linux-kernel - Re: [RFC] x86_64: A real proposal for iret-less return to kernel

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Message-ID: <CALCETrVPVcyTd8FYV90DPhKvGW1GhHxkUPObpXK69AhwwqLi1w@mail.gmail.com>
Date:	Wed, 21 May 2014 14:35:59 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC] x86_64: A real proposal for iret-less return to kernel

On Wed, May 21, 2014 at 2:25 PM, Jiri Kosina <jkosina@...e.cz> wrote:
> On Wed, 21 May 2014, Borislav Petkov wrote:
>
>> > ISTM the do_machine_check code ought to consider any kill-worthy MCE
>> > from kernel space to be non-recoverable, but I want to keep the scope
>> > of these patches under control.
>>
>> MCA has a bit called RIPV which, if set, signals that RIP is valid and
>> it is safe to return provided we've taken proper care of handling even
>> non-correctable errors (memory poisoning, etc).
>
> Yeah, but it tries to send SIGBUS from MCE context. And if MCE triggered
> at the time the CPU was already holding sighand->siglock for that
> particular task, it'll deadlock against itself.
>

If RIPV is set but we interrupted *kernel* code, SIGBUS doesn't seem
like the right solution anyway.

Are there any machine check exceptions for which it makes sense to
continue right where we left off without a signal?  Is CMIC such a
beast?  Can CMIC be delivered when interrupts are off?

--Andy
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