linux-kernel - [PATCH 3/8] x86/boot: Mark start_secondary() with _

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Message-Id: <20241105114522.068976491@infradead.org>
Date: Tue, 05 Nov 2024 12:39:04 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
 peterz@...radead.org,
 alyssa.milburn@...el.com,
 scott.d.constable@...el.com,
 joao@...rdrivepizza.com,
 andrew.cooper3@...rix.com,
 jpoimboe@...nel.org,
 alexei.starovoitov@...il.com,
 ebiggers@...nel.org,
 samitolvanen@...gle.com,
 kees@...nel.org
Subject: [PATCH 3/8] x86/boot: Mark start_secondary() with __noendbr

The handoff between the boot stubs and start_secondary() are before IBT is
enabled and is definitely not subject to kCFI. As such, suppress all that for
this function.

Notably when the ENDBR poison would become fatal (ud1 instead of nop) this will
trigger a tripple fault because we haven't set up the IDT to handle #UD yet.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/kernel/smpboot.c |    3 ++-
 include/linux/objtool.h   |   13 ++++++++++---
 2 files changed, 12 insertions(+), 4 deletions(-)

--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -228,7 +228,7 @@ static void ap_calibrate_delay(void)
 /*
  * Activate a secondary processor.
  */
-static void notrace start_secondary(void *unused)
+static void notrace __noendbr start_secondary(void *unused)
 {
 	/*
 	 * Don't put *anything* except direct CPU state initialization
@@ -313,6 +313,7 @@ static void notrace start_secondary(void
 	wmb();
 	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
 }
+NOENDBR_SYMBOL(start_secondary);
 
 /*
  * The bootstrap kernel entry code has set these up. Save them for
--- a/include/linux/objtool.h
+++ b/include/linux/objtool.h
@@ -45,12 +45,18 @@
 #define STACK_FRAME_NON_STANDARD_FP(func)
 #endif
 
-#define ANNOTATE_NOENDBR					\
-	"986: \n\t"						\
+#define __ANNOTATE_NOENDBR(label)				\
 	".pushsection .discard.noendbr\n\t"			\
-	".long 986b\n\t"					\
+	".long " #label "\n\t"					\
 	".popsection\n\t"
 
+#define NOENDBR_SYMBOL(func)					\
+	asm(__ANNOTATE_NOENDBR(func))
+
+#define ANNOTATE_NOENDBR					\
+	"986: \n\t"						\
+	__ANNOTATE_NOENDBR(986b)
+
 #define ASM_REACHABLE							\
 	"998:\n\t"							\
 	".pushsection .discard.reachable\n\t"				\
@@ -157,6 +163,7 @@
 #define STACK_FRAME_NON_STANDARD_FP(func)
 #define ANNOTATE_NOENDBR
 #define ASM_REACHABLE
+#define NOENDBR_SYMBOL(func)
 #else
 #define ANNOTATE_INTRA_FUNCTION_CALL
 .macro UNWIND_HINT type:req sp_reg=0 sp_offset=0 signal=0