On Wed, Jul 5, 2017 at 10:23 AM, Andy Lutomirski <l...@kernel.org> wrote: > Right. But I think the approach that we're all taking here is a bit > nutty. We all realize that this issue is a longstanding *GCC* bug > [1], but we're acting like it's a Big Deal (tm) kernel bug that Must > Be Fixed (tm) and therefore is allowed to break ABI. My security hat > is normally pretty hard-line, but I think it may be time to call BS. > > Imagine if Kees had sent some symlink hardening patch that was > default-on and broke a stock distro. Or if I had sent a vsyscall > hardening patch that broke real code. It would get reverted right > away, probably along with a diatribe about how we should have known > better. I think this stack gap stuff is the same thing. It's not a > security fix -- it's a hardening patch. > > Looking at it that way, I think a new inherited-on-exec flag is nucking futs. > > I'm starting to think that the right approach is to mostly revert all > this stuff (the execve fixes are fine). Then start over and think > about it as hardening. I would suggest the following approach: > > - The stack gap is one page, just like it's been for years. > - As a hardening feature, if the stack would expand within 64k or > whatever of a non-MAP_FIXED mapping, refuse to expand it. (This might > have to be a non-hinted mapping, not just a non-MAP_FIXED mapping.) > The idea being that, if you deliberately place a mapping under the > stack, you know what you're doing. If you're like LibreOffice and do > something daft and are thus exploitable, you're on your own. > - As a hardening measure, don't let mmap without MAP_FIXED position > something within 64k or whatever of the bottom of the stack unless a > MAP_FIXED mapping is between them. > > And that's all. It's not like a 64k gap actually fixes these bugs for > real -- it just makes them harder to exploit. > > [1] The code that GCC generates for char buf[bug number] and alloca() > is flat-out wrong. Everyone who's ever thought about it all all knows > it and has known about it for years, but no one cared to fix it.
As part of that should we put restrictions on the environment of set*id exec too? Part of the risks demonstrated by Qualys was that allowing a privilege-elevating binary to inherit rlimits can have lead to the nasty memory layout side-effects. That would fall into the "hardening" bucket as well. And if it turns out there is some set*id binary out there that can't run with "only", e.g., 128MB of stack, we can make it configurable... -Kees -- Kees Cook Pixel Security
