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Date: Wed, 21 May 2014 23:55:47 +0200
From: Borislav Petkov <bp@alien8.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Jiri Kosina <jkosina@suse.cz>, Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Andi Kleen <andi@firstfloor.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@kernel.org>,
        Tony Luck <tony.luck@intel.com>
Subject: Re: [RFC] x86_64: A real proposal for iret-less return to kernel
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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.
--