linux-kernel - [PATCH] drm/prime: Fix use after free in mmap with drm_gem_ttm

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Date:   Thu, 30 Sep 2021 09:00:07 +1000
From:   Anand K Mistry <amistry@...gle.com>
To:     dri-devel@...ts.freedesktop.org
Cc:     nartemiev@...gle.com, Anand K Mistry <amistry@...gle.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] drm/prime: Fix use after free in mmap with drm_gem_ttm_mmap

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@...gle.com>
---

 drivers/gpu/drm/drm_prime.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 2a54f86856af..e1854fd24bb0 100644
--- 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_object *obj, struct vm_area_struct *vma)
 	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;
 	}
 
-- 
2.33.0.800.g4c38ced690-goog