Re: bugs in page colouring code

Rik van Riel
2012-06-14T12:58:15Z

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?

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

> 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?

>> 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.

>> 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...

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