[PATCH] crypto: af_alg - fix NULL pointer dereference in af_alg_free_areq_sgls()

[Eric Biggers]

If allocating the ->tsgl member of 'struct af_alg_async_req' failed,
during cleanup we dereferenced the NULL ->tsgl pointer in
af_alg_free_areq_sgls(), because ->tsgl_entries was nonzero.

Fix it by only freeing the ->tsgl list if it is non-NULL.

This affected both algif_skcipher and algif_aead.

Fixes: e870456d8e7c ("crypto: algif_skcipher - overhaul memory management")
Fixes: d887c52d6ae4 ("crypto: algif_aead - overhaul memory management")
Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v4.14+
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
 crypto/af_alg.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index 358749c38894..415a54ced4d6 100644
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -672,14 +672,15 @@ void af_alg_free_areq_sgls(struct af_alg_async_req *areq)
 	}
 
 	tsgl = areq->tsgl;
-	for_each_sg(tsgl, sg, areq->tsgl_entries, i) {
-		if (!sg_page(sg))
-			continue;
-		put_page(sg_page(sg));
-	}
+	if (tsgl) {
+		for_each_sg(tsgl, sg, areq->tsgl_entries, i) {
+			if (!sg_page(sg))
+				continue;
+			put_page(sg_page(sg));
+		}
 
-	if (areq->tsgl && areq->tsgl_entries)
 		sock_kfree_s(sk, tsgl, areq->tsgl_entries * sizeof(*tsgl));
+	}
 }
 EXPORT_SYMBOL_GPL(af_alg_free_areq_sgls);
 
-- 
2.15.0