linux-kernel - Re: [PATCH 2/5] x86, mpx: hook #BR exception handler to allocate bound tables

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Message-ID: <52D95F77.4030908@zytor.com>
Date:	Fri, 17 Jan 2014 08:51:03 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>,
	"Ren, Qiaowei" <qiaowei.ren@...el.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/5] x86, mpx: hook #BR exception handler to allocate
 bound tables

On 01/17/2014 08:47 AM, Borislav Petkov wrote:
> 
> Right, so Steve and I played a couple of scenarios in IRC with this. So
> #BR is comparable with #PF, AFAICT, and as expected we don't take any
> locks when handling page faults in kernel space as we might deadlock.
> 
> Now, what happens if a thread is sleeping on some lock down that
> GFP_KERNEL allocation path and another thread gets a #BR and goes that
> same mmap_pgoff path and tries to grab that same lock?

It goes to sleep.  Same as if we take a page fault and have to page
something in.

> Also, what happens if you take a #BR in NMI context, say the NMI
> handler?

You should never, ever do that.  We should never take a #BR in the
kernel, full stop -- if we do it is panic time.

> All I'm trying to say is, it might not be such a good idea to sleep in a
> fault handler...

A fault handler from user space is really nothing other than a different
kind of system call.  It is nothing magic about it.

	-hpa

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