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

[Guenter Roeck <linux@roeck-us.net>]

On Thu, Feb 28, 2019 at 10:01:45PM +0000, Chris Wilson wrote:
> 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@microsoft.com>
> > > > > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> > > > > Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > > > > Cc: <stable@vger.kernel.org> # v4.0+
> > > > > Cc: Akash Goel <akash.goel@intel.com>
> > > > > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > > > > Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> > > > > Cc: Adam Zabrocki <adamza@microsoft.com>
> > > > > Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > > > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@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

I won't ring any bells; I don't play such games. I'll make a local change
in our kernel to fix the problem, quoting your statement that less than
page-sized requests were never supposed to be supported, and add a note
that we'll have to handle this with a local patch going forward.

Guenter
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