rcu/nocb: Fix possible invalid rdp's->nocb_cb_kthread pointer access

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: rcu/nocb: Fix possible invalid rdp's->nocb_cb_kthread pointer access In the preparation stage of CPU online, if the corresponding the rdp's->nocb_cb_kthread does not exist, will be created, there is a situation where the rdp's rcuop kthreads creation fails, and then de-offload this CPU's rdp, does not assign this CPU's rdp->nocb_cb_kthread pointer, but this rdp's->nocb_gp_rdp and rdp's->rdp_gp->nocb_gp_kthread is still valid. This will cause the subsequent re-offload operation of this offline CPU, which will pass the conditional check and the kthread_unpark() will access invalid rdp's->nocb_cb_kthread pointer. This commit therefore use rdp's->nocb_gp_kthread instead of rdp_gp's->nocb_gp_kthread for safety check.

References

• https://git.kernel.org/stable/c/b097ae798298885695c339d390b48b4e39619fa7
• https://git.kernel.org/stable/c/3da45ec1e485a1a5ad31fe9ddd467c7ee5ae4ef9
• https://git.kernel.org/stable/c/cce3d027227c69e85896af9fbc6fa9af5c68f067
• https://git.kernel.org/stable/c/1c951683a720b17c9ecaad1932bc95b29044611f
• https://git.kernel.org/stable/c/9b5ec8e6b31755288a07b3abeeab8cd38e9d3c9d
• https://git.kernel.org/stable/c/1bba3900ca18bdae28d1b9fa10f16a8f8cb2ada1