Re: [PATCH v2 01/17] mm/gup: Fixup p*_access_permitted()

[Peter Zijlstra]

On Thu, Dec 14, 2017 at 09:54:50PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 14, 2017 at 12:44:58PM -0800, Dave Hansen wrote:
> > On 12/14/2017 06:37 AM, Peter Zijlstra wrote:
> > > I'm also looking at pte_access_permitted() in handle_pte_fault(); that
> > > looks very dodgy to me. How does that not result in endlessly CoW'ing
> > > the same page over and over when we have a PKEY disallowing write access
> > > on that page?
> > 
> > I'm not seeing the pte_access_permitted() in handle_pte_fault().  I
> > assume that's something you added in this series.
> 
> No, Dan did in 5c9d2d5c269c4.
> 
> > But, one of the ways that we keep pkeys from causing these kinds of
> > repeating loops when interacting with other things is this hunk in the
> > page fault code:
> > 
> > > static inline int
> > > access_error(unsigned long error_code, struct vm_area_struct *vma)
> > > {
> > ...
> > >         /*
> > >          * Read or write was blocked by protection keys.  This is
> > >          * always an unconditional error and can never result in
> > >          * a follow-up action to resolve the fault, like a COW.
> > >          */
> > >         if (error_code & PF_PK)
> > >                 return 1;
> > 
> > That short-circuits the page fault pretty quickly.  So, basically, the
> > rule is: if the hardware says you tripped over pkey permissions, you
> > die.  We don't try to do anything to the underlying page *before* saying
> > that you die.
> 
> That only works when you trip the fault from hardware. Not if you do a
> software fault using gup().
> 
> AFAIK __get_user_pages(FOLL_FORCE|FOLL_WRITE|FOLL_GET) will loop
> indefinitely on the case I described.

Note that my patch actually fixes this by making can_follow_write_pte()
not return NULL (we'll take the CoW fault irrespective of PKEYs) and
then on the second go-around, we'll find a writable PTE but return
-EFAULT from follow_page_mask() because of PKEY and terminate.

But as is, follow_page_mask() will return NULL because either !write or
PKEY, faultin_page()->handle_mm_fault() will see !write because of PKEY
go into the CoW path, we rety follow_page_mask() it will _still_ return
NULL because PKEY, again to the fault, again retry, again ....