Re: bugs in page colouring code

[Borislav Petkov <bp@amd64.org>]

On Thu, Jun 14, 2012 at 08:57:50AM -0400, Rik van Riel wrote:
> On 06/14/2012 06:36 AM, Borislav Petkov wrote:
> >On Wed, Jun 13, 2012 at 03:29:36PM -0400, Rik van Riel wrote:
> 
> >>For one, there are separate kernel boot arguments to control whether
> >>32 and 64 bit processes need to have their addresses aligned for
> >>page colouring.
> >>
> >>Do we really need that?
> >
> >Yes.
> 
> What do we need it for?
> 
> I can see wanting a big knob to disable page colouring
> globally for both 32 and 64 bit processes, but why do
> we need to control it separately?

Right, so for 32-bit we have 8 bits for ASLR and with our workaround, if
enabled on 32-bit, the randomness goes down to 5 bits. Thus, we wanted
to have it flexible and so the user can choose between randomization and
performance, depending on what he cares about.

> I am not too keen on x86 keeping a slightly changed private copy of
> arch_align_addr :)

x86 is special, you know that, right? :-)

> >Mind you, this is only enabled on AMD F15h - all other x86 simply can't
> >tweak it without code change.
> >
> >>Would it be a problem if I discarded that code, in order to get to one
> >>common cache colouring implementation?
> >
> >Sorry, but, we'd like to keep it in.
> 
> What is it used for?

>From <Documentation/kernel-parameters.txt>:

	align_va_addr=	[X86-64]
			Align virtual addresses by clearing slice [14:12] when
			allocating a VMA at process creation time. This option
			gives you up to 3% performance improvement on AMD F15h
			machines (where it is enabled by default) for a
			CPU-intensive style benchmark, and it can vary highly in
			a microbenchmark depending on workload and compiler.

			32: only for 32-bit processes
			64: only for 64-bit processes
			on: enable for both 32- and 64-bit processes
			off: disable for both 32- and 64-bit processes

> >>Secondly, MAP_FIXED never checks for page colouring alignment. I
> >>assume the cache aliasing on AMD Bulldozer is merely a performance
> >>issue, and we can simply ignore page colouring for MAP_FIXED?
> >
> >Right, AFAICR, MAP_FIXED is not generally used for shared libs (correct
> >me if I'm wrong here, my memory is very fuzzy about it) and since we see
> >the perf issue with shared libs, this was fine.
> 
> Try stracing /bin/mount one of these days. A whole bunch
> of libraries are mapped with MAP_FIXED :)
> 
> However, I expect that on x86 many applications expect
> MAP_FIXED to just work, and enforcing that would be
> more trouble than it's worth.

Right, but if MAP_FIXED mappings succeed, then all processes sharing
that mapping will have it at the same virtual address, correct? And
if so, then we don't have the aliasing issue either so MAP_FIXED is a
don't-care from that perspective.

> >>That will be easy to get right in an architecture-independent
> >>implementation.
> >>
> >>
> >>A third issue is this:
> >>
> >>         if (!(current->flags&  PF_RANDOMIZE))
> >>                 return addr;
> >>
> >>Do we really want to skip page colouring merely because the
> >>application does not have PF_RANDOMIZE set?  What is this
> >>conditional supposed to do?
> >
> >Linus said that without this we are probably breaking old userspace
> >which can't stomach ASLR so we had to respect such userspace which
> >clears that flag.
> 
> I wonder if that is true, since those userspace programs
> probably run fine on ARM, MIPS and other architectures...

Well, I'm too young to know that :) Reportedly, those were some obscure
old binaries and we added the PF_RANDOMIZE check out of caution, so as
to not break them, if at all.

-- 
Regards/Gruss,
Boris.

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