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

[Andy Lutomirski]

> On Jul 5, 2017, at 12:32 PM, Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote:
> 
>> 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).  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.
> 
> 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?

I mean one page, with semantics as close to previous (4.11) behavior as practical.

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

FWIW, even this last part may be problematic.  It'll break anything that tries to allocate many small MAP_GROWSDOWN stacks on 32-bit.  Hopefully nothing does this, but maybe Java does.

> Ben.
> 
>> 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.
> -- 
> Ben Hutchings
> Anthony's Law of Force: Don't force it, get a larger hammer.
>