[PATCH 2/2] x86/fineibt: Poison ENDBR at +0

[Peter Zijlstra <peterz@infradead.org>]

Alyssa noticed that when building the kernel with CFI_CLANG+IBT and
booting on IBT enabled hardware obtain FineIBT, the indirect functions
look like:

  __cfi_foo:
	endbr64
	subl	$hash, %r10d
	jz	1f
	ud2
	nop
  1:
  foo:
	endbr64

This is because clang currently does not supress ENDBR emission for
functions it provides a __cfi prologue symbol for.

Having this second ENDBR however makes it possible to elide the CFI
check. Therefore, we should poison this second ENDBR (if present) when
switching to FineIBT mode.

Fixes: 931ab63664f0 ("x86/ibt: Implement FineIBT")
Reported-by: "Milburn, Alyssa" <alyssa.milburn@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/alternative.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -940,6 +940,17 @@ static int cfi_rewrite_preamble(s32 *sta
 	return 0;
 }
 
+static void cfi_rewrite_endbr(s32 *start, s32 *end)
+{
+	s32 *s;
+
+	for (s = start; s < end; s++) {
+		void *addr = (void *)s + *s;
+
+		poison_endbr(addr+16, false);
+	}
+}
+
 /* .retpoline_sites */
 static int cfi_rand_callers(s32 *start, s32 *end)
 {
@@ -1034,14 +1045,19 @@ static void __apply_fineibt(s32 *start_r
 		return;
 
 	case CFI_FINEIBT:
+		/* place the FineIBT preamble at func()-16 */
 		ret = cfi_rewrite_preamble(start_cfi, end_cfi);
 		if (ret)
 			goto err;
 
+		/* rewrite the callers to target func()-16 */
 		ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
 		if (ret)
 			goto err;
 
+		/* now that nobody targets func()+0, remove ENDBR there */
+		cfi_rewrite_endbr(start_cfi, end_cfi);
+
 		if (builtin)
 			pr_info("Using FineIBT CFI\n");
 		return;