linux-kernel - [PATCH 1/7] x86/entry: Fix #UD vs WARN more

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Date:   Thu, 18 Jun 2020 16:44:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, dvyukov@...gle.com,
        elver@...gle.com, andreyknvl@...gle.com, mark.rutland@....com,
        mhelsley@...are.com, rostedt@...dmis.org, jthierry@...hat.com,
        mbenes@...e.cz, peterz@...radead.org
Subject: [PATCH 1/7] x86/entry: Fix #UD vs WARN more

vmlinux.o: warning: objtool: exc_invalid_op()+0x47: call to probe_kernel_read() leaves .noinstr.text section

Since we use UD2 as a short-cut for 'CALL __WARN', treat it as such.
Have the bare exception handler do the report_bug() thing.

Fixes: 15a416e8aaa7 ("x86/entry: Treat BUG/WARN as NMI-like entries")
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
---
 arch/x86/kernel/traps.c |   50 +++++++++++++++++++++---------------------------
 1 file changed, 22 insertions(+), 28 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -216,40 +216,34 @@ static inline void handle_invalid_op(str
 		      ILL_ILLOPN, error_get_trap_addr(regs));
 }
 
+static noinstr bool handle_bug(struct pt_regs *regs)
+{
+	bool handled = false;
+
+	/*
+	 * All lies, just get the WARN/BUG out.
+	 */
+	instrumentation_begin();
+	if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
+		regs->ip += LEN_UD2;
+		handled = true;
+	}
+	instrumentation_end();
+
+	return handled;
+}
+
 DEFINE_IDTENTRY_RAW(exc_invalid_op)
 {
 	bool rcu_exit;
 
 	/*
-	 * Handle BUG/WARN like NMIs instead of like normal idtentries:
-	 * if we bugged/warned in a bad RCU context, for example, the last
-	 * thing we want is to BUG/WARN again in the idtentry code, ad
-	 * infinitum.
+	 * We use UD2 as a short encoding for 'CALL __WARN', as such
+	 * handle it before exception entry to avoid recursive WARN
+	 * in case exception entry is the one triggering WARNs.
 	 */
-	if (!user_mode(regs) && is_valid_bugaddr(regs->ip)) {
-		enum bug_trap_type type;
-
-		nmi_enter();
-		instrumentation_begin();
-		trace_hardirqs_off_finish();
-		type = report_bug(regs->ip, regs);
-		if (regs->flags & X86_EFLAGS_IF)
-			trace_hardirqs_on_prepare();
-		instrumentation_end();
-		nmi_exit();
-
-		if (type == BUG_TRAP_TYPE_WARN) {
-			/* Skip the ud2. */
-			regs->ip += LEN_UD2;
-			return;
-		}
-
-		/*
-		 * Else, if this was a BUG and report_bug returns or if this
-		 * was just a normal #UD, we want to continue onward and
-		 * crash.
-		 */
-	}
+	if (!user_mode(regs) && handle_bug(regs))
+		return;
 
 	rcu_exit = idtentry_enter_cond_rcu(regs);
 	instrumentation_begin();