linux-kernel - Re: [PATCH 4/4] net/core/dev.c : Race condition in net_tx

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Date:	Wed, 25 Feb 2015 10:16:34 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ameen Ali <ameenali023@...il.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] net/core/dev.c : Race condition in net_tx_action.

On Wed, 2015-02-25 at 19:50 +0200, Ameen Ali wrote:
> Transmitting asynchronously on all the network devices available we will  notice the following behaviour:
> a) The instruction "if (sd->completion_queue) {" saves on a CPU register the pointer value (register contents is used for the comparison)
> b) The interupt is disabled (using "local_irq_disable")
> c) when the content of "clist" is updated, the register is used, instead of re-read the "completion_queue" variable.
> 
> So, when a low-level tx interrupt arrives after the latching of "completion_queue", but before "local_irq_disable",
>  the value stored in "clist" reflect the situation before low-level tx interrupt, resulting in a sk_buff leak
> ---
>  net/core/dev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 8f9710c..db3e59e 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3413,7 +3413,7 @@ EXPORT_SYMBOL(netif_rx_ni);
>  
>  static void net_tx_action(struct softirq_action *h)
>  {
> -	struct softnet_data *sd = this_cpu_ptr(&softnet_data);
> +	volatile struct softnet_data *sd = &__get_cpu_var(softnet_data);
>  
>  	if (sd->completion_queue) {
>  		struct sk_buff *clist;

Seems real bug is elsewhere. This is becoming a FAQ.

Which arch are you using, and which compiler ?

volatile are highly discouraged in favor of ACCESS_ONCE, READ_ONCE(),
WRITE_ONCE() : read
Documentation/volatile-considered-harmful.txt:19:safe

local_irq_disable() acts as a barrier the compiler should reload the
value from memory.



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