bpf: Disable migrate when kprobe_multi attach to access bpf_prog_active

The syscall link_create not protected by bpf_disable_instrumentation,
accessing percpu data bpf_prog_active should use cpu local_lock when
kprobe_multi program attach.

Fixes: 0dcac27 ("bpf: Add multi kprobe link")
Signed-off-by: Tao Chen <[email protected]>

Author: Tao Chen

kernel/trace/bpf_trace.c

@@ -2728,23 +2728,23 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
 	struct pt_regs *regs;
 	int err;
 
+	migrate_disable();
 	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
 		bpf_prog_inc_misses_counter(link->link.prog);
 		err = 1;
 		goto out;
 	}
 
-	migrate_disable();
 	rcu_read_lock();
 	regs = ftrace_partial_regs(fregs, bpf_kprobe_multi_pt_regs_ptr());
 	old_run_ctx = bpf_set_run_ctx(&run_ctx.session_ctx.run_ctx);
 	err = bpf_prog_run(link->link.prog, regs);
 	bpf_reset_run_ctx(old_run_ctx);
 	rcu_read_unlock();
-	migrate_enable();
 
  out:
 	__this_cpu_dec(bpf_prog_active);
+	migrate_enable();
 	return err;
 }