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

[Chris Wilson]

Quoting Joonas Lahtinen (2019-01-07 08:56:55)
> 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.
> 
> 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>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 062c8395557c..f1d594a53978 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1680,6 +1680,15 @@ i915_gem_sw_finish_ioctl(struct drm_device *dev, void *data,
>         return 0;
>  }
>  
> +static inline bool
> +match_gem_vma(struct vm_area_struct *vma, struct file *filp,
> +             unsigned long addr, unsigned long size)

With the exception of there isn't anything gem specific here,

> +{
> +       return vma && vma->vm_file == filp &&
> +              vma->vm_start == addr &&
> +              (vma->vm_end - vma->vm_start) == size;

and we can break this up into separate ifs with a forgiving compiler,

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

I still couldn't see an easy way of passing pgprot bits into do_mmap to
avoid the race entirely.
-Chris