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

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Date:	Fri, 17 Jan 2014 17:47:36 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"Ren, Qiaowei" <qiaowei.ren@...el.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	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 Fri, Jan 17, 2014 at 02:47:15PM +0000, Ren, Qiaowei wrote:
> > do_bounds
> > |->do_mpx_bt_fault
> >    |->allocate_bt
> >       |->sys_mmap_pgoff
> >          |->vm_mmap_pgoff
> >             |->do_mmap_pgoff
> >                |->mmap_region
> >                   |-> kmem_cache_zalloc(vm_area_cachep, GFP_KERNEL);
> > 
> Sorry for my late reply.
>
> Petkov, could you please detail the problem? Memory allocation can't
> be done in the eception handler? I guess it is like do_page_fault(),
> right?

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?

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

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

-- 
Regards/Gruss,
    Boris.

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