[PATCH 6.0 076/157] drm/shmem-helper: Avoid vm_open error paths

Greg Kroah-Hartman
2022-12-12T13:41:29Z

From: Rob Clark <robdclark@chromium.org>

commit 09bf649a74573cb596e211418a4f8008f265c5a9 upstream.

vm_open() is not allowed to fail. Fortunately we are guaranteed that
the pages are already pinned, thanks to the initial mmap which is now
being cloned into a forked process, and only need to increment the
refcnt. So just increment it directly. Previously if a signal was
delivered at the wrong time to the forking process, the
mutex_lock_interruptible() could fail resulting in the pages_use_count
not being incremented.

Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
Cc: stable@vger.kernel.org
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221130185748.357410-3-robdclark@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/gpu/drm/drm_gem_shmem_helper.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -571,12 +571,20 @@ static void drm_gem_shmem_vm_open(struct
{
struct drm_gem_object *obj = vma->vm_private_data;
struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
- int ret;

WARN_ON(shmem->base.import_attach);

- ret = drm_gem_shmem_get_pages(shmem);
- WARN_ON_ONCE(ret != 0);
+ mutex_lock(&shmem->pages_lock);
+
+ /*
+ * We should have already pinned the pages when the buffer was first
+ * mmap'd, vm_open() just grabs an additional reference for the new
+ * mm the vma is getting copied into (ie. on fork()).
+ */
+ if (!WARN_ON_ONCE(!shmem->pages_use_count))
+ shmem->pages_use_count++;
+
+ mutex_unlock(&shmem->pages_lock);

drm_gem_vm_open(vma);
}