Re: [patch 2/2] mm: lockless get_user_pages

[Nick Piggin <npiggin@suse.de>]

On Sat, May 24, 2008 at 01:44:32AM +0200, Nick Piggin wrote:
> On Fri, May 23, 2008 at 01:31:12PM +0100, apw@shadowen.org wrote:
> > On Fri, May 23, 2008 at 04:27:33AM +0200, Nick Piggin wrote:
> 
>  
> > [...]
> > > > I did wonder if we could also check _PAGE_BIT_USER bit as by my reading
> > > > that would only ever be set on user pages, and by rejecting pages without
> > > > that set we could prevent any kernel pages being returned basically
> > > > for free.
> > > 
> > > I still do want the access_ok check to avoid any possible issues with
> > > kernel page table modifications... but checking for the user bit would
> > > be another good sanity check, good idea. 
> > 
> > Definatly not advocating removing any checks at all.  Just thinking the
> > addition is one more safety net should any one of the checks be flawed.
> > Security being a pig to prove at the best of times.
> 
> It isn't a bad idea at all. I'll see what I can do.

Oh, hmm, I was already checking the _PAGE_USER bit anyway ;)

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