netdev - Re: [PATCH ipsec v2] xfrm: prevent ipcomp scratch buffer race condition

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Date:	Tue, 15 Oct 2013 15:44:29 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Kubecek <mkubecek@...e.cz>
Cc:	Steffen Klassert <steffen.klassert@...unet.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH ipsec v2] xfrm: prevent ipcomp scratch buffer race
 condition

On Tue, 2013-10-15 at 23:40 +0200, Michal Kubecek wrote:
> In ipcomp_compress(), sortirq is enabled too early, allowing the
> per-cpu scratch buffer to be rewritten by ipcomp_decompress()
> (called on the same CPU in softirq context) between populating
> the buffer and copying the compressed data to the skb.
> 
> Add similar protection into ipcomp_decompress() as it can be
> called from process context as well (even if such scenario seems
> a bit artificial).
> 
> v2: as pointed out by Steffen Klassert, if we also move the
> local_bh_disable() before reading the per-cpu pointers, we can
> get rid of get_cpu()/put_cpu().
> 
> Signed-off-by: Michal Kubecek <mkubecek@...e.cz>
> ---
>  net/xfrm/xfrm_ipcomp.c | 29 ++++++++++++++++++-----------
>  1 file changed, 18 insertions(+), 11 deletions(-)
> 
> diff --git a/net/xfrm/xfrm_ipcomp.c b/net/xfrm/xfrm_ipcomp.c
> index 2906d52..9fe4f42 100644
> --- a/net/xfrm/xfrm_ipcomp.c
> +++ b/net/xfrm/xfrm_ipcomp.c
> @@ -45,12 +45,17 @@ static int ipcomp_decompress(struct xfrm_state *x, struct sk_buff *skb)
>  	const int plen = skb->len;
>  	int dlen = IPCOMP_SCRATCH_SIZE;
>  	const u8 *start = skb->data;
> -	const int cpu = get_cpu();
> -	u8 *scratch = *per_cpu_ptr(ipcomp_scratches, cpu);
> -	struct crypto_comp *tfm = *per_cpu_ptr(ipcd->tfms, cpu);
> -	int err = crypto_comp_decompress(tfm, start, plen, scratch, &dlen);
> +	struct crypto_comp *tfm;
> +	u8 *scratch;
> +	int cpu;
> +	int err;
>  	int len;
>  
> +	local_bh_disable();
> +	cpu = smp_processor_id();
> +	scratch = *per_cpu_ptr(ipcomp_scratches, cpu);
> +	tfm = *per_cpu_ptr(ipcd->tfms, cpu);

Have you tried this_cpu_ptr() instead ?




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