Date:	Wed, 09 Jun 2010 16:12:09 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Sven Wegener <sven.wegener@...aler.net>
CC:	Julian Anastasov <ja@....bg>, Simon Horman <horms@...ge.net.au>,
	Wensong Zhang <wensong@...ux-vs.org>, netdev@...r.kernel.org,
	lvs-devel@...r.kernel.org
Subject: Re: [PATCH] ipvs: Add missing locking during connection table hashing
 and unhashing

Sven Wegener wrote:
> The code that hashes and unhashes connections from the connection table
> is missing locking of the connection being modified, which opens up a
> race condition and results in memory corruption when this race condition
> is hit.
>
> Here is what happens in pretty verbose form:
>
> CPU 0					CPU 1
> ------------				------------
> An active connection is terminated and
> we schedule ip_vs_conn_expire() on this
> CPU to expire this connection.
>
> 					IRQ assignment is changed to this CPU,
> 					but the expire timer stays scheduled on
> 					the other CPU.
>
> 					New connection from same ip:port comes
> 					in right before the timer expires, we
> 					find the inactive connection in our
> 					connection table and get a reference to
> 					it. We proper lock the connection in
> 					tcp_state_transition() and read the
> 					connection flags in set_tcp_state().
>
> ip_vs_conn_expire() gets called, we
> unhash the connection from our
> connection table and remove the hashed
> flag in ip_vs_conn_unhash(), without
> proper locking!
>
> 					While still holding proper locks we
> 					write the connection flags in
> 					set_tcp_state() and this sets the hashed
> 					flag again.
>
> ip_vs_conn_expire() fails to expire the
> connection, because the other CPU has
> incremented the reference count. We try
> to re-insert the connection into our
> connection table, but this fails in
> ip_vs_conn_hash(), because the hashed
> flag has been set by the other CPU. We
> re-schedule execution of
> ip_vs_conn_expire(). Now this connection
> has the hashed flag set, but isn't
> actually hashed in our connection table
> and has a dangling list_head.
>
> 					We drop the reference we held on the
> 					connection and schedule the expire timer
> 					for timeouting the connection on this
> 					CPU. Further packets won't be able to
> 					find this connection in our connection
> 					table.
>
> 					ip_vs_conn_expire() gets called again,
> 					we think it's already hashed, but the
> 					list_head is dangling and while removing
> 					the connection from our connection table
> 					we write to the memory location where
> 					this list_head points to.
>
> The result will probably be a kernel oops at some other point in time.
>
> Signed-off-by: Sven Wegener <sven.wegener@...aler.net>
> Cc: stable@...nel.org
> Acked-by: Simon Horman <horms@...ge.net.au>
> ---
>  net/netfilter/ipvs/ip_vs_conn.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
>
> This race condition is pretty subtle, but it can be triggered remotely.
> It needs the IRQ assignment change or another circumstance where packets
> coming from the same ip:port for the same service are being processed on
> different CPUs. And it involves hitting the exact time at which
> ip_vs_conn_expire() gets called. It can be avoided by making sure that
> all packets from one connection are always processed on the same CPU and
> can be made harder to exploit by changing the connection timeouts to
> some custom values.
>
>   

Applied, thanks.

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