bpf: Invoke __bpf_prog_exit_sleepable_recur() on recursion in kern_sys_bpf().

If __bpf_prog_enter_sleepable_recur() detects recursion then it returns
0 without undoing rcu_read_lock_trace(), migrate_disable() or
decrementing the recursion counter. This is fine in the JIT case because
the JIT code will jump in the 0 case to the end and invoke the matching
exit trampoline (__bpf_prog_exit_sleepable_recur()).

This is not the case in kern_sys_bpf() which returns directly to the
caller with an error code.

Add __bpf_prog_exit_sleepable_recur() as clean up in the recursion case.

Fixes: b1d18a75 ("bpf: Extend sys_bpf commands for bpf_syscall programs.")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20230830080405.251926-2-bigeasy@linutronix.de

kernel/bpf/syscall.c

@@ -5505,6 +5505,7 @@ int kern_sys_bpf(int cmd, union bpf_attr *attr, unsigned int size)
 		run_ctx.saved_run_ctx = NULL;
 		if (!__bpf_prog_enter_sleepable_recur(prog, &run_ctx)) {
 			/* recursion detected */
+			__bpf_prog_exit_sleepable_recur(prog, 0, &run_ctx);
 			bpf_prog_put(prog);
 			return -EBUSY;
 		}