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    SubjectRe: bugs in page colouring code
    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...

    --
    All rights reversed


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