netdev - Re: Double free of dst_entry in ipv4_dst

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Message-ID: <20151214112802.Horde.BK3A-grfQxyYIrszzKdCZg1@ltc.linux.ibm.com>
Date:	Mon, 14 Dec 2015 11:28:02 -0500
From:	dwilder@...ibm.com
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, predeep@...ibm.com, mjtarsel@...ibm.com
Subject: Re: Double free of dst_entry in ipv4_dst_destroy()

Eric -
With this patch applied the test ran clean for 2 days.

Thanks for your help.

Quoting Eric Dumazet <eric.dumazet@...il.com>:

> On Fri, 2015-12-11 at 07:48 -0800, Eric Dumazet wrote:
>> On Fri, 2015-12-11 at 06:23 -0800, Eric Dumazet wrote:
>> > On Sun, 2015-12-06 at 17:58 -0800, Eric Dumazet wrote:
>> > > On Sun, 2015-12-06 at 13:03 -0800, Eric Dumazet wrote:
>> > >
>> > > > But then when later we promote a skb->dst to a refctounted one
>> > > > (skb_dst_force(), we might make sure we abort the operation  
>> if __refcnt
>> > > > == 0 ( and DST_NOCACHE is in dst->flags)
>> > > >
>> > >
>> > > Minimum patch would be :
>> > >
>> >
>> > Here is a more complete patch, it should fix the issue I think :
>>
>> Hmm, I'll send a v3, I forgot to test DST_NOCACHE properly.
>>
>
> David, please test the following patch, thanks !
>
>  include/net/dst.h   |   33 +++++++++++++++++++++++++++++++++
>  include/net/sock.h  |    2 +-
>  net/ipv4/tcp_ipv4.c |    5 ++---
>  net/ipv6/tcp_ipv6.c |    3 +--
>  4 files changed, 37 insertions(+), 6 deletions(-)
>
> diff --git a/include/net/dst.h b/include/net/dst.h
> index 1279f9b09791..c7329dcd90cc 100644
> --- a/include/net/dst.h
> +++ b/include/net/dst.h
> @@ -322,6 +322,39 @@ static inline void skb_dst_force(struct sk_buff *skb)
>  	}
>  }
>
> +/**
> + * dst_hold_safe - Take a reference on a dst if possible
> + * @dst: pointer to dst entry
> + *
> + * This helper returns false if it could not safely
> + * take a reference on a dst.
> + */
> +static inline bool dst_hold_safe(struct dst_entry *dst)
> +{
> +	if (dst->flags & DST_NOCACHE)
> +		return atomic_inc_not_zero(&dst->__refcnt);
> +	dst_hold(dst);
> +	return true;
> +}
> +
> +/**
> + * skb_dst_force_safe - makes sure skb dst is refcounted
> + * @skb: buffer
> + *
> + * If dst is not yet refcounted and not destroyed, grab a ref on it.
> + */
> +static inline void skb_dst_force_safe(struct sk_buff *skb)
> +{
> +	if (skb_dst_is_noref(skb)) {
> +		struct dst_entry *dst = skb_dst(skb);
> +
> +		if (!dst_hold_safe(dst))
> +			dst = NULL;
> +
> +		skb->_skb_refdst = (unsigned long)dst;
> +	}
> +}
> +
>
>  /**
>   *	__skb_tunnel_rx - prepare skb for rx reinsert
> diff --git a/include/net/sock.h b/include/net/sock.h
> index eaef41433d7a..18322bded064 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -816,7 +816,7 @@ void sk_stream_write_space(struct sock *sk);
>  static inline void __sk_add_backlog(struct sock *sk, struct sk_buff *skb)
>  {
>  	/* dont let skb dst not refcounted, we are going to leave rcu lock */
> -	skb_dst_force(skb);
> +	skb_dst_force_safe(skb);
>
>  	if (!sk->sk_backlog.tail)
>  		sk->sk_backlog.head = skb;
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index db003438aaf5..d8841a2f1569 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -1493,7 +1493,7 @@ bool tcp_prequeue(struct sock *sk, struct sk_buff *skb)
>  	if (likely(sk->sk_rx_dst))
>  		skb_dst_drop(skb);
>  	else
> -		skb_dst_force(skb);
> +		skb_dst_force_safe(skb);
>
>  	__skb_queue_tail(&tp->ucopy.prequeue, skb);
>  	tp->ucopy.memory += skb->truesize;
> @@ -1721,8 +1721,7 @@ void inet_sk_rx_dst_set(struct sock *sk, const  
> struct sk_buff *skb)
>  {
>  	struct dst_entry *dst = skb_dst(skb);
>
> -	if (dst) {
> -		dst_hold(dst);
> +	if (dst && dst_hold_safe(dst)) {
>  		sk->sk_rx_dst = dst;
>  		inet_sk(sk)->rx_dst_ifindex = skb->skb_iif;
>  	}
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index e7aab561b7b4..6b8a8a9091fa 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -93,10 +93,9 @@ static void inet6_sk_rx_dst_set(struct sock *sk,  
> const struct sk_buff *skb)
>  {
>  	struct dst_entry *dst = skb_dst(skb);
>
> -	if (dst) {
> +	if (dst && dst_hold_safe(dst)) {
>  		const struct rt6_info *rt = (const struct rt6_info *)dst;
>
> -		dst_hold(dst);
>  		sk->sk_rx_dst = dst;
>  		inet_sk(sk)->rx_dst_ifindex = skb->skb_iif;
>  		inet6_sk(sk)->rx_dst_cookie = rt6_get_cookie(rt);
>
>
> --
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