netdev - Re: [PATCH net v3] net: bridge: fix vlan stats use-after-free on destruction

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Message-ID: <137746b0-1655-3704-a6f1-54274615d605@cumulusnetworks.com>
Date:   Fri, 24 Apr 2020 10:26:51 +0300
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     netdev@...r.kernel.org, roopa@...ulusnetworks.com,
        davem@...emloft.net, bridge@...ts.linux-foundation.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH net v3] net: bridge: fix vlan stats use-after-free on
 destruction

On 24/04/2020 03:05, Stephen Hemminger wrote:
> On Fri, 16 Nov 2018 18:50:01 +0200
> Nikolay Aleksandrov <nikolay@...ulusnetworks.com> wrote:
> 
>> Syzbot reported a use-after-free of the global vlan context on port vlan
>> destruction. When I added per-port vlan stats I missed the fact that the
>> global vlan context can be freed before the per-port vlan rcu callback.
>> There're a few different ways to deal with this, I've chosen to add a
>> new private flag that is set only when per-port stats are allocated so
>> we can directly check it on destruction without dereferencing the global
>> context at all. The new field in net_bridge_vlan uses a hole.
>>
>> v2: cosmetic change, move the check to br_process_vlan_info where the
>>     other checks are done
>> v3: add change log in the patch, add private (in-kernel only) flags in a
>>     hole in net_bridge_vlan struct and use that instead of mixing
>>     user-space flags with private flags
>>
>> Fixes: 9163a0fc1f0c ("net: bridge: add support for per-port vlan stats")
>> Reported-by: syzbot+04681da557a0e49a52e5@...kaller.appspotmail.com
>> Signed-off-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
> 
> Why not just use v->stats itself as the flag.
> Since free of NULL is a nop it would be cleaner?
> 

v->stats is *always* set while the vlan is published/visible, that's a guarantee
I don't want to break because I'll have to add null checks in the fast-path.

By the way this is a thread from 2018. :-)

Cheers,
 Nik