netdev - Re: [PATCH net] ipv6: fix udpv6 sendmsg crash caused by too small MTU

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Date:   Wed, 10 Jan 2018 10:56:47 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Mike Maloney <maloneykernel@...il.com>, netdev@...r.kernel.org,
        davem@...emloft.net
Cc:     Mike Maloney <maloney@...gle.com>
Subject: Re: [PATCH net] ipv6: fix udpv6 sendmsg crash caused by too small
 MTU

On Wed, 2018-01-10 at 12:45 -0500, Mike Maloney wrote:
> From: Mike Maloney <maloney@...gle.com>
> 
> The logic in __ip6_append_data() assumes that the MTU is at least large
> enough for the headers.  A device's MTU may be adjusted after being
> added while sendmsg() is processing data, resulting in
> __ip6_append_data() seeing any MTU.  For an mtu smaller than the size of
> the fragmentation header, the math results in a negative 'maxfraglen',
> which causes problems when refragmenting any previous skb in the
> skb_write_queue, leaving it possibly malformed.
> 
> Instead sendmsg returns EINVAL when the mtu is calculated to be less
> than IPV6_MIN_MTU.
> 

> Reported-by: syzbot <syzkaller@...glegroups.com>
> Signed-off-by: Mike Maloney <maloney@...gle.com>
> 
> 

Reviewed-by: Eric Dumazet <edumazet@...gle.com>