linux-kernel - [PATCH 56/93] drm: fix DRM_IOCTL_MODE

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Message-Id: <1379707768-1804-57-git-send-email-kamal@canonical.com>
Date:	Fri, 20 Sep 2013 13:08:51 -0700
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	David Herrmann <dh.herrmann@...il.com>,
	Dave Airlie <airlied@...hat.com>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 56/93] drm: fix DRM_IOCTL_MODE_GETFB handle-leak

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

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

From: David Herrmann <dh.herrmann@...il.com>

commit 101b96f32956ee99bf1468afaf572b88cda9f88b upstream.

DRM_IOCTL_MODE_GETFB is used to retrieve information about a given
framebuffer ID. It is a read-only helper and was thus declassified for
unprivileged access in:

  commit a14b1b42477c5ef089fcda88cbaae50d979eb8f9
  Author: Mandeep Singh Baines <mandeep.baines@...il.com>
  Date:   Fri Jan 20 12:11:16 2012 -0800

      drm: remove master fd restriction on mode setting getters

However, alongside width, height and stride information,
DRM_IOCTL_MODE_GETFB also passes back a handle to the underlying buffer of
the framebuffer. This handle allows users to mmap() it and read or write
into it. Obviously, this should be restricted to DRM-Master.

With the current setup, *any* process with access to /dev/dri/card0 (which
means any process with access to hardware-accelerated rendering) can
access the current screen framebuffer and modify it ad libitum.

For backwards-compatibility reasons we want to keep the
DRM_IOCTL_MODE_GETFB call unprivileged. Besides, it provides quite useful
information regarding screen setup. So we simply test whether the caller
is the current DRM-Master and if not, we return 0 as handle, which is
always invalid. A following DRM_IOCTL_GEM_CLOSE on this handle will fail
with EINVAL, but we accept this. Users shouldn't test for errors during
GEM_CLOSE, anyway. And it is still better as a failing MODE_GETFB call.

v2: add capable(CAP_SYS_ADMIN) check for compatibility with i-g-t

Signed-off-by: David Herrmann <dh.herrmann@...il.com>
Reviewed-by: Chris Wilson <chris@...is-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@...hat.com>
[ kamal: backport to 3.8 (context) ]
Signed-off-by: Kamal Mostafa <kamal@...onical.com>

Conflicts:
	drivers/gpu/drm/drm_crtc.c
---
 drivers/gpu/drm/drm_crtc.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index bcb2c0a..8663a83 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -2464,7 +2464,22 @@ int drm_mode_getfb(struct drm_device *dev,
 	r->depth = fb->depth;
 	r->bpp = fb->bits_per_pixel;
 	r->pitch = fb->pitches[0];
-	fb->funcs->create_handle(fb, file_priv, &r->handle);
+	if (fb->funcs->create_handle) {
+		if (file_priv->is_master || capable(CAP_SYS_ADMIN)) {
+			ret = fb->funcs->create_handle(fb, file_priv,
+						       &r->handle);
+		} else {
+			/* GET_FB() is an unprivileged ioctl so we must not
+			 * return a buffer-handle to non-master processes! For
+			 * backwards-compatibility reasons, we cannot make
+			 * GET_FB() privileged, so just return an invalid handle
+			 * for non-masters. */
+			r->handle = 0;
+			ret = 0;
+		}
+	} else {
+		ret = -ENODEV;
+	}

 out:
 	mutex_unlock(&dev->mode_config.mutex);
-- 
1.8.1.2

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