netdev - Re: [PATCH v2 net] ipv6: fixes rt6_probe() and fib6_nh->last

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Date:   Thu, 07 Nov 2019 16:14:05 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     edumazet@...gle.com
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com,
        syzkaller@...glegroups.com, dsahern@...il.com
Subject: Re: [PATCH v2 net] ipv6: fixes rt6_probe() and fib6_nh->last_probe
 init

From: Eric Dumazet <edumazet@...gle.com>
Date: Thu,  7 Nov 2019 09:26:19 -0800

> 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.
> 
> [1]
> BUG: KCSAN: data-race in find_match / find_match
 ...
> Fixes: cc3a86c802f0 ("ipv6: Change rt6_probe to take a fib6_nh")
> Fixes: f547fac624be ("ipv6: rate-limit probes for neighbourless routes")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Reviewed-by: David Ahern <dsahern@...il.com>
> ---
> v2: Added a 2nd Fixes: tag to help stable backports and Reviewed-by: from David Ahern

Applied and queued up for -stable, thanks Eric.