linux-kernel - Re: [PATCH 7/8] x86, microcode, intel: guard against misaligned microcode data

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Message-ID: <20141107234806.GG5180@pd.tnic>
Date:	Sat, 8 Nov 2014 00:48:06 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	linux-kernel@...r.kernel.org, H Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH 7/8] x86, microcode, intel: guard against misaligned
 microcode data

On Fri, Nov 07, 2014 at 08:54:25PM -0200, Henrique de Moraes Holschuh wrote:
> Well, it might well be lost in the noise given how slow a microcode update
> is.
> 
> What I mean is that the early microcode driver "won't waste cpu time moving
> data that is already aligned".

Yes, don't say "faster" but explain that we can save us the shuffling
of microcode data back and forth in the early loader if the microcode
update is 16-byte aligned in the initrd.

> That WARN_ONCE() should only trigger if a bug shows its ugly face.  If it is
> triggering in any other case, this patch is broken.
> 
> mc_intel->bits in the late driver really shouldn't depend at all on any
> alignment from initramfs or whatever: that path is supposed to run on
> microcode that came from the firmware loader, or the deprecated microcode

Right, the early path is apply_microcode_early() - I misread that part,
sorry.

> device, or which was memcpy'd from the initramfs to a kmalloc()'d area by
> save_microcode() in the early driver.  All of these will always be 16-byte
> aligned AFAIK.

Why will it always be 16-byte aligned? And if it is going to be always
16-byte aligned, why do we need the WARN_ONCE() at all?

> So, the early driver gets alignment code as it will usually have to deal
> with unaligned microcode data, and the late driver (which should never see
> unaligned microcode data) gets a WARN_ONCE to alert us if something broke in
> the kernel code.
> 
> Should I change the WARN_ONCE message to:
> 
> WARN_ONCE(... "kernel bug: microcode data incorrectly aligned") ?

No, you should either give a possible scenario where an unaligned
pointer can really happen or, alternatively, if you can prove that the
condition will never happen, drop the WARN_ONCE completely.

And please no improbable
maybe-it-could-once-in-a-blue-moon-when-the-stars-align-happen cases.

Oh, and even if the WARN_ONCE triggers, what can we do about it? If we
can't do anything about it, then we have a problem. If we can, then we
better do it and change the WARN_ONCE to an if-check which, if positive,
executes code that fixes the situation.

IOW, what I'm trying to say is, can we make the kmalloc'ed memory
16-byte aligned and if so, then copy the microcode data there and be
happy. And I think we can... :-)

> Neither do I.  But this has zero footprint on the resulting kernel, and
> might save someone 10 years from now from trying to debug initramfs data
> corruption.

... and someone might waste a lot of time 10 years from now trying
to fathom why the hell that thing was added, only to realize that
someone was being unreasonably overzealous. No, we don't add code for
hypothetical cases which are absolutely improbable.

-- 
Regards/Gruss,
    Boris.

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