netdev - Re: [PATCH next 0/3] blackhole device to invalidate dst

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Message-ID: <5441f3f1-0672-fbb1-e875-7f8ceb68d719@gmail.com>
Date:   Sat, 22 Jun 2019 09:35:53 -0600
From:   David Ahern <dsahern@...il.com>
To:     Mahesh Bandewar <maheshb@...gle.com>,
        Netdev <netdev@...r.kernel.org>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Michael Chan <michael.chan@...adcom.com>,
        Daniel Axtens <dja@...ens.net>,
        Mahesh Bandewar <mahesh@...dewar.net>
Subject: Re: [PATCH next 0/3] blackhole device to invalidate dst

On 6/21/19 6:45 PM, Mahesh Bandewar wrote:
> When we invalidate dst or mark it "dead", we assign 'lo' to
> dst->dev. First of all this assignment is racy and more over,
> it has MTU implications.
> 
> The standard dev MTU is 1500 while the Loopback MTU is 64k. TCP
> code when dereferencing the dst don't check if the dst is valid
> or not. TCP when dereferencing a dead-dst while negotiating a
> new connection, may use dst device which is 'lo' instead of
> using the correct device. Consider the following scenario:
> 

Why doesn't the TCP code (or any code) check if a cached dst is valid?
That's the whole point of marking it dead - to tell users not to rely on
it.