Re: [PATCH 2/5] x86, mpx: hook #BR exception handler to allocatebound tables

From: H. Peter Anvin
Date: Fri Jan 17 2014 - 11:56:39 EST

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.


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