From b61907bb42409adf9b3120f741af7c57dd7e3db2 Mon Sep 17 00:00:00 2001
From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Mon, 9 Oct 2017 23:30:02 +0800
Subject: [PATCH] crypto: shash - Fix zero-length shash ahash digest crash
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Git-commit: b61907bb42409adf9b3120f741af7c57dd7e3db2
Patch-mainline: v4.14-rc5
References: bsc#1051510

The shash ahash digest adaptor function may crash if given a
zero-length input together with a null SG list.  This is because
it tries to read the SG list before looking at the length.

This patch fixes it by checking the length first.

Cc: <stable@vger.kernel.org>
Reported-by: Stephan Müller<smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Stephan Müller <smueller@chronox.de>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 crypto/shash.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/crypto/shash.c
+++ b/crypto/shash.c
@@ -276,12 +276,14 @@ static int shash_async_finup(struct ahas
 
 int shash_ahash_digest(struct ahash_request *req, struct shash_desc *desc)
 {
-	struct scatterlist *sg = req->src;
-	unsigned int offset = sg->offset;
 	unsigned int nbytes = req->nbytes;
+	struct scatterlist *sg;
+	unsigned int offset;
 	int err;
 
-	if (nbytes < min(sg->length, ((unsigned int)(PAGE_SIZE)) - offset)) {
+	if (nbytes &&
+	    (sg = req->src, offset = sg->offset,
+	     nbytes < min(sg->length, ((unsigned int)(PAGE_SIZE)) - offset))) {
 		void *data;
 
 		data = kmap_atomic(sg_page(sg));