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

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Date:	Wed, 21 May 2014 15:13:16 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Borislav Petkov <bp@...en8.de>, 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>
Subject: Re: [RFC] x86_64: A real proposal for iret-less return to kernel

On Wed, May 21, 2014 at 3:01 PM, Luck, Tony <tony.luck@...el.com> wrote:
>> 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.
>
> We added code for recoverable errors to get out of the MC context
> before trying to lookup the page and send the signal.  Bottom of
> do_machine_check():
>
>         if (cfg->tolerant < 3) {
>                 if (no_way_out)
>                         mce_panic("Fatal machine check on current CPU", &m, msg);
>                 if (worst == MCE_AR_SEVERITY) {
>                         /* schedule action before return to userland */
>                         mce_save_info(m.addr, m.mcgstatus & MCG_STATUS_RIPV);
>                         set_thread_flag(TIF_MCE_NOTIFY);
>                 } else if (kill_it) {
>                         force_sig(SIGBUS, current);
>                 }
>         }
>
> That TIF_MCE_NOTIFY prevents the return to user mode, and we end up in mce_notify_process().

Why is this necessary?

If the MCE hit kernel code, then we're going to die anyway.  If the
MCE hit user code, then we should be in a completely sensible context
and we can just send the signal.

--Andy

>
> The "force_sig()" there is legacy code - and perhaps should just move off to mce_notify_process()
> too (need to save "worst" so it will know what to do).
>
> -Tony



-- 
Andy Lutomirski
AMA Capital Management, LLC
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