Re: [patch] wan/x25_asy: integer overflow in x25_asy_change_mtu()

On Thu, Jul 17, 2014 at 08:45:58AM +0000, David Laight wrote:

From: Dan Carpenter

quoted

If "newmtu * 2 + 4" is too large then it can cause an integer overflow
leading to memory corruption.  Btw, "newmtu" is not allowed to be a
negative number because of the check in dev_set_mtu(), so that's ok.

This still allows large numbers to be used to allocate almost all of
kernel memory - causing massive issues elsewhere.

I'd have thought a 'sanity' limit on the mtu would be more appropriate.
I've no idea which mtu is being changed here, and I can't even remember
the x.25 protocol well enough if it is an x.25 level 3 limit.
But I suspect that a 'sanity' bound to 1MB won't cause any grief.

I agree that a sanity check is probably better but I don't think kmalloc
can allocate more than 128k (or something.  It's arch dependent as
well).  So using 1MB is almost no different from my original patch.

What's a better, smaller limit?

regards,
dan carpenter