[PATCH v2] drm/vmwgfx: Work around VMW_ALLOC_DMABUF

[Meng Tang <tangmeng@uniontech.com>]

A privilege escalation vulnerability was found in vmwgfx driver
in drivers/gpu/drm/vmwgfx/vmwgfx_drv.c in GPU component of Linux
kernel with device file '/dev/dri/renderD128 (or Dxxx)'. This flaw
allows a local attacker with a user account on the system to gain
privilege, causing a denial of service(DoS).

This vulnerability can be quickly verified by the following code
logic:
...
dri_fd = open("/dev/dri/renderD128", O_RDWR);
ret = ioctl(dri_fd, 0xC0186441, &arg);
if (ret == 0) {
	printf("[*] VMW_ALLOC_DMABUF Success!\n");
}
...

Submit this commit to fix it.

Signed-off-by: Meng Tang <tangmeng@uniontech.com>
---
 drivers/gpu/drm/vmwgfx/vmwgfx_drv.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
index bd02cb0e6837..115787697957 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
@@ -1263,6 +1263,10 @@ static long vmw_generic_ioctl(struct file *filp, unsigned int cmd,
 			if (!drm_is_current_master(file_priv) &&
 			    !capable(CAP_SYS_ADMIN))
 				return -EACCES;
+		} else if (nr == DRM_COMMAND_BASE + DRM_VMW_ALLOC_DMABUF) {
+			if (!drm_is_current_master(file_priv) &&
+			    !capable(CAP_SYS_ADMIN))
+				return -EPERM;
 		}
 
 		if (unlikely(ioctl->cmd != cmd))
-- 
2.20.1