The BPF helper bpf_cgroup_from_id() calls kernfs_find_and_get_node_by_id()
which acquires kernfs_idr_lock, which is an non-raw non-IRQ-safe lock. This
can lead to deadlocks as bpf_cgroup_from_id() can be called from any BPF
programs including e.g. the ones that attach to functions which are holding
the scheduler rq lock.

Consider the following BPF program:

  SEC("fentry/__set_cpus_allowed_ptr_locked")
  int BPF_PROG(__set_cpus_allowed_ptr_locked, struct task_struct *p,
	       struct affinity_context *affn_ctx, struct rq *rq, struct rq_flags *rf)
  {
	  struct cgroup *cgrp = bpf_cgroup_from_id(p->cgroups->dfl_cgrp->kn->id);

	  if (cgrp) {
		  bpf_printk("%d[%s] in %s", p->pid, p->comm, cgrp->kn->name);
		  bpf_cgroup_release(cgrp);
	  }
	  return 0;
  }

__set_cpus_allowed_ptr_locked() is called with rq lock held and the above
BPF program calls bpf_cgroup_from_id() within leading to the following
lockdep warning:

  =====================================================
  WARNING: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected
  6.7.0-rc3-work-00053-g07124366a1d7-dirty #147 Not tainted
  -----------------------------------------------------
  repro/1620 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
  ffffffff833b3688 (kernfs_idr_lock){+.+.}-{2:2}, at: kernfs_find_and_get_node_by_id+0x1e/0x70

		and this task is already holding:
  ffff888237ced698 (&rq->__lock){-.-.}-{2:2}, at: task_rq_lock+0x4e/0xf0
  which would create a new lock dependency:
   (&rq->__lock){-.-.}-{2:2} -> (kernfs_idr_lock){+.+.}-{2:2}
  ...
   Possible interrupt unsafe locking scenario:

	 CPU0                    CPU1
	 ----                    ----
    lock(kernfs_idr_lock);
				 local_irq_disable();
				 lock(&rq->__lock);
				 lock(kernfs_idr_lock);
    <Interrupt>
      lock(&rq->__lock);

		 *** DEADLOCK ***
  ...
  Call Trace:
   dump_stack_lvl+0x55/0x70
   dump_stack+0x10/0x20
   __lock_acquire+0x781/0x2a40
   lock_acquire+0xbf/0x1f0
   _raw_spin_lock+0x2f/0x40
   kernfs_find_and_get_node_by_id+0x1e/0x70
   cgroup_get_from_id+0x21/0x240
   bpf_cgroup_from_id+0xe/0x20
   bpf_prog_98652316e9337a5a___set_cpus_allowed_ptr_locked+0x96/0x11a
   bpf_trampoline_6442545632+0x4f/0x1000
   __set_cpus_allowed_ptr_locked+0x5/0x5a0
   sched_setaffinity+0x1b3/0x290
   __x64_sys_sched_setaffinity+0x4f/0x60
   do_syscall_64+0x40/0xe0
   entry_SYSCALL_64_after_hwframe+0x46/0x4e

Let's fix it by protecting kernfs_node and kernfs_root with RCU and making
kernfs_find_and_get_node_by_id() acquire rcu_read_lock() instead of
kernfs_idr_lock.

This adds an rcu_head to kernfs_node making it larger by 16 bytes on 64bit.
Combined with the preceding rearrange patch, the net increase is 8 bytes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andrea Righi <andrea.righi@canonical.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20240109214828.252092-4-tj@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>