x86/kprobes: Update kcb status flag after singlestepping

Fix kprobes to update kcb (kprobes control block) status flag to
KPROBE_HIT_SSDONE even if the kp->post_handler is not set.

This bug may cause a kernel panic if another INT3 user runs right
after kprobes because kprobe_int3_handler() misunderstands the
INT3 is kprobe's single stepping INT3.

Fixes: 6256e668 ("x86/kprobes: Use int3 instead of debug trap for single-step")
Reported-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Daniel Müller <deso@posteo.net>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20220727210136.jjgc3lpqeq42yr3m@muellerd-fedora-PC2BDTX9
Link: https://lore.kernel.org/r/165942025658.342061.12452378391879093249.stgit@devnote2

arch/x86/kernel/kprobes/core.c

@@ -814,16 +814,20 @@ set_current_kprobe(struct kprobe *p, struct pt_regs *regs,
 static void kprobe_post_process(struct kprobe *cur, struct pt_regs *regs,
 			       struct kprobe_ctlblk *kcb)
 {
-	if ((kcb->kprobe_status != KPROBE_REENTER) && cur->post_handler) {
-		kcb->kprobe_status = KPROBE_HIT_SSDONE;
-		cur->post_handler(cur, regs, 0);
-	}
-
 	/* Restore back the original saved kprobes variables and continue. */
-	if (kcb->kprobe_status == KPROBE_REENTER)
+	if (kcb->kprobe_status == KPROBE_REENTER) {
+		/* This will restore both kcb and current_kprobe */
 		restore_previous_kprobe(kcb);
-	else
+	} else {
+		/*
+		 * Always update the kcb status because
+		 * reset_curent_kprobe() doesn't update kcb.
+		 */
+		kcb->kprobe_status = KPROBE_HIT_SSDONE;
+		if (cur->post_handler)
+			cur->post_handler(cur, regs, 0);
 		reset_current_kprobe();
+	}
 }
 NOKPROBE_SYMBOL(kprobe_post_process);