In the Linux kernel, the following vulnerability has been resolved: bpf, cgroup: Fix storage...

In the Linux kernel, the following vulnerability has been resolved:

bpf, cgroup: Fix storage null-ptr-deref after replacing prog

Syzkaller reported a storage null-ptr-deref issue after replacing prog. This occurs in the following scenario:

1. prog A, an empty prog, is attached to a cgrp.
2. prog B uses BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE and calls the bpf_get_local_storage helper.
3. link_update is called to replace prog A with prog B.

The reason is that __cgroup_bpf_replace fails to alloc and assign the required cgrp storage for the incoming replacement prog. Consequently, the new prog inherits an uninit storage, leading to null-ptr-deref panic when kick the new prog.

Fix this by rejecting a link update if new_prog's cgroup storage is incompatible with link->prog.