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

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Date:	Wed, 21 May 2014 23:59:21 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Borislav Petkov <bp@...en8.de>
cc:	Andy Lutomirski <luto@...capital.net>,
	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, 21 May 2014, Borislav Petkov wrote:

> > 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)?
> 
> Let me quote Jiri:
> 
> (1) task sends signal to itself
> (2) it acquires sighand->siglock so that it's able to queue the signal
> (3) MCE triggers
> (4) it tries to send a signal to the same task
> (5) it tries to acquire sighand->siglock and loops forever

Ah, alright, but due to what mce_severity() does, this can't happen, 
because if the current CPU is in the kernel (which is obviously implied by 
holding a spinlock), it never proceeds sending the signal, becase 
no_way_out gets set and mce_panic() invoked.

-- 
Jiri Kosina
SUSE Labs
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