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 :-) Signed-off-by: Peter Zijlstra (Intel) <[email protected]> --- arch/x86/kernel/machine_kexec_64.c | 4 +++ arch/x86/kvm/vmx/vmenter.S | 4 +++ 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 | 42 ++++++++++++++++++++++++++++++++++++ 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 @@ -453,6 +453,10 @@ 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 @@ -361,6 +361,10 @@ SYM_FUNC_END(vmread_error_trampoline) .section .text, "ax" +#ifndef CONFIG_X86_FRED + SYM_FUNC_START(vmx_do_interrupt_irqoff) VMX_DO_EVENT_IRQOFF CALL_NOSPEC _ASM_ARG1 SYM_FUNC_END(vmx_do_interrupt_irqoff) + +#endif --- 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 @@ -2390,6 +2390,8 @@ 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 */ @@ -2431,6 +2433,15 @@ 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"); + return -1; + } + insn->sym->nocfi = 1; + break; + default: ERROR_INSN(insn, "Unknown annotation type: %d", type); return -1; @@ -4000,6 +4011,37 @@ 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_NOTYPE || + 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; struct section *group_sec;
