linux-kernel - [PATCH 3.16.y-ckt 087/104] drm/vmwgfx: Fix kernel NULL pointer dereference on older hardware

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Message-Id: <1445867006-18048-88-git-send-email-luis.henriques@canonical.com>
Date:	Mon, 26 Oct 2015 13:43:09 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Thomas Hellstrom <thellstrom@...are.com>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 087/104] drm/vmwgfx: Fix kernel NULL pointer dereference on older hardware

3.16.7-ckt19 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Thomas Hellstrom <thellstrom@...are.com>

commit ed7d78b2da32198ca4c70172e3b63c6b3e2c570b upstream.

The commit "drm/vmwgfx: Fix up user_dmabuf refcounting", while fixing a
kernel crash introduced a NULL pointer dereference on older hardware.
Fix this.

Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
Reviewed-by: Sinclair Yeh <syeh@...are.com>
Reviewed-by: Brian Paul <brianp@...are.com>
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 drivers/gpu/drm/vmwgfx/vmwgfx_surface.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
index 9e190db6cc37..17a4107639b2 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
@@ -653,7 +653,8 @@ static void vmw_user_surface_base_release(struct ttm_base_object **p_base)
 	struct vmw_resource *res = &user_srf->srf.res;
 
 	*p_base = NULL;
-	ttm_base_object_unref(&user_srf->backup_base);
+	if (user_srf->backup_base)
+		ttm_base_object_unref(&user_srf->backup_base);
 	vmw_resource_unreference(&res);
 }
 
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