From: Guillaume Nault <g.na...@alphalink.fr>
Date: Fri, 1 Mar 2013 16:02:02 +0100
> The sendmsg() syscall handler for PPPoL2TP doesn't decrease the socket
> reference counter after successful transmissions. Any successful
> sendmsg() call from userspace will then increase the reference counter
> forever, thus preventing the kernel's session and tunnel data from
> being freed later on.
>
> The problem only happens when writing directly on L2TP sockets.
> PPP sockets attached to L2TP are unaffected as the PPP subsystem
> uses pppol2tp_xmit() which symmetrically increase/decrease reference
> counters.
>
> This patch adds the missing call to sock_put() before returning from
> pppol2tp_sendmsg().
>
> Cc: <sta...@vger.kernel.org>
> Signed-off-by: Guillaume Nault <g.na...@alphalink.fr>
Looking at how this code works, it is such a terrible design. This
whole reference counting issue exists purely because
pppol2tp_sock_to_session() grabs the 'sk' reference.
In all but one case, it need not do this.
The socket system calls have an implicit reference to 'sk' via
socket->sk. If you can get into the system call and socket->sk
is non-NULL then 'sk' is NOT going anywhere.
And all of these system call handlers have this pattern:
session = pppol2tp_sock_to_session(sk);
...
sock_put(sk);
The only case where the reference count is really needed is that
sequence in pppol2tp_release().
Long term the right thing to do here is stop having this session
grabber function take the 'sk' reference. Then in pppol2tp_release
we'll grab a reference explicitly. At all the other call sites we
then blast aweay all of the sock_put(sk) paths.
Anyways, for now I'll apply this patch and queue it up for -stable,
thanks.
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