[PATCH 5.15 247/279] drm/prime: Fix use after free in mmap with drm_gem_ttm_mmap

[Greg Kroah-Hartman]

From: Anand K Mistry <amistry@xxxxxxxxxx>

commit 8244a3bc27b3efd057da154b8d7e414670d5044f upstream.

drm_gem_ttm_mmap() drops a reference to the gem object on success. If
the gem object's refcount == 1 on entry to drm_gem_prime_mmap(), that
drop will free the gem object, and the subsequent drm_gem_object_get()
will be a UAF. Fix by grabbing a reference before calling the mmap
helper.

This issue was forseen when the reference dropping was adding in
commit 9786b65bc61ac ("drm/ttm: fix mmap refcounting"):
  "For that to work properly the drm_gem_object_get() call in
  drm_gem_ttm_mmap() must be moved so it happens before calling
  obj->funcs->mmap(), otherwise the gem refcount would go down
  to zero."

Signed-off-by: Anand K Mistry <amistry@xxxxxxxxxx>
Fixes: 9786b65bc61a ("drm/ttm: fix mmap refcounting")
Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx> # v5.5+
Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20210930085932.1.I8043d61cc238e0168e2f4ca5f4783223434aa587@changeid
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/drm_prime.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -719,11 +719,13 @@ int drm_gem_prime_mmap(struct drm_gem_ob
 	if (obj->funcs && obj->funcs->mmap) {
 		vma->vm_ops = obj->funcs->vm_ops;
 
+		drm_gem_object_get(obj);
 		ret = obj->funcs->mmap(obj, vma);
-		if (ret)
+		if (ret) {
+			drm_gem_object_put(obj);
 			return ret;
+		}
 		vma->vm_private_data = obj;
-		drm_gem_object_get(obj);
 		return 0;
 	}