linux-kernel - Crash with SO_REUSEPORT and ef456144da8ef507c8cf504284b6042e9201a05c

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Date:	Tue, 19 Jan 2016 11:57:54 -0400
From:	Marc Dionne <marc.c.dionne@...il.com>
To:	netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Craig Gallek <kraig@...gle.com>
Cc:	eric.dumazet@...gle.com
Subject: Crash with SO_REUSEPORT and ef456144da8ef507c8cf504284b6042e9201a05c

I shared this one with Craig but I thought I'd put it out to a wider audience.

Trying to run the current kernel mainline on a test system I found
that any attempt to run many of our executables would crash the
system.  The networking code in all of these opens and listens on
multiple UDP sockets set with SO_REUSEPORT.  We also like to bind the
first socket before setting SO_REUSEPORT so we can catch some cases
where the port is actually in use by someone else (for instance a
previous incarnation of the same service).

This is easily reproduced with this sequence:
- create 2 sockets A and B
- bind socket A to an address
- set SO_REUSEPORT on socket A
- set SO_REUSEPORT on socket B
- bind socket B to the same address as A

The sk_reuseport_cb structure is only allocated at bind time if
SO_REUSEPORT is already set, so A doesn't have one.  When we bind B, A
is found as a match that has SO_REUSEPORT and reuseport_add_sock will
try to use the NULL sk_reuseport_cb structure from A, causing a crash.

Not sure what the best fix is, but seems like the structure could be
either allocated (if not already done) when setting SO_REUSEPORT, or
when we find it to be NULL in reuseport_add_sock (but locking may be
an issue there).  I was able to test that allocating sk_reuseport_cb
when setting SO_REUSEPORT makes things behave normally again; see
attached patch.  That's surely not a correct/complete fix as B (in the
scenario above) will have an unnecessary sk_reuseport_cb which will
trigger a warning and should be dealt with.

Thanks,
Marc

View attachment "reuseport.patch" of type "text/x-patch" (2068 bytes)