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

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Date:	Wed, 30 Dec 2015 13:45:15 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
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 1:24 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> 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.

Great!

This seems to be a zero-day. Should we CC stable@...r.kernel.org ?

Code in 03c8efc1ffeb6b82a22c1af8dd908af349563314 (Oct 19, 2010) contained:

+       sock_init_data(newsock, sk2);
+
+       err = type->accept(ask->private, sk2);
+       if (err) {
+               sk_free(sk2);
+               goto unlock;
+       }

There were no sock_graft call, but sock_init_data also sets
newsock->sk = sk2, which seems to be enough for the double-free.




> ---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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