[Intel-gfx] [CI, v3, 1/2] drm/i915: Prevent a race during I915_GEM_MMAP ioctl with WC set

[Chris Wilson]

Quoting Guenter Roeck (2019-02-28 21:32:41)
> On Thu, Feb 28, 2019 at 11:12:49AM -0800, Guenter Roeck wrote:
> > Hi,
> > 
> > On Thu, Feb 07, 2019 at 10:54:53AM +0200, Joonas Lahtinen wrote:
> > > Make sure the underlying VMA in the process address space is the
> > > same as it was during vm_mmap to avoid applying WC to wrong VMA.
> > > 
> > > A more long-term solution would be to have vm_mmap_locked variant
> > > in linux/mmap.h for when caller wants to hold mmap_sem for an
> > > extended duration.
> > > 
> > 
> > It seems like we may have a regression due to this patch. I am still
> > debugging, but I have a question; please see below.
> > 
> > Thanks,
> > Guenter
> > 
> > > v2:
> > > - Refactor the compare function
> > > 
> > > Fixes: 1816f9236303 ("drm/i915: Support creation of unbound wc user mappings for objects")
> > > Reported-by: Adam Zabrocki <adamza at microsoft.com>
> > > Suggested-by: Linus Torvalds <torvalds at linux-foundation.org>
> > > Signed-off-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > > Cc: <stable at vger.kernel.org> # v4.0+
> > > Cc: Akash Goel <akash.goel at intel.com>
> > > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > > Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> > > Cc: Adam Zabrocki <adamza at microsoft.com>
> > > Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com> #v1
> > > ---
> > >  drivers/gpu/drm/i915/i915_gem.c | 12 +++++++++++-
> > >  1 file changed, 11 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > > index 05ce9176ac4e..52639f749908 100644
> > > --- a/drivers/gpu/drm/i915/i915_gem.c
> > > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > > @@ -1681,6 +1681,16 @@ i915_gem_sw_finish_ioctl(struct drm_device *dev, void *data,
> > >     return 0;
> > >  }
> > >  
> > > +static inline bool
> > > +__vma_matches(struct vm_area_struct *vma, struct file *filp,
> > > +         unsigned long addr, unsigned long size)
> > > +{
> > > +   if (vma->vm_file != filp)
> > > +           return false;
> > > +
> > > +   return vma->vm_start == addr && (vma->vm_end - vma->vm_start) == size;
> > 
> > Shouldn't this be:
> >       return vma->vm_start == addr && (vma->vm_end - vma->vm_start + 1) == size;
> > instead ?
> > 
> 
> Answer is no .. because vm_end points to the first byte after the
> end address.
> 
> The actual values are:
> 
> start=7d288f7f9000 end=7d288f84d000 end-start=54000 size=53400
> 
> meaning the size field passed in the ioctl is smaller than the total length
> of the area.
> 
> Question is now: Is the request/ioctl indeed invalid, ie does the requested
> size have to match the vma size ?

Yes. The vma is page-aligned, your request isn't. What happens next is
undefined behaviour, and almost certainly not what you expect -- you
can't access the last bits of your framebuffer.
-Chris