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

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Date:	Tue, 14 Jul 2015 18:25:32 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
CC:	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: 4.2-rc2: early boot memory corruption from FPU rework

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@...ux.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).

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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