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

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Message-ID: <094aedc7-3303-7f27-25eb-a32523faa5b7@gmail.com>
Date:   Thu, 7 Nov 2019 09:37:07 -0700
From:   David Ahern <dsahern@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     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 11/6/19 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?