Re: [PATCH v2 3/3] pppoatm: protect against freeing of vcc

On Tue, Nov 27, 2012 at 05:16:32PM +0000, David Woodhouse wrote:

Krzysztof, you've fixed a bunch of races... but I think there's one
still left.

An ATM driver will often have code like this, which gets called from
arbitrary contexts:
        if (vcc->pop)
                vcc->pop(vcc, skb);

Now, what happens if pppoatm_send(vcc, NULL) happens after the address
of vcc->pop (currently pppoatm_pop) has been loaded, but before the
function is actually called?

You tear down all the setup and set vcc->user_back to NULL. And then
pppoatm_pop() gets called. And promptly crashes because pvcc is NULL.

A lot of these problems exist for br2684 too, and in prodding at it a
little I can consistently crash the system by sending a flood of
outbound packets while I kill the br2684ctl program. I end up in
br2684_pop() with vcc->user_back == NULL. In looking to see how you'd
fixed that in pppoatm, I realised that you haven't... :)

Yes, I missed that one - it's even worse, I introduced that bug 
in "[PATCH 1/7] atm: detach protocol before closing vcc". Before that
patch that scenario shouldn't happen because vcc was closed before
calling pppoatm_send(vcc, NULL) - the driver should provide appropriate
synchronization.

I think that we should just drop that patch. With later changes it's not
necessary - the pppoatm_send() can be safely called while closing vcc.

Krzysiek