Messages in this thread | | | Date | Thu, 14 Jun 2012 08:57:50 -0400 | From | Rik van Riel <> | Subject | Re: bugs in page colouring code |
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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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