The patch below does not apply to the 4.2-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From ed7d78b2da32198ca4c70172e3b63c6b3e2c570b Mon Sep 17 00:00:00 2001
From: Thomas Hellstrom <[email protected]>
Date: Mon, 12 Oct 2015 01:49:39 -0700
Subject: [PATCH] drm/vmwgfx: Fix kernel NULL pointer dereference on older
 hardware

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

Cc: <[email protected]>
Signed-off-by: Thomas Hellstrom <[email protected]>
Reviewed-by: Sinclair Yeh <[email protected]>
Reviewed-by: Brian Paul <[email protected]>

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c 
b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
index 64b50409fa07..03f63c749c02 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
@@ -657,7 +657,8 @@ 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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