[PATCH v2 13/13] objtool: Validate kCFI calls

Peter Zijlstra
2025-04-30T11:26:47Z

Validate that all indirect calls adhere to kCFI rules. Notably doing
nocfi indirect call to a cfi function is broken.

Apparently some Rust 'core' code violates this and explodes when ran
with FineIBT.

All the ANNOTATE_NOCFI_SYM sites are prime targets for attackers.

- runtime EFI is especially henous because it also needs to disable
IBT. Basically calling unknown code without CFI protection at
runtime is a massice security issue.

- Kexec image handover; if you can exploit this, you get to keep it :-)

- KVM, for the interrupt injection calling IDT gates directly.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/kernel/machine_kexec_64.c | 4 +++
arch/x86/kvm/vmx/vmenter.S | 5 ++++
arch/x86/platform/efi/efi_stub_64.S | 4 +++
drivers/misc/lkdtm/perms.c | 5 ++++
include/linux/objtool.h | 10 ++++++++
include/linux/objtool_types.h | 1
tools/include/linux/objtool_types.h | 1
tools/objtool/check.c | 41 ++++++++++++++++++++++++++++++++++++
tools/objtool/include/objtool/elf.h | 1
9 files changed, 72 insertions(+)

--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -421,6 +421,10 @@ void __nocfi machine_kexec(struct kimage

__ftrace_enabled_restore(save_ftrace_enabled);
}
+/*
+ * Handover to the next kernel, no CFI concern.
+ */
+ANNOTATE_NOCFI_SYM(machine_kexec);

/* arch-dependent functionality related to kexec file-based syscall */

--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -363,5 +363,10 @@ SYM_FUNC_END(vmread_error_trampoline)
.section .text, "ax"

SYM_FUNC_START(vmx_do_interrupt_irqoff)
+ /*
+ * Calling an IDT gate directly; annotate away the CFI concern for now.
+ * Should be fixed if possible.
+ */
+ ANNOTATE_NOCFI_SYM
VMX_DO_EVENT_IRQOFF CALL_NOSPEC _ASM_ARG1
SYM_FUNC_END(vmx_do_interrupt_irqoff)
--- a/arch/x86/platform/efi/efi_stub_64.S
+++ b/arch/x86/platform/efi/efi_stub_64.S
@@ -11,6 +11,10 @@
#include <asm/nospec-branch.h>

SYM_FUNC_START(__efi_call)
+ /*
+ * The EFI code doesn't have any CFI, annotate away the CFI violation.
+ */
+ ANNOTATE_NOCFI_SYM
pushq %rbp
movq %rsp, %rbp
and $~0xf, %rsp
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -9,6 +9,7 @@
#include <linux/vmalloc.h>
#include <linux/mman.h>
#include <linux/uaccess.h>
+#include <linux/objtool.h>
#include <asm/cacheflush.h>
#include <asm/sections.h>

@@ -86,6 +87,10 @@ static noinline __nocfi void execute_loc
func();
pr_err("FAIL: func returned\n");
}
+/*
+ * Explicitly doing the wrong thing for testing.
+ */
+ANNOTATE_NOCFI_SYM(execute_location);

static void execute_user_location(void *dst)
{
--- a/include/linux/objtool.h
+++ b/include/linux/objtool.h
@@ -184,6 +184,15 @@
* WARN using UD2.
*/
#define ANNOTATE_REACHABLE(label) __ASM_ANNOTATE(label, ANNOTYPE_REACHABLE)
+/*
+ * This should not be used; it annotates away CFI violations. There are a few
+ * valid use cases like kexec handover to the next kernel image, and there is
+ * no security concern there.
+ *
+ * There are also a few real issues annotated away, like EFI because we can't
+ * control the EFI code.
+ */
+#define ANNOTATE_NOCFI_SYM(sym) asm(__ASM_ANNOTATE(sym, ANNOTYPE_NOCFI))

#else
#define ANNOTATE_NOENDBR ANNOTATE type=ANNOTYPE_NOENDBR
@@ -194,6 +203,7 @@
#define ANNOTATE_INTRA_FUNCTION_CALL ANNOTATE type=ANNOTYPE_INTRA_FUNCTION_CALL
#define ANNOTATE_UNRET_BEGIN ANNOTATE type=ANNOTYPE_UNRET_BEGIN
#define ANNOTATE_REACHABLE ANNOTATE type=ANNOTYPE_REACHABLE
+#define ANNOTATE_NOCFI_SYM ANNOTATE type=ANNOTYPE_NOCFI
#endif

#if defined(CONFIG_NOINSTR_VALIDATION) && \
--- a/include/linux/objtool_types.h
+++ b/include/linux/objtool_types.h
@@ -65,5 +65,6 @@ struct unwind_hint {
#define ANNOTYPE_IGNORE_ALTS 6
#define ANNOTYPE_INTRA_FUNCTION_CALL 7
#define ANNOTYPE_REACHABLE 8
+#define ANNOTYPE_NOCFI 9

#endif /* _LINUX_OBJTOOL_TYPES_H */
--- a/tools/include/linux/objtool_types.h
+++ b/tools/include/linux/objtool_types.h
@@ -65,5 +65,6 @@ struct unwind_hint {
#define ANNOTYPE_IGNORE_ALTS 6
#define ANNOTYPE_INTRA_FUNCTION_CALL 7
#define ANNOTYPE_REACHABLE 8
+#define ANNOTYPE_NOCFI 9

#endif /* _LINUX_OBJTOOL_TYPES_H */
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2388,6 +2388,8 @@ static int __annotate_ifc(struct objtool

static int __annotate_late(struct objtool_file *file, int type, struct instruction *insn)
{
+ struct symbol *sym;
+
switch (type) {
case ANNOTYPE_NOENDBR:
/* early */
@@ -2429,6 +2431,15 @@ static int __annotate_late(struct objtoo
insn->dead_end = false;
break;

+ case ANNOTYPE_NOCFI:
+ sym = insn->sym;
+ if (!sym) {
+ ERROR_INSN(insn, "dodgy NOCFI annotation");
+ break;
+ }
+ insn->sym->nocfi = 1;
+ break;
+
default:
ERROR_INSN(insn, "Unknown annotation type: %d", type);
return -1;
@@ -3998,6 +4009,36 @@ static int validate_retpoline(struct obj
warnings++;
}

+ if (!opts.cfi)
+ return warnings;
+
+ /*
+ * kCFI call sites look like:
+ *
+ * movl $(-0x12345678), %r10d
+ * addl -4(%r11), %r10d
+ * jz 1f
+ * ud2
+ * 1: cs call __x86_indirect_thunk_r11
+ *
+ * Verify all indirect calls are kCFI adorned by checking for the
+ * UD2. Notably, doing __nocfi calls to regular (cfi) functions is
+ * broken.
+ */
+ list_for_each_entry(insn, &file->retpoline_call_list, call_node) {
+ struct symbol *sym = insn->sym;
+
+ if (sym && sym->type == STT_FUNC && !sym->nocfi) {
+ struct instruction *prev =
+ prev_insn_same_sym(file, insn);
+
+ if (!prev || prev->type != INSN_BUG) {
+ WARN_INSN(insn, "no-cfi indirect call!");
+ warnings++;
+ }
+ }
+ }
+
return warnings;
}

--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -70,6 +70,7 @@ struct symbol {
u8 local_label : 1;
u8 frame_pointer : 1;
u8 ignore : 1;
+ u8 nocfi : 1;
struct list_head pv_target;
struct reloc *relocs;
};