drm/i915: Prevent a race during I915_GEM_MMAP ioctl with WC set

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.

v2:
- Refactor the compare function

Fixes: 1816f92 ("drm/i915: Support creation of unbound wc user mappings for objects")
Reported-by: Adam Zabrocki <[email protected]>
Suggested-by: Linus Torvalds <[email protected]>
Signed-off-by: Joonas Lahtinen <[email protected]>
Cc: <[email protected]> # v4.0+
Cc: Akash Goel <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Adam Zabrocki <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit 5c4604e)
Signed-off-by: Jani Nikula <[email protected]>

drivers/gpu/drm/i915/i915_gem.c

@@ -1824,6 +1824,16 @@ 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;
+}
+
 /**
  * i915_gem_mmap_ioctl - Maps the contents of an object, returning the address
  *			 it is mapped to.
@@ -1882,7 +1892,7 @@ i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
 			return -EINTR;
 		}
 		vma = find_vma(mm, addr);
-		if (vma)
+		if (vma && __vma_matches(vma, obj->base.filp, addr, args->size))
 			vma->vm_page_prot =
 				pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
 		else