[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 21:57:03)
> On Thu, Feb 28, 2019 at 01:32:41PM -0800, Guenter Roeck wrote:
> > 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 ? This used to work until this patch was
> > applied, and the change causes our test code to fail (and possibly minigbm,
> > which is used by the test code). That doesn't mean that our code is correct
> > (I see some related local changes in our version of minigbm), but it is
> > annoying, and I am being asked to revert this patch as regression
> > from our kernel releases.
> > 
> 
> In i915_gem_create():
> 
>         size = roundup(size, PAGE_SIZE);
>         if (size == 0)
>                 return -EINVAL;
> 
> This suggests to me that the requested size can be smaller than the

Not really, the ABI has never handled less than page-sized requests.
It's a mistake from the very beginning that it was not rejected as being
the invalid size it was.

> allocated size, which in turn suggests that the check
>         (vma->vm_end - vma->vm_start) == size;
> is wrong. Either it should be
>         (vma->vm_end - vma->vm_start) >= size;
> or possibly
>         (vma->vm_end - vma->vm_start) == roundup(size, PAGE_SIZE);
> 
> Any comments/feedback/thoughts ?

It's a violation of mmap(2).

Is probably what we will have to do if you ring the regression bell loud
enough, and do not see the folly of your ways. :-p
-Chris