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

From: Ben Hutchings
Date: Wed Jul 05 2017 - 19:51:30 EST

On Wed, 2017-07-05 at 13:53 -0700, Andy Lutomirski wrote:
> 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:
> > > 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.
> >
> FWIW, even this last part may be problematic.ÂÂIt'll break anything
> that tries to allocate many small MAP_GROWSDOWN stacks on 32-
> bit.ÂÂHopefully nothing does this, but maybe Java does.

glibc (NPTL) does not. Java (at least Hotspot in OpenJDK 6,7, 8) does
not. LinuxThreads *does* and is used by uclibc. dietlibc *does*. I
would be surprised if either was used for applications with very many
threads, but then this issue has thrown up a lot of surprises.


Ben Hutchings
Man invented language to satisfy his deep need to complain. - Lily

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