Re: [PATCH 0/3] RFC: add init_allocations=1 boot option

[Alexander Potapenko <glider@google.com>]

On Tue, Apr 23, 2019 at 8:49 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, Apr 18, 2019 at 8:42 AM Alexander Potapenko <glider@google.com> wrote:
> >
> > Following the recent discussions here's another take at initializing
> > pages and heap objects with zeroes. This is needed to prevent possible
> > information leaks and make the control-flow bugs that depend on
> > uninitialized values more deterministic.
> >
> > The patchset introduces a new boot option, init_allocations, which
> > makes page allocator and SL[AOU]B initialize newly allocated memory.
> > init_allocations=0 doesn't (hopefully) add any overhead to the
> > allocation fast path (no noticeable slowdown on hackbench).
>
> I continue to prefer to have a way to both at-allocation
> initialization _and_ poison-on-free, so let's not redirect this to
> doing it only at free time.
There's a problem with poison-on-free.
By default SLUB stores the freelist pointer (not sure if it's the only
piece of data) in the memory chunk itself, so newly allocated memory
is dirty despite it has been zeroed out previously.
We could probably zero out the bits used by the allocator when
allocating the memory chunk, but it sounds hacky (yet saves us 8 bytes
on every allocation)
A cleaner solution would be to unconditionally relocate the free
pointer by short-circuiting
https://elixir.bootlin.com/linux/latest/source/mm/slub.c#L3531
Surprisingly, this doesn't work, because now the sizes of slub caches
are a bit off, and create_unique_id() in slub.c returns clashing sysfs
names.

> We're going to need both hooks when doing
> Memory Tagging, so let's just get it in place now. The security
> benefits on tagging, IMO, easily justify a 1-2% performance hit. And
> likely we'll see this improve with new hardware.
>
> > With only the the first of the proposed patches the slowdown numbers are:
> >  - 1.1% (stdev 0.2%) sys time slowdown building Linux kernel
> >  - 3.1% (stdev 0.3%) sys time slowdown on af_inet_loopback benchmark
> >  - 9.4% (stdev 0.5%) sys time slowdown on hackbench
> >
> > The second patch introduces a GFP flag that allows to disable
> > initialization for certain allocations. The third page is an example of
> > applying it to af_unix.c, which helps hackbench greatly.
> >
> > Slowdown numbers for the whole patchset are:
> >  - 1.8% (stdev 0.8%) on kernel build
> >  - 6.5% (stdev 0.2%) on af_inet_loopback
>
> Any idea why thes two went _up_?
>
> >  - 0.12% (stdev 0.6%) on hackbench
>
> Well that's quite an improvement. :)
>
> --
> Kees Cook



-- 
Alexander Potapenko
Software Engineer

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