netdev - Re: [PATCH net] net: Revert "net: avoid one atomic operation in skb

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Message-ID: <1416598161.8629.120.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 21 Nov 2014 11:29:21 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Sabrina Dubroca <sd@...asysnail.net>
Cc:	Chris Mason <clm@...com>, David Miller <davem@...emloft.net>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net] net: Revert "net: avoid one atomic operation in
 skb_clone()"

On Fri, 2014-11-21 at 19:05 +0100, Sabrina Dubroca wrote:
> Hello Eric,
> 
> 2014-11-21, 09:04:11 -0800, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > Not sure what I was thinking, but doing anything after
> > releasing a refcount is suicidal or/and embarrassing.
> > 
> > By the time we set skb->fclone to SKB_FCLONE_FREE, another cpu
> > could have released last reference and freed whole skb.
> > 
> > We potentially corrupt memory or trap if CONFIG_DEBUG_PAGEALLOC is set.
> > 
> > Reported-by: Chris Mason <clm@...com>
> > Fixes: ce1a4ea3f1258 ("net: avoid one atomic operation in skb_clone()")
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > ---
> >  net/core/skbuff.c |   23 ++++++-----------------
> >  1 file changed, 6 insertions(+), 17 deletions(-)
> > 
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index c16615bfb61e..be4c7deed971 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -552,20 +552,13 @@ static void kfree_skbmem(struct sk_buff *skb)
> >  	case SKB_FCLONE_CLONE:
> >  		fclones = container_of(skb, struct sk_buff_fclones, skb2);
> >  
> > -		/* Warning : We must perform the atomic_dec_and_test() before
> > -		 * setting skb->fclone back to SKB_FCLONE_FREE, otherwise
> > -		 * skb_clone() could set clone_ref to 2 before our decrement.
> > -		 * Anyway, if we are going to free the structure, no need to
> > -		 * rewrite skb->fclone.
> > +		/* The clone portion is available for
> > +		 * fast-cloning again.
> >  		 */
> > -		if (atomic_dec_and_test(&fclones->fclone_ref)) {
> > +		skb->fclone = SKB_FCLONE_UNAVAILABLE;
> 
> Shouldn't that be SKB_FCLONE_FREE ?

Absolutely :(

I'll send a v2, thanks !


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