netdev - Re: [PATCH 1/1] net: possible use after free in dst

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Message-ID: <1452088644.8255.165.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 06 Jan 2016 05:57:24 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Francesco Ruggeri <fruggeri@...stanetworks.com>
Cc:	fruggeri@...sta.com, edumazet@...gle.com, davem@...emloft.net,
	netdev@...r.kernel.org
Subject: Re: [PATCH 1/1] net: possible use after free in dst_release

On Wed, 2016-01-06 at 00:18 -0800, Francesco Ruggeri wrote:
> dst_release should not access dst->flags after decrementing
> __refcnt to 0. The dst_entry may be in dst_busy_list and
> dst_gc_task may dst_destroy it before dst_release gets a chance
> to access dst->flags.
> 
> Signed-off-by: Francesco Ruggeri <fruggeri@...sta.com>
> ---
>  net/core/dst.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dst.c b/net/core/dst.c
> index e6dc772..a1656e3 100644
> --- a/net/core/dst.c
> +++ b/net/core/dst.c
> @@ -301,12 +301,13 @@ void dst_release(struct dst_entry *dst)
>  {
>  	if (dst) {
>  		int newrefcnt;
> +		unsigned short nocache = dst->flags & DST_NOCACHE;
>  
>  		newrefcnt = atomic_dec_return(&dst->__refcnt);
>  		if (unlikely(newrefcnt < 0))
>  			net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
>  					     __func__, dst, newrefcnt);
> -		if (!newrefcnt && unlikely(dst->flags & DST_NOCACHE))
> +		if (!newrefcnt && unlikely(nocache))
>  			call_rcu(&dst->rcu_head, dst_destroy_rcu);
>  	}
>  }

You're right.

My previous fix (commit d69bbf88c8d0) was okay only for DST_NOCACHE dst.

Fixes: d69bbf88c8d0 ("net: fix a race in dst_release()")
Fixes: 27b75c95f10d ("net: avoid RCU for NOCACHE dst")
Acked-by: Eric Dumazet <edumazet@...gle.com>

Thanks !


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