linux-kernel - Re: [RFC PATCH] x86, entry: Switch stacks on a paranoid entry from userspace

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Date:	Tue, 11 Nov 2014 22:36:29 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Tony Luck <tony.luck@...el.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [RFC PATCH] x86, entry: Switch stacks on a paranoid entry from
 userspace

A very big hmmm...

On Tue, Nov 11, 2014 at 12:56:52PM -0800, Andy Lutomirski wrote:
> This causes all non-NMI kernel entries from userspace to run on the
> normal kernel stack.

So one of the reasons #MC has its own stack is because we need a
known-good stack in such situations. What if the normal kernel stack is
corrupted too due to a #MC?

> This means that machine check recovery can happen in non-atomic
> context.  It also obviates the need for the paranoid_userspace path.
> 
> Borislav has referred to this idea as the tail wagging the dog.  I
> think that's okay -- the dog was pretty ugly.

And I still am not sure about this: so the #MC handler makes implicit
assumptions that while it is running nothing is going to interrupt it
and it can access MCA MSRs. If you switch to process context, another
#MC will preempt it and overwrite MCA MSRs. Which is a no-no.

So unless I'm missing something - and I probably am - I don't think
we can run #MC handler in process context. #MC is the highest prio
abort-type exception along with processor reset for a reason.

Thanks.

-- 
Regards/Gruss,
    Boris.

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