netdev - Re: [PATCH net-next 2/2] xfrm: Fix unaligned access in xfrm_notify

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Date:	Wed, 21 Oct 2015 06:10:43 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	steffen.klassert@...unet.com
Cc:	sowmini.varadhan@...cle.com, linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	herbert@...dor.apana.org.au, dhowells@...hat.com,
	zohar@...ux.vnet.ibm.com, David.Woodhouse@...el.com
Subject: Re: [PATCH net-next 2/2] xfrm: Fix unaligned access in
 xfrm_notify_sa() for DELSA

From: Steffen Klassert <steffen.klassert@...unet.com>
Date: Wed, 21 Oct 2015 08:57:04 +0200

> On Mon, Oct 19, 2015 at 05:23:29PM -0400, Sowmini Varadhan wrote:
>> On sparc, deleting established SAs (e.g., by restarting ipsec
>> at the peer) results in unaligned access messages via
>> xfrm_del_sa -> km_state_notify -> xfrm_send_state_notify().
>> Use an aligned pointer to xfrm_usersa_info for this case.
>> 
>> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
>> ---
>>  net/xfrm/xfrm_user.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>> 
>> diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
>> index a8de9e3..158ef4a 100644
>> --- a/net/xfrm/xfrm_user.c
>> +++ b/net/xfrm/xfrm_user.c
>> @@ -2659,7 +2659,7 @@ static int xfrm_notify_sa(struct xfrm_state *x, const struct km_event *c)
>>  		if (attr == NULL)
>>  			goto out_free_skb;
>>  
>> -		p = nla_data(attr);
>> +		p = PTR_ALIGN(nla_data(attr), __alignof__(*p));
> 
> Hm, this breaks userspace notifications on 64-bit systems.
> Userspace expects this to be aligned to 4, with your patch
> it is aligned to 8 on 64-bit.

That's correct, netlink attributes are fundamentally only 4 byte
aligned and this cannot be changed.  nla_data() is exactly
where the attribute must be placed, aligned or not.

The only workaround is, when designing netlink attributes.  Various
netlink libraries have workarounds for accessing, for example, 64-bit
stats which are going to be unaligned in netlink messages.
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