Re: general protection fault in af_alg_free_areq_sgls

[Eric Biggers]

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 <ebigg...@google.com>
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 <syzkal...@googlegroups.com>
Cc: <sta...@vger.kernel.org> # v4.14+
Signed-off-by: Eric Biggers <ebigg...@google.com>
---
 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