[tip: x86/urgent] x86/fineibt: Poison ENDBR at +0

[tip-bot2 for Peter Zijlstra]

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

Commit-ID:     04505bbbbb15da950ea0239e328a76a3ad2376e0
Gitweb:        https://git.kernel.org/tip/04505bbbbb15da950ea0239e328a76a3ad2376e0
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 15 Jun 2023 21:35:48 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 10 Jul 2023 09:52:25 +02:00

x86/fineibt: Poison ENDBR at +0

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

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

This is because the compiler generates code for kCFI+IBT. In that case
the caller does the hash check and will jump to +0, so there must be
an ENDBR there. The compiler doesn't know about FineIBT at all; also
it is possible to actually use kCFI+IBT when booting with 'cfi=kcfi'
on IBT enabled hardware.

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

Fixes: 931ab63664f0 ("x86/ibt: Implement FineIBT")
Reported-by: "Milburn, Alyssa" <alyssa.milburn@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230615193722.194131053@xxxxxxxxxxxxx
---
 arch/x86/kernel/alternative.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 04b25a2..d77aaab 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1068,6 +1068,17 @@ static int cfi_rewrite_preamble(s32 *start, s32 *end)
 	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)
 {
@@ -1162,14 +1173,19 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
 		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;