bpf: Assign bpf_tramp_run_ctx::saved_run_ctx before recursion check.

__bpf_prog_enter_recur() assigns bpf_tramp_run_ctx::saved_run_ctx before
performing the recursion check which means in case of a recursion
__bpf_prog_exit_recur() uses the previously set bpf_tramp_run_ctx::saved_run_ctx
value.

__bpf_prog_enter_sleepable_recur() assigns bpf_tramp_run_ctx::saved_run_ctx
after the recursion check which means in case of a recursion
__bpf_prog_exit_sleepable_recur() uses an uninitialized value. This does not
look right. If I read the entry trampoline code right, then bpf_tramp_run_ctx
isn't initialized upfront.

Align __bpf_prog_enter_sleepable_recur() with __bpf_prog_enter_recur() and
set bpf_tramp_run_ctx::saved_run_ctx before the recursion check is made.
Remove the assignment of saved_run_ctx in kern_sys_bpf() since it happens
a few cycles later.

Fixes: e384c7b ("bpf, x86: Create bpf_tramp_run_ctx on the caller thread's stack")
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/20230830080405.251926-3-bigeasy@linutronix.de

Author: Sebastian Andrzej Siewior

kernel/bpf/syscall.c

@@ -5502,7 +5502,6 @@ int kern_sys_bpf(int cmd, union bpf_attr *attr, unsigned int size)
 		}
 
 		run_ctx.bpf_cookie = 0;
-		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);

kernel/bpf/trampoline.c

@@ -926,13 +926,12 @@ u64 notrace __bpf_prog_enter_sleepable_recur(struct bpf_prog *prog,
 	migrate_disable();
 	might_fault();
 
+	run_ctx->saved_run_ctx = bpf_set_run_ctx(&run_ctx->run_ctx);
+
 	if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
 		bpf_prog_inc_misses_counter(prog);
 		return 0;
 	}
-
-	run_ctx->saved_run_ctx = bpf_set_run_ctx(&run_ctx->run_ctx);
-
 	return bpf_prog_start_time();
 }