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

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Message-ID: <20140521215547.GJ25130@pd.tnic>
Date:	Wed, 21 May 2014 23:55:47 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
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 02:52:55PM -0700, Andy Lutomirski 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

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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