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... -- All rights reversed -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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