linux-kernel - Re: [PATCH] efi: runtime-wrappers: run UEFI Runtime Services with interrupts enabled

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Date:	Mon, 8 Feb 2016 11:37:58 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Matt Fleming <matt@...eblueprint.co.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] efi: runtime-wrappers: run UEFI Runtime Services with
 interrupts enabled

On Feb 4, 2016 5:58 AM, "Ard Biesheuvel" <ard.biesheuvel@...aro.org> wrote:
>
> OK, since Sai has confirmed that Windows leaves interrupts enabled when
> calling the EFI variable store related runtime services, we should be able
> to do the same for Linux, or at least be slightly more confident that we
> won't have to back out this change later.

Could this use a mutex instead of a spinlock?

Can someone with a mixed mode setup read a variable in a loop and make
sure it doesn't crash and burn?  It should work fine, but explicit
testing would be nice.  (It's interesting mainly because doing a mixed
mode call with interrupts on can result in a non-IST CPL0 to CPL0
exception delivery, which won't result in a stack switch.  This could
easily trigger a stack overflow, logic bug, microcode bug, or
as-yet-unknown CPU "feature".

Hmm.  We should also audit the mixed mode entry code to make sure that
the high bits of RSP are explicitly clear before switching into compat
mode.  If I had to make a guess about how CPUs behave, I'd guess
pessimistically: Intel CPUs clear the high bits of RSP when switching
into long mode due to interrupt delivery, and AMD CPUs leave them set
just to mess with us.

Also, a WARN_ON(in_interrupt()) somewhere might be a good sanity check.

--Andy