Subject: [PATCH 2/3] ARM: cacheflush: don't bother rounding to nearest vma

From: [email-masked] (Catalin Marinas)
Date: Wed, 27 Mar 2013 13:08:36 +0000

On Wed, Mar 27, 2013 at 12:43:52PM +0000, Will Deacon wrote:
> On Wed, Mar 27, 2013 at 12:21:59PM +0000, Catalin Marinas wrote:
> > On Wed, Mar 27, 2013 at 12:15:12PM +0000, Will Deacon wrote:
> > > On Wed, Mar 27, 2013 at 11:09:38AM +0000, Catalin Marinas wrote:
> > > > While this would work, it introduces a possibility of DoS where an
> > > > application passes bigger valid range (kernel linear mapping) and the
> > > > kernel code would not be preempted (CONFIG_PREEMPT disabled). IIRC,
> > > > that's why Russell reject such patch a while back.
> > > 
> > > Hmm, I'm not sure I buy that argument. Firstly, you can't just pass a kernel
> > > linear mapping address -- we'll fault straight away because it's not a
> > > userspace address.
> > 
> > Fault where?
> 
> I was expecting something like an access_ok check, but you're right, we
> don't have one (and I guess that's not strictly needed given that flushing
> isn't destructive). I still find it a bit scary that we allow userspace to
> pass kernel addresses through though -- especially if there's something like
> a DMA or CPU suspend operation running on another core.

Currently we don't allow kernel addresses since we require a valid vma.
If we drop the vma search, we should add an access_ok.

> > > Secondly, what's to stop an application from mmaping a large area into
> > > a single VMA and giving rise to the same situation? Finally,
> > > interrupts are enabled during this operation, so I don't understand
> > > how you can trigger a DoS, irrespective of the preempt configuration.
> > 
> > You can prevent context switching to other threads. But I agree, with a
> > large vma (which is already faulted in), you can get similar behaviour.
> 
> So the easy fix is to split the range up into chunks and call cond_resched
> after processing each one.

This would work.

-- 
Catalin