On Thu, Jul 6, 2017 at 9:48 PM, Andy Lutomirski <l...@kernel.org> wrote: > How about a much simpler solution: don't read rlimit at all in > copy_strings(), let alone try to enforce it. Instead, just before the > point of no return, check how much stack space is already used and, if > it's more than an appropriate threshold (e.g. 1/4 of the rlimit), > abort. Sure, this adds overhead if we're going to abort, but does > that really matter?
We should avoid using up tons of memory and then failing. Better to cap it as we use it. Plumbing a sane value into this shouldn't be hard at all. Just making this a hardcoded 2MB seems sane (1/4 of 8MB). > I don't see why using rlimit for layout control makes any sense > whatsoever. Is there some historical reason we need that? As far as > I can see (on insufficient inspection) is that the kernel is trying to > guarantee that, if we have so much arg crap that our remaining stack > is less than 128k, then we don't exceed our limit by a little bit. IIUC, this is a big deal on 32-bit. Unlimited stack triggers top-down mmap instead of bottom-up. I mean, I'd be delighted to get rid of this, but I thought it was relied on by userspace. -Kees -- Kees Cook Pixel Security
