linux-kernel - Re: crypto: use-after-free in alg

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Date:	Wed, 30 Dec 2015 20:24:17 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	linux-crypto@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	syzkaller <syzkaller@...glegroups.com>
Subject: Re: crypto: use-after-free in alg_bind

On Wed, Dec 30, 2015 at 11:58:58AM +0100, Dmitry Vyukov wrote:
>
> I forgot to diff include/crypto/if_alg.h, but the changes are there
> (otherwise all references to refcnt would not compile). Also I moved
> ask->refcnt checks to alg_setsockopt to fix the deadlock, I believe
> that's the missing chunks you refer to. I can retest if you wish, but
> I don't think that my changes can affect the reported use-after-free.
> Do you?

OK I see the problem now.  When accept fails we free the socket
twice.

---8<---
Subject: crypto: af_alg - Fix socket double-free when accept fails

When we fail an accept(2) call we will end up freeing the socket
twice, once due to the direct sk_free call and once again through
newsock.

This patch fixes this by removing the sk_free call.

Cc: stable@...r.kernel.org
Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index 7b5b592..eaf98e2 100644
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -285,10 +285,8 @@ int af_alg_accept(struct sock *sk, struct socket *newsock)
 	security_sk_clone(sk, sk2);
 
 	err = type->accept(ask->private, sk2);
-	if (err) {
-		sk_free(sk2);
+	if (err)
 		goto unlock;
-	}
 
 	sk2->sk_family = PF_ALG;
 
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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