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Date:   Wed, 31 May 2017 17:13:42 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        James Morris <jmorris@...ei.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Patrick McHardy <kaber@...sh.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: net/ipv4: use-after-free in add_grec

On Wed, 2017-05-31 at 16:55 -0700, Eric Dumazet wrote:

> The issue here is the timer firing while ip_mc_clear_src() has been
> already called.
> 
> My patch should fix the problem.
> 
> Or another one using del_timer_sync() instead of del_timer() in
> igmp_stop_timer(), but such a change would be more invasive,
> since the del_timer_sync() would need to happen while im->lock
> spinlock is not held.

BTW, I guess that Andrey could try to add a delay to trigger the bug
more often.

diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index 44fd86de2823dd17de16276a8ec01b190e69b8b4..84fff17daab0832c470a613b29b2aaade07cec0a 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -798,7 +798,7 @@ static void igmp_timer_expire(unsigned long data)
 	}
 	im->reporter = 1;
 	spin_unlock(&im->lock);
-
+	udelay(10000);
 	if (IGMP_V1_SEEN(in_dev))
 		igmp_send_report(in_dev, im, IGMP_HOST_MEMBERSHIP_REPORT);
 	else if (IGMP_V2_SEEN(in_dev))


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