netdev - Re: [PATCH net] inet: protect against too small mtu values.

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Date:   Sat, 07 Dec 2019 11:55:43 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     edumazet@...gle.com
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com,
        syzkaller@...glegroups.com
Subject: Re: [PATCH net] inet: protect against too small mtu values.

From: Eric Dumazet <edumazet@...gle.com>
Date: Thu,  5 Dec 2019 20:43:46 -0800

> syzbot was once again able to crash a host by setting a very small mtu
> on loopback device.
> 
> Let's make inetdev_valid_mtu() available in include/net/ip.h,
> and use it in ip_setup_cork(), so that we protect both ip_append_page()
> and __ip_append_data()
> 
> Also add a READ_ONCE() when the device mtu is read.
> 
> Pairs this lockless read with one WRITE_ONCE() in __dev_set_mtu(),
> even if other code paths might write over this field.
> 
> Add a big comment in include/linux/netdevice.h about dev->mtu
> needing READ_ONCE()/WRITE_ONCE() annotations.
> 
> Hopefully we will add the missing ones in followup patches.
 ...
> Fixes: 1470ddf7f8ce ("inet: Remove explicit write references to sk/inet in ip_append_data")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: syzbot <syzkaller@...glegroups.com>

Applied and queued up for -stable, thanks.