linux-kernel - Re: [PATCH 5/8] x86, microcode, intel: don't check extsig entry checksum

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Date:	Fri, 7 Nov 2014 18:49:17 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	Fenghua Yu <fenghua.yu@...el.com>, linux-kernel@...r.kernel.org,
	H Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH 5/8] x86, microcode, intel: don't check extsig entry
 checksum

On Fri, Oct 31, 2014 at 03:14:40PM -0200, Henrique de Moraes Holschuh wrote:
> > I don't know where you come up with this but the code you're removing
> > was added in 2013:
> > 
> > 	e666dfa273db ("x86/microcode_intel_lib.c: Early update ucode on Intel's CPU")
> 
> Was it?  As far as I know, it is "heavily based" (i.e. copied and adapted)
> on code from the regular microcode driver, and not rewritten from scrach.
> 
> If I am correct, this really is code from 2003, changed by 11 years worth of
> file renaming, file splits, and refactoring.
> 
> It took me about 30 minutes to track the origin of this code in the regular
> Intel microcode driver.  As far as I can tell, it was added to version 1.12
> of the original driver, somewhere between v2.6.0-test7 and v2.6.0-test8.
> 
> The Intel engineers credited for the original change are: Nitin Kamble
> <nitin.a.kamble@...el.com> and Jun Nakajima <jun.nakajima@...el.com>.
> (information from the changelog at the top of the driver source).
> 
> You can see one of the earliest versions of the code here:
> https://git.kernel.org/cgit/linux/kernel/git/history/history.git/tree/arch/i386/kernel/microcode.c?id=v2.6.0-test8
> 
> The extended table checksum gets calculated around line 323.  The possibly
> problematic logic is in line 341 and 345, and matches what the modern driver
> does.

Ok so e666dfa273db only adds code so I assumed it was new.

> I am not so sure.  Besides, it is entirely possible that the Intel SDM got
> updated in that area after the september 2003 code drop.  This is very hard
> to verify outside of Intel.   Or, maybe, the issue was detected internally
> at Intel, and it was decided that the issue should be shelved until the day
> extended microcode tables are actually required (which didn't happen yet).

Yeah, I wouldn't want to remove code from the loader without Intel
people saying something first.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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