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Re: 4.2-rc2: early boot memory corruption from FPU rework

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Actually we could statically associate a biggerbuffer based on the XCR0 features we support.  That would preclude dynamic enabling and really just adds complexity for no good reason.

On July 14, 2015 12:46:17 PM PDT, Dave Hansen <dave.hansen@linux.intel.com> wrote:
>On 05/05/2015 10:49 AM, Ingo Molnar wrote:
>> @@ -574,12 +573,10 @@ static void setup_init_fpu_buf(void)
>>  	on_boot_cpu = 0;
>>  
>>  	/*
>> -	 * Setup init_xstate_buf to represent the init state of
>> +	 * Setup init_xstate_ctx to represent the init state of
>>  	 * all the features managed by the xsave
>>  	 */
>> -	init_xstate_buf = alloc_bootmem_align(xstate_size,
>> -					      __alignof__(struct xsave_struct));
>> -	fx_finit(&init_xstate_buf->i387);
>> +	fx_finit(&init_xstate_ctx.i387);
>
>This is causing memory corruption in 4.2-rc2.
>
>We do not know the size of the 'init_xstate_buf' before we boot.  It's
>completely enumerated in CPUID leaves but it is not static by any
>means.
> This commit when applied (3e5e126774) tries to replace the dynamic
>allocation with a static one.  When we do the first 'xrstor' (in
>copy_xregs_to_kernel_booting()) it overruns init_fpstate and corrupts
>the next chunk of memory (which is xfeatures_mask in my case).
>
>I'm seeing this on a system with states not represented in
>XSTATE_RESERVE (XSTATE_ZMM_Hi256 / XSTATE_OPMASK / XSTATE_Hi16_ZMM).
>The systems affected are not widely available, but this is something
>that we absolutely do not want to see regress.
>
>This bug could also occur if a future CPU decided to change the amount
>of storage allocated for a given xstate feature (which would be
>architecturally OK).
>
>According to the commit:
>
>>     This removes the last bootmem allocation from the FPU init path,
>allowing
>>     it to be called earlier in the boot sequence.
>
>so we can't easily just revert this, although I'm not 100% that this is
>before bootmem is availalble.
>
>This patch works around the problem, btw:
>
>	https://www.sr71.net/~dave/intel/bloat-xsave-gunk-2.patch
>
>One curiosity here is that the bisect for this actually turned up the
>patch that disables 'XSAVES' support.  When we used 'XSAVES' and the
>"compacted" format, we managed to fit in to the buffer and things
>worked
>(accidentally).

-- 
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4.2-rc2: early boot memory corruption from FPU rework Dave Hansen <dave.hansen@linux.intel.com> - 2015-07-14 21:50 +0200
  Re: 4.2-rc2: early boot memory corruption from FPU rework "H. Peter Anvin" <hpa@zytor.com> - 2015-07-15 03:30 +0200
  Re: 4.2-rc2: early boot memory corruption from FPU rework Ingo Molnar <mingo@kernel.org> - 2015-07-15 13:10 +0200
    [REGRESSION] 4.2-rc2: early boot memory corruption from FPU rework Dave Hansen <dave.hansen@linux.intel.com> - 2015-07-16 02:40 +0200
      Re: [REGRESSION] 4.2-rc2: early boot memory corruption from FPU rework Linus Torvalds <torvalds@linux-foundation.org> - 2015-07-16 04:40 +0200
      Re: [REGRESSION] 4.2-rc2: early boot memory corruption from FPU rework Linus Torvalds <torvalds@linux-foundation.org> - 2015-07-16 05:00 +0200
      Re: [REGRESSION] 4.2-rc2: early boot memory corruption from FPU  rework Ingo Molnar <mingo@kernel.org> - 2015-07-17 09:50 +0200
        Re: [REGRESSION] 4.2-rc2: early boot memory corruption from FPU  rework Ingo Molnar <mingo@kernel.org> - 2015-07-17 11:00 +0200

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