crypto: shash - Fix zero-length shash ahash digest crash

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>

1 files changed, 5 insertions, 3 deletions

diff --git a/crypto/shash.c b/crypto/shash.c
index 8fcecc66741d..325a14da5827 100644
--- a/crypto/shash.c
+++ b/crypto/shash.c
@@ -275,12 +275,14 @@ static int shash_async_finup(struct ahash_request *req)
 
 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));