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

From: Ben Hutchings
Date: Wed Jul 05 2017 - 15:33:27 EST

On Wed, 2017-07-05 at 10:23 -0700, Andy Lutomirski wrote:
> 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).ÂÂThen start over and think
> about it as hardening.ÂÂI would suggest the following approach:
> Â- The stack gap is one page, just like it's been for years.

Given that in the following points you say that something sounding like
a stack gap would be "64k or whatever", what does "the stack gap" mean
in this first point?

> Â- As a hardening feature, if the stack would expand within 64k or
> whatever of a non-MAP_FIXED mapping, refuse to expand it.ÂÂ(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.ÂÂIf you're like LibreOffice and do
> something daft and are thus exploitable, you're on your own.
> Â- 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.

Having tested patches along these lines, I think the above would avoid
the reported regressions.


> And that's all.ÂÂIt'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.ÂÂEveryone who's ever thought about it all all knows
> it and has known about it for years, but no one cared to fix it.
Ben Hutchings
Anthony's Law of Force: Don't force it, get a larger hammer.

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