drm/exynos: call find_vma with the mmap_sem held

Performing vma lookups without taking the mm->mmap_sem is asking for
trouble.  While doing the search, the vma in question can be modified or
even removed before returning to the caller.  Take the lock (exclusively)
in order to avoid races while iterating through the vmacache and/or
rbtree.

Signed-off-by: Jonathan Gonzalez V <[email protected]>
Signed-off-by: Davidlohr Bueso <[email protected]>
Cc: Inki Dae <[email protected]>
Cc: Joonyoung Shim <[email protected]>
Cc: David Airlie <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

drivers/gpu/drm/exynos/exynos_drm_g2d.c

@@ -467,21 +467,25 @@ static dma_addr_t *g2d_userptr_get_dma_addr(struct drm_device *drm_dev,
 		goto err_free;
 	}
 
+	down_read(&current->mm->mmap_sem);
 	vma = find_vma(current->mm, userptr);
 	if (!vma) {
+		up_read(&current->mm->mmap_sem);
 		DRM_ERROR("failed to get vm region.\n");
 		ret = -EFAULT;
 		goto err_free_pages;
 	}
 
 	if (vma->vm_end < userptr + size) {
+		up_read(&current->mm->mmap_sem);
 		DRM_ERROR("vma is too small.\n");
 		ret = -EFAULT;
 		goto err_free_pages;
 	}
 
 	g2d_userptr->vma = exynos_gem_get_vma(vma);
 	if (!g2d_userptr->vma) {
+		up_read(&current->mm->mmap_sem);
 		DRM_ERROR("failed to copy vma.\n");
 		ret = -ENOMEM;
 		goto err_free_pages;
@@ -492,10 +496,12 @@ static dma_addr_t *g2d_userptr_get_dma_addr(struct drm_device *drm_dev,
 	ret = exynos_gem_get_pages_from_userptr(start & PAGE_MASK,
 						npages, pages, vma);
 	if (ret < 0) {
+		up_read(&current->mm->mmap_sem);
 		DRM_ERROR("failed to get user pages from userptr.\n");
 		goto err_put_vma;
 	}
 
+	up_read(&current->mm->mmap_sem);
 	g2d_userptr->pages = pages;
 
 	sgt = kzalloc(sizeof(*sgt), GFP_KERNEL);