Re: [PATCH V2] ipv6: fix race condition regarding dst->expires and dst->from.

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2013-02-20 at 19:29 +0900, YOSHIFUJI Hideaki wrote:
> Eric Dumazet wrote:
> | Some strange crashes happen in rt6_check_expired(), with access
> | to random addresses.
> |
> | At first glance, it looks like the RTF_EXPIRES and
> | stuff added in commit 1716a96101c49186b
> | (ipv6: fix problem with expired dst cache)
> | are racy : same dst could be manipulated at the same time
> | on different cpus.
> |
> | At some point, our stack believes rt->dst.from contains a dst pointer,
> | while its really a jiffie value (as rt->dst.expires shares the same area
> | of memory)
> |
> | rt6_update_expires() should be fixed, or am I missing something ?
> |
> | CC Neil because of https://bugzilla.redhat.com/show_bug.cgi?id=892060
> 
> Because we do not have any locks for dst_entry, we cannot change
> essential structure in the entry; e.g., we cannot change reference
> to other entity.
> 
> To fix this issue, split 'from' and 'expires' field in dst_entry
> out of union.  Once it is 'from' is assigned in the constructor,
> keep the reference until the very last stage of the life time of
> the object.
> 
> Of course, it is unsafe to change 'from', so make rt6_set_from simple
> just for fresh entries.
> 
> Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
> Reported-by: Neil Horman <nhorman@tuxdriver.com>
> CC: Gao Feng <gaofeng@cn.fujitsu.com>
> Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
> ---

This seems good to me, but I cant test it at this moment.

I CC Steinar as he reported one crash to me.

Thanks Yoshifuji !

Reviewed-by: Eric Dumazet <edumazet@google.com>

Reported-by: Steinar H. Gunderson <sesse@google.com>