[tip: x86/entry] x86/entry: Fix #UD vs WARN more

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the x86/entry branch of tip:

Commit-ID:     145a773aef83181d47ebab21bb33c89233aadb1e
Gitweb:        https://git.kernel.org/tip/145a773aef83181d47ebab21bb33c89233aadb1e
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Tue, 16 Jun 2020 13:28:36 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 25 Jun 2020 13:45:40 +02:00

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@xxxxxxxxxxxxx>
Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200622114713.GE577403@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 arch/x86/kernel/traps.c | 72 +++++++++++++++++++++-------------------
 1 file changed, 38 insertions(+), 34 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index a7d1570..1d9ea21 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -84,17 +84,16 @@ static inline void cond_local_irq_disable(struct pt_regs *regs)
 		local_irq_disable();
 }
 
-int is_valid_bugaddr(unsigned long addr)
+__always_inline int is_valid_bugaddr(unsigned long addr)
 {
-	unsigned short ud;
-
 	if (addr < TASK_SIZE_MAX)
 		return 0;
 
-	if (probe_kernel_address((unsigned short *)addr, ud))
-		return 0;
-
-	return ud == INSN_UD0 || ud == INSN_UD2;
+	/*
+	 * We got #UD, if the text isn't readable we'd have gotten
+	 * a different exception.
+	 */
+	return *(unsigned short *)addr == INSN_UD2;
 }
 
 static nokprobe_inline int
@@ -216,40 +215,45 @@ static inline void handle_invalid_op(struct pt_regs *regs)
 		      ILL_ILLOPN, error_get_trap_addr(regs));
 }
 
-DEFINE_IDTENTRY_RAW(exc_invalid_op)
+static noinstr bool handle_bug(struct pt_regs *regs)
 {
-	bool rcu_exit;
+	bool handled = false;
+
+	if (!is_valid_bugaddr(regs->ip))
+		return handled;
 
 	/*
-	 * 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.
+	 * All lies, just get the WARN/BUG out.
 	 */
-	if (!user_mode(regs) && is_valid_bugaddr(regs->ip)) {
-		enum bug_trap_type type;
+	instrumentation_begin();
+	/*
+	 * Since we're emulating a CALL with exceptions, restore the interrupt
+	 * state to what it was at the exception site.
+	 */
+	if (regs->flags & X86_EFLAGS_IF)
+		raw_local_irq_enable();
+	if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
+		regs->ip += LEN_UD2;
+		handled = true;
+	}
+	if (regs->flags & X86_EFLAGS_IF)
+		raw_local_irq_disable();
+	instrumentation_end();
 
-		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();
+	return handled;
+}
 
-		if (type == BUG_TRAP_TYPE_WARN) {
-			/* Skip the ud2. */
-			regs->ip += LEN_UD2;
-			return;
-		}
+DEFINE_IDTENTRY_RAW(exc_invalid_op)
+{
+	bool rcu_exit;
 
-		/*
-		 * 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.
-		 */
-	}
+	/*
+	 * 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) && handle_bug(regs))
+		return;
 
 	rcu_exit = idtentry_enter_cond_rcu(regs);
 	instrumentation_begin();