netdev - [RFC] possible bug in inet->opt handling

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Message-ID: <1302881994.3613.34.camel@edumazet-laptop>
Date:	Fri, 15 Apr 2011 17:39:54 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	netdev <netdev@...r.kernel.org>
Subject: [RFC] possible bug in inet->opt handling

In commit 903ab86d19 (udp: Add lockless transmit path), we added a
fastpath to avoid taking socket lock if we dont use corking.

Prior work were commit 1c32c5ad6fac8c (inet: Add ip_make_skb and
ip_finish_skb) and commit 1470ddf7f8cecf776921e5 (inet: Remove explicit
write references to sk/inet in ip_append_data)

Problem is ip_make_skb() calls ip_setup_cork() and
ip_setup_cork() possibly makes a copy of ipc->opt (struct ip_options),
without any protection against another thread manipulating inet->opt.

Another thread can change inet->opt pointer and free old one... kaboom.

This was discovered by code analysis (I am trying to remove the zeroing
of cork variable in ip_make_skb(), since its a bit expensive and
probably useless)

Note : race was there before Herbert patches.

My plan is to add RCU protection on inet->opt, unless someone has better
idea ?

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