Re: [PATCH] mm: larger stack guard gap, between vmas

From: Andy Lutomirski
Date: Wed Jul 05 2017 - 20:20:21 EST

On Wed, Jul 5, 2017 at 4:50 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Wed, Jul 5, 2017 at 10:23 AM, Andy Lutomirski <luto@xxxxxxxxxx> 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...

Yes. I think it's ridiculous that you can change rlimits and then
exec a setuid thing. It's not so easy to fix, though. Maybe track,
per-task, inherited by clone and exec, what the rlimits were the last
time the process had privilege and reset to those limits when running
something setuid. But a better approach might be to have some sysctls
that say what the rlimits become when doing setuid.

We need per-user-ns sysctls for stuff like this, and we don't really
have them...