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 12:10:54 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	"Ren, Qiaowei" <qiaowei.ren@...el.com>,
	"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>
Subject: Re: [PATCH 2/5] x86, mpx: hook #BR exception handler to allocate
 bound tables

On Fri, 17 Jan 2014 17:47:36 +0100
Borislav Petkov <bp@...en8.de> wrote:

> 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...
> 

Or do what #PF does. Check if the fault happened in the kernel and go
one path (probably follow what do_fault() does), otherwise if it is
userspace, it's ok to sleep or grab locks or whatever you want.

-- Steve
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