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Re: [PATCH] x86/fpu: move FPU state into separate cache

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On Wed, Mar 29, 2017 at 1:45 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 03/29/17 13:39, Kees Cook wrote:
>> This removes ARCH_WANTS_DYNAMIC_TASK_STRUCT from x86, leaving only s390
>> still defining this config.
>>
>> In order to support future structure layout randomization of the
>> task_struct, none of the structure fields are allowed to have a specific
>> position or dynamic size. To enable randomization of task_struct on
>> x86, the FPU state must be moved to its own dynamically sized cache,
>> and dereferenced from the task_struct.
>>
>> This change is nearly identical to what was done in grsecurity to support
>> structure layout randomization. Hopefully I found all the needed changes.
>> This passes allyesconfig, and boot tests.
>
> Is this really what we want to happen?  It seems much more sane to
> simply make them adjacent; they don't need to be part of the same
> structure (in practice, there are three objects: thread_info,
> task_struct, and the FPU state.)

They're adjacent already, which poses a problem for the struct layout
randomization plugin, since adjacency may no longer be true (after
layout randomization). This adjacency (or not) isn't really the
problem: it's that FPU state size is only known at runtime. Another
solution would be to have FPU state be a fixed size...

-Kees

-- 
Kees Cook
Pixel Security

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[PATCH] x86/fpu: move FPU state into separate cache Kees Cook <keescook@chromium.org> - 2017-03-29 22:50 +0200
  Re: [PATCH] x86/fpu: move FPU state into separate cache "H. Peter Anvin" <hpa@zytor.com> - 2017-03-29 23:10 +0200
    Re: [PATCH] x86/fpu: move FPU state into separate cache Kees Cook <keescook@chromium.org> - 2017-03-29 23:20 +0200
      Re: [PATCH] x86/fpu: move FPU state into separate cache Dave Hansen <dave.hansen@linux.intel.com> - 2017-03-29 23:20 +0200
      Re: [PATCH] x86/fpu: move FPU state into separate cache Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-29 23:40 +0200
        Re: [PATCH] x86/fpu: move FPU state into separate cache Andy Lutomirski <luto@amacapital.net> - 2017-03-29 23:40 +0200
          Re: [PATCH] x86/fpu: move FPU state into separate cache Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-29 23:50 +0200
            Re: [PATCH] x86/fpu: move FPU state into separate cache hpa@zytor.com - 2017-03-30 00:50 +0200
              Re: [PATCH] x86/fpu: move FPU state into separate cache Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-30 02:00 +0200
                Re: [PATCH] x86/fpu: move FPU state into separate cache Kees Cook <keescook@chromium.org> - 2017-03-30 04:00 +0200
        Re: [PATCH] x86/fpu: move FPU state into separate cache Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-29 23:40 +0200

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