bpf: Fix NULL pointer dereference in bpf_task_from_vpid()

In the Linux kernel, the following vulnerability has been resolved: bpf: Fix NULL pointer dereference in bpf_task_from_vpid() bpf_task_from_vpid() looks up a task in the pid namespace of the current task, via find_task_by_vpid(): find_task_by_vpid(vpid) find_task_by_pid_ns(vpid, task_active_pid_ns(current)) find_pid_ns(nr, ns) -> idr_find(&ns->idr, nr) cgroup_skb programs run in softirq, which may interrupt a task that is itself in do_exit(). Once that task has passed exit_notify() -> release_task() -> __unhash_process(), its thread_pid is cleared, so task_active_pid_ns(current) returns NULL and find_pid_ns() dereferences &NULL->idr: BUG: kernel NULL pointer dereference, address: 0000000000000050 RIP: 0010:idr_find+0x11/0x30 lib/idr.c:176 Call Trace: <IRQ> find_pid_ns kernel/pid.c:370 [inline] find_task_by_pid_ns+0x3b/0xe0 kernel/pid.c:485 bpf_task_from_vpid+0x5b/0x200 kernel/bpf/helpers.c:2916 bpf_prog_run_array_cg+0x17e/0x530 kernel/bpf/cgroup.c:81 __cgroup_bpf_run_filter_skb+0x12b/0x250 kernel/bpf/cgroup.c:1612 sk_filter_trim_cap+0x1dc/0x4c0 net/core/filter.c:148 tcp_v4_rcv+0x18d1/0x2200 net/ipv4/tcp_ipv4.c:2223 </IRQ> <TASK> do_exit+0xa63/0x1270 kernel/exit.c:1010 get_signal+0x141c/0x1530 kernel/signal.c:3037 Bail out when current has no pid namespace.