Re: general protection fault in af_alg_free_areq_sgls

On Mon, Nov 27, 2017 at 10:56:47AM -0800, syzbot wrote:

Hello,

syzkaller hit the following crash on
b0a84f19a5161418d4360cd57603e94ed489915e
git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
compiler: gcc (GCC) 7.1.1 20170620
.config is attached
Raw console output is attached.

Unfortunately, I don't have any reproducer for this bug yet.

This was probably caused by taking the path where areq->tsgl could not be
allocated.  (syzkaller probably reached it after injecting a memory allocation
failure.)  The following should fix it:

---8<---

From 1a7a7f86f09c50652f1fff75b8d3a32712826b32 Mon Sep 17 00:00:00 2001

From: Eric Biggers <redacted>
Date: Tue, 28 Nov 2017 00:46:24 -0800
Subject: [PATCH] crypto: af_alg - fix NULL pointer dereference in
 af_alg_free_areq_sgls()

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 <redacted>
Cc: <redacted> # v4.14+
Signed-off-by: Eric Biggers <redacted>
---
 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