netdev - Re: [PATCH net] ipv6: fixes rt6_probe() and fib6_nh->last

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Message-ID: <aa337d1c-28cb-6e63-6603-f9d54b51d2c9@gmail.com>
Date:   Thu, 7 Nov 2019 09:50:44 -0700
From:   David Ahern <dsahern@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ipv6: fixes rt6_probe() and fib6_nh->last_probe init

On 11/7/19 9:45 AM, Eric Dumazet wrote:
> On Thu, Nov 7, 2019 at 8:37 AM David Ahern <dsahern@...il.com> wrote:
>>
>> On 11/6/19 7:45 PM, Eric Dumazet wrote:
>>> While looking at a syzbot KCSAN report [1], I found multiple
>>> issues in this code :
>>>
>>> 1) fib6_nh->last_probe has an initial value of 0.
>>>
>>>    While probably okay on 64bit kernels, this causes an issue
>>>    on 32bit kernels since the time_after(jiffies, 0 + interval)
>>>    might be false ~24 days after boot (for HZ=1000)
>>>
>>> 2) The data-race found by KCSAN
>>>    I could use READ_ONCE() and WRITE_ONCE(), but we also can
>>>    take the opportunity of not piling-up too many rt6_probe_deferred()
>>>    works by using instead cmpxchg() so that only one cpu wins the race.
>>>
>>
>> ...
>>
>>> Fixes: cc3a86c802f0 ("ipv6: Change rt6_probe to take a fib6_nh")
>>
>> That commit only moves the location of last_probe, from fib6_info into
>> fib6_nh. Given that I would expect the same problem to exist with the
>> previous code. Agree? Point being should this be backported to older
>> stable releases since said commit is new to 5.2?
> 
> Yes, the commit adding last probe went in 4.19
> 
> Fixes: f547fac624be ("ipv6: rate-limit probes for neighbourless routes")
> 

Reviewed-by: David Ahern <dsahern@...il.com>