drm/dp_mst: Ensure mst_primary pointer is valid in drm_dp_mst_handle_up_req()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: drm/dp_mst: Ensure mst_primary pointer is valid in drm_dp_mst_handle_up_req() While receiving an MST up request message from one thread in drm_dp_mst_handle_up_req(), the MST topology could be removed from another thread via drm_dp_mst_topology_mgr_set_mst(false), freeing mst_primary and setting drm_dp_mst_topology_mgr::mst_primary to NULL. This could lead to a NULL deref/use-after-free of mst_primary in drm_dp_mst_handle_up_req(). Avoid the above by holding a reference for mst_primary in drm_dp_mst_handle_up_req() while it's used. v2: Fix kfreeing the request if getting an mst_primary reference fails.

References

• https://git.kernel.org/stable/c/f61b2e5e7821f868d6afc22382a66a30ee780ba0
• https://git.kernel.org/stable/c/9735d40f5fde9970aa46e828ecc85c32571d58a2
• https://git.kernel.org/stable/c/ce55818b2d3a999f886af91679589e4644ff1dc8
• https://git.kernel.org/stable/c/e54b00086f7473dbda1a7d6fc47720ced157c6a8