Subject: Re: [regression] nf_iterate(), BUG: unable to handle kernel NULL pointer dereference

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 1 Aug 2008 14:10:50 -0700

On Thu, Jul 24, 2008 at 02:13:42PM -0700, Linus Torvalds wrote:
> 
> 
> On Thu, 24 Jul 2008, Patrick McHardy wrote:
> > 
> > To fix this I think we need a __krealloc() that doesn't
> > free the old memory, especially since it must not be
> > freed immediately because it may still be used in a RCU
> > read side (see the last part in the patch attached to
> > this mail (based on a kernel without your patch)).
> 
> Hmm. Don't you need to fix some of the ordering of the initialization too?
> 
> If there are possible readers that happen in parallel with changing this 
> thing, don't you need to protect the update of "ext->len" against the 
> actual changes? And the readers should probably have a read barrier 
> between checking "len" and actually looking at the values? Finally, why do 
> the "ct->ext" dereference thing, when we know it has to be equal to "new"?
> 
> ie something like this on the writing side (in _addition_ to both the 
> patches already seen), but I didn't do the reading side (ie there are no 
> "smp_rmb()"'s on the reading side)
> 
> And no, I don't know the code, so I don't know who/what can read those 
> things with RCU, so maybe there is some reason why the actual data doesn't 
> need protecting. But I somehow doubt it.

There certainly needs to be an rcu_assign_pointer() or a smp_wmb()
in there somewhere -- whenever it is that the new structure becomes
accessible to RCU readers.  I looked around a bit, but couldn't tell
when this stuff becomes accessible to readers...

							Thanx, Paul

> 		Linus
> 
> ---
>  net/netfilter/nf_conntrack_extend.c |    9 +++++----
>  1 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/net/netfilter/nf_conntrack_extend.c b/net/netfilter/nf_conntrack_extend.c
> index 3469bc7..135e095 100644
> --- a/net/netfilter/nf_conntrack_extend.c
> +++ b/net/netfilter/nf_conntrack_extend.c
> @@ -115,10 +115,11 @@ void *__nf_ct_ext_add(struct nf_conn *ct, enum nf_ct_ext_id id, gfp_t gfp)
>  		ct->ext = new;
>  	}
> 
> -	ct->ext->offset[id] = newoff;
> -	ct->ext->len = newlen;
> -	memset((void *)ct->ext + newoff, 0, newlen - newoff);
> -	return (void *)ct->ext + newoff;
> +	new->offset[id] = newoff;
> +	memset((void *)new + newoff, 0, newlen - newoff);
> +	smp_wmb();
> +	new->len = newlen;
> +	return (void *)new + newoff;
>  }
>  EXPORT_SYMBOL(__nf_ct_ext_add);
> 
> --
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