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

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Message-ID: <CANn89i+U05o3T8dMCyKM15QmYp43w3Uc5dHKvZ-z2WHB5zYNLQ@mail.gmail.com>
Date:   Thu, 7 Nov 2019 08:43:30 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Ahern <dsahern@...il.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 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?