drm: fix use-after-free read in drm_mode_create_lease_ioctl()

commit 12d43deb1ee639d01a2a8d2a7a4cc8ad31224475 upstream.

fd_install() moves the reference given to it into the file descriptor table
of the current process. If the current process is multithreaded, then
immediately after fd_install(), another thread can close() the file
descriptor and cause the file's resources to be cleaned up.

Since the reference to "lessee" is held by the file, we must not access
"lessee" after the fd_install() call.

As far as I can tell, to reach this codepath, the caller must have an open
file descriptor to a DRI device in master mode. I'm not sure what the
requirements for that are.

Signed-off-by: Jann Horn <jannh@google.com>
Fixes: 62884cd386b8 ("drm: Add four ioctls for managing drm mode object leases [v7]")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181001153117.216923-1-jannh@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
index d638c0fb3418a4404b390fd12c668b4f02157a23..23a5643a4b98763f7f7cd36a9fff5ba4fd02aadd 100644 (file)
--- a/drivers/gpu/drm/drm_lease.c
+++ b/drivers/gpu/drm/drm_lease.c
@@ -566,14 +566,14 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
        lessee_priv->is_master = 1;
        lessee_priv->authenticated = 1;
 
-       /* Hook up the fd */
-       fd_install(fd, lessee_file);
-
        /* Pass fd back to userspace */
        DRM_DEBUG_LEASE("Returning fd %d id %d\n", fd, lessee->lessee_id);
        cl->fd = fd;
        cl->lessee_id = lessee->lessee_id;
 
+       /* Hook up the fd */
+       fd_install(fd, lessee_file);
+
        DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
        return 0;