieve the last item from ctx->tsgl list if the list is empty.

This patch fix it by checking if the list is empty or not at first.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
---
 crypto/algif_skcipher.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/crypto/algif_skcipher.c b/crypto/algif_skcipher.c
index a1c4f0a..bfa702e 100644
--- a/crypto/algif_skcipher.c
+++ b/crypto/algif_skcipher.c
@@ -73,9 +73,10 @@ static int skcipher_alloc_sgl(struct sock *sk)
 	struct skcipher_sg_list *sgl;
 	struct scatterlist *sg = NULL;
 
-	sgl = list_entry(ctx->tsgl.prev, struct skcipher_sg_list, list);
-	if (!list_empty(&ctx->tsgl))
+	if (!list_empty(&ctx->tsgl)) {
+		sgl = list_entry(ctx->tsgl.prev, struct skcipher_sg_list, list);
 		sg = sgl->sg;
+	}
 
 	if (!sg || sgl->cur >= MAX_SGL_ENTS) {
 		sgl = sock_kmalloc(sk, sizeof(*sgl) +
-- 
1.8.3.2