netdev - Re: net: Fix skb_set_peeked use-after-free bug

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Message-ID: <55C1F77C.70302@yandex-team.ru>
Date:	Wed, 05 Aug 2015 14:46:04 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	Herbert Xu <herbert@...dor.apana.org.au>,
	Brenden Blanco <bblanco@...mgrid.com>
CC:	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: net: Fix skb_set_peeked use-after-free bug

On 04.08.2015 10:42, Herbert Xu wrote:
> Brenden Blanco <bblanco@...mgrid.com> wrote:
>>> [  318.244596] BUG: unable to handle kernel NULL pointer dereference
>>> at 000000000000008e
>>> [  318.245182] IP: [<ffffffff81455e7c>] __skb_recv_datagram+0xbc/0x5a0
>>
>> Replying to myself, and adding commit interested parties...
>>
>> I went through the git log for the function in question, and
>> positively identified that the following commit introduces the crash:
>>
>> 738ac1e net: Clone skb before setting peeked flag
>>
>> Null dereference is at line 224 of net/core/datagram.c (according to
>> my objdump dis-assembly):
>
> Sorry, brain fart on my part.  Please try this patch.
>
> ---8<---
> The commit 738ac1ebb96d02e0d23bc320302a6ea94c612dec ("net: Clone
> skb before setting peeked flag") introduced a use-after-free bug
> in skb_recv_datagram.  This is because skb_set_peeked may create
> a new skb and free the existing one.  As it stands the caller will
> continue to use the old freed skb.
>
> This patch fixes it by making skb_set_peeked return the new skb
> (or the old one if unchanged).
>
> Fixes: 738ac1ebb96d ("net: Clone skb before setting peeked flag")
> Reported-by: Brenden Blanco <bblanco@...mgrid.com>
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

Seems correct. You can add:

Reviewed-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>

Your skb_set_peeked() doesn't set prev/next to NULL when
unlinks old skb from the queue unlike to __skb_unlink().
Isn't big deal but nulling might be useful.

>
> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index 4967262..617088a 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -131,12 +131,12 @@ out_noerr:
>   	goto out;
>   }
>
> -static int skb_set_peeked(struct sk_buff *skb)
> +static struct sk_buff *skb_set_peeked(struct sk_buff *skb)
>   {
>   	struct sk_buff *nskb;
>
>   	if (skb->peeked)
> -		return 0;
> +		return skb;
>
>   	/* We have to unshare an skb before modifying it. */
>   	if (!skb_shared(skb))
> @@ -144,7 +144,7 @@ static int skb_set_peeked(struct sk_buff *skb)
>
>   	nskb = skb_clone(skb, GFP_ATOMIC);
>   	if (!nskb)
> -		return -ENOMEM;
> +		return ERR_PTR(-ENOMEM);
>
>   	skb->prev->next = nskb;
>   	skb->next->prev = nskb;
> @@ -157,7 +157,7 @@ static int skb_set_peeked(struct sk_buff *skb)
>   done:
>   	skb->peeked = 1;
>
> -	return 0;
> +	return skb;
>   }
>
>   /**
> @@ -229,8 +229,9 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
>   					continue;
>   				}
>
> -				error = skb_set_peeked(skb);
> -				if (error)
> +				skb = skb_set_peeked(skb);
> +				error = PTR_ERR(skb);
> +				if (IS_ERR(skb))
>   					goto unlock_err;
>
>   				atomic_inc(&skb->users);
>


-- 
Konstantin
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