[tip: x86/core] x86: Fix {int3,ibt}_selftest() vs LTO

[tip-bot2 for Peter Zijlstra]

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

Commit-ID:     c7d90e15b8950009d0d4e8f3503b09b2ea6d527c
Gitweb:        https://git.kernel.org/tip/c7d90e15b8950009d0d4e8f3503b09b2ea6d527c
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 10 Mar 2022 11:16:03 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 11 Mar 2022 13:05:08 +01:00

x86: Fix {int3,ibt}_selftest() vs LTO

Both these selftests define a symbol in inline asm which goes
side-ways if the asm gets duplicated due to inlining. Nick actually
saw this happen with a Clang LTO build.

Mark the two selftests noinline to ensure this cannot happen, as
suggestd by David.

While there, update the comment for int3_selftest() and increase coding
style consistency between the two.

Fixes: 103c0093ceb6 ("x86/ibt: Add IBT feature, MSR and #CP handling")
Reported-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Suggested-by: David Laight <David.Laight@xxxxxxxxxx>,
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> # llvm build, non-IBT boot
Link: https://lkml.kernel.org/r/YinP49gEl2zUVekz@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 arch/x86/kernel/alternative.c | 8 ++++----
 arch/x86/kernel/traps.c       | 7 ++++---
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d6c41f8..820c43a 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -781,7 +781,8 @@ int3_exception_notify(struct notifier_block *self, unsigned long val, void *data
 	return NOTIFY_STOP;
 }
 
-static void __init int3_selftest(void)
+/* Must be noinline to ensure uniqueness of int3_selftest_ip. */
+static noinline void __init int3_selftest(void)
 {
 	static __initdata struct notifier_block int3_exception_nb = {
 		.notifier_call	= int3_exception_notify,
@@ -794,9 +795,8 @@ static void __init int3_selftest(void)
 	/*
 	 * Basically: int3_magic(&val); but really complicated :-)
 	 *
-	 * Stick the address of the INT3 instruction into int3_selftest_ip,
-	 * then trigger the INT3, padded with NOPs to match a CALL instruction
-	 * length.
+	 * INT3 padded with NOP to CALL_INSN_SIZE. The int3_exception_nb
+	 * notifier above will emulate CALL for us.
 	 */
 	asm volatile ("int3_selftest_ip:\n\t"
 		      ANNOTATE_NOENDBR
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 89fb299..755c23b 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -213,7 +213,7 @@ DEFINE_IDTENTRY(exc_overflow)
 
 static __ro_after_init bool ibt_fatal = true;
 
-void ibt_selftest_ip(void); /* code label defined in asm below */
+extern void ibt_selftest_ip(void); /* code label defined in asm below */
 
 enum cp_error_code {
 	CP_EC        = (1 << 15) - 1,
@@ -237,7 +237,7 @@ DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
 	if (WARN_ON_ONCE(user_mode(regs) || (error_code & CP_EC) != CP_ENDBR))
 		return;
 
-	if (unlikely(regs->ip == (unsigned long)ibt_selftest_ip)) {
+	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_ip)) {
 		regs->ax = 0;
 		return;
 	}
@@ -251,7 +251,8 @@ DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
 	BUG();
 }
 
-bool ibt_selftest(void)
+/* Must be noinline to ensure uniqueness of ibt_selftest_ip. */
+noinline bool ibt_selftest(void)
 {
 	unsigned long ret;