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

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Message-ID: <CALCETrUr75gGj3E5cSZdGRpX-wd6pDzDruMGave+GbnpWaptRA@mail.gmail.com>
Date:	Wed, 21 May 2014 14:52:55 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	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>,
	Tony Luck <tony.luck@...el.com>
Subject: Re: [RFC] x86_64: A real proposal for iret-less return to kernel

On Wed, May 21, 2014 at 2:48 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, May 21, 2014 at 02:35:59PM -0700, Andy Lutomirski wrote:
>> 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?
>
> I think you mean CMCI and that's not even reported with a MCE exception
> - there's a separate APIC interrupt for that.
>
> I think this signal thing is for killing processes which have poisoned
> memory but this memory can contained within that process and the
> physical page frame can be poisoned so that it doesn't get used ever
> again. In any case, this is an example for an uncorrectable error which
> needs action from us but doesn't necessarily have to kill the whole
> machine.
>
> This is supposed to be more graceful instead of consuming the corrupted
> data and sending it out to disk.
>
> But sending signals from #MC context is definitely a bad idea. I think
> we had addressed this with irq_work at some point but my memory is very
> hazy.

Why is it a problem if user_mode_vm(regs)?  Conversely, why is sending
a signal a remotely reasonable thing to do if !user_mode_vm(regs)?

--Andy
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