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

[Joonas Lahtinen]

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)
+{
+	return vma && vma->vm_file == filp &&
+	       vma->vm_start == addr &&
+	       (vma->vm_end - vma->vm_start) == size;
+}
+
 /**
  * i915_gem_mmap_ioctl - Maps the contents of an object, returning the address
  *			 it is mapped to.
@@ -1738,7 +1747,7 @@ i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
 			return -EINTR;
 		}
 		vma = find_vma(mm, addr);
-		if (vma)
+		if (match_gem_vma(vma, obj->base.filp, addr, args->size))
 			vma->vm_page_prot =
 				pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
 		else
-- 
2.17.2