netdev - RE: [PATCH 3/3] netfilter: x_tables: fix ordering of jumpstack allocation and table update

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Date:	Wed, 23 Oct 2013 10:45:04 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Pablo Neira Ayuso" <pablo@...filter.org>,
	<netfilter-devel@...r.kernel.org>
Cc:	<davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: RE: [PATCH 3/3] netfilter: x_tables: fix ordering of jumpstack allocation and table update

> Subject: [PATCH 3/3] netfilter: x_tables: fix ordering of jumpstack allocation and table update
...
> Meanwhile, CPU0 is handling the network receive path and ends up in
> ipt_do_table, resulting in:
> 
> 	private = table->private;
> 
> 	[...]
> 
> 	jumpstack  = (struct ipt_entry **)private->jumpstack[cpu];
> 
> On weakly ordered memory architectures, the writes to table->private
> and newinfo->jumpstack from CPU1 can be observed out of order by CPU0.
> Furthermore, on architectures which don't respect ordering of address
> dependencies (i.e. Alpha), the reads from CPU0 can also be re-ordered.

Which reads might be out of order?
AFAICT they are strongly sequenced because they second depends on the
value read by the first.
So I don't see why the read barrier is needed.

I presume the above code is tied to a single cpu.

...
> 
> -	table->private = newinfo;
>  	newinfo->initial_entries = private->initial_entries;
> +	/*
> +	 * Ensure contents of newinfo are visible before assigning to
> +	 * private.
> +	 */
> +	smp_wmb();
> +	table->private = newinfo;

Those writes were in the wrong order on all systems.
Also gcc needs to be told not to reorder the writes even on non-smp
systems (if the code might be pre-empted).
So an asm volatile (:::"memory") is needed there even if no specific
synchronisation instruction is needed.

	David



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