linux-hardening - Re: [RFC PATCH 21/21] x86: Add support for CONFIG_CFI

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Date:   Sat, 30 Apr 2022 11:24:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev
Subject: Re: [RFC PATCH 21/21] x86: Add support for CONFIG_CFI_CLANG

On Fri, Apr 29, 2022 at 01:36:44PM -0700, Sami Tolvanen wrote:
> Add CONFIG_CFI_CLANG error handling and allow the config to be selected
> on x86_64.

Might be useful to have an example output of all ths somewhere, because
unless I go build my own clang again, I can't tell from these patches
what actual codegen looks like.

Going from the below, I seem to be able to reverse engineer some of it:

  .long \signature
  int3
  int3
my_func:
  ENDBR
  ...
  ret

And then the callsites look like (clang *always* uses r11, right?):


  cmpl	\signature, -6(%r11)
  je	1f
  ud2
1:
  call __x86_indirect_thunk_r11



> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/x86/Kconfig               |  1 +
>  arch/x86/include/asm/linkage.h |  7 ++++++
>  arch/x86/kernel/traps.c        | 39 +++++++++++++++++++++++++++++++++-
>  3 files changed, 46 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index b0142e01002e..01db5c5c4dde 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -108,6 +108,7 @@ config X86
>  	select ARCH_SUPPORTS_PAGE_TABLE_CHECK	if X86_64
>  	select ARCH_SUPPORTS_NUMA_BALANCING	if X86_64
>  	select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP	if NR_CPUS <= 4096
> +	select ARCH_SUPPORTS_CFI_CLANG		if X86_64
>  	select ARCH_SUPPORTS_LTO_CLANG
>  	select ARCH_SUPPORTS_LTO_CLANG_THIN
>  	select ARCH_USE_BUILTIN_BSWAP
> diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h
> index 85865f1645bd..d20acf5ebae3 100644
> --- a/arch/x86/include/asm/linkage.h
> +++ b/arch/x86/include/asm/linkage.h
> @@ -25,6 +25,13 @@
>  #define RET	ret
>  #endif
>  
> +#ifdef CONFIG_CFI_CLANG
> +#define __CFI_TYPE(name)			\
> +	.fill 10, 1, 0x90 ASM_NL		\
> +	.4byte __kcfi_typeid_##name ASM_NL	\
> +	.fill 2, 1, 0xcc
> +#endif
> +
>  #else /* __ASSEMBLY__ */
>  
>  #ifdef CONFIG_SLS
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 1563fb995005..b9e46e6ed83b 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -40,6 +40,7 @@
>  #include <linux/hardirq.h>
>  #include <linux/atomic.h>
>  #include <linux/ioasid.h>
> +#include <linux/cfi.h>
>  
>  #include <asm/stacktrace.h>
>  #include <asm/processor.h>
> @@ -295,6 +296,41 @@ static inline void handle_invalid_op(struct pt_regs *regs)
>  		      ILL_ILLOPN, error_get_trap_addr(regs));
>  }
>  
> +#ifdef CONFIG_CFI_CLANG
> +void *arch_get_cfi_target(unsigned long addr, struct pt_regs *regs)
> +{
> +	char buffer[MAX_INSN_SIZE];
> +	int offset;
> +	struct insn insn;
> +	unsigned long *target;

Reverse xmas please..

> +
> +	/*
> +	 * The expected CFI check instruction sequence:
> +	 *   cmpl    <id>, -6(%reg)	; 7 bytes
> +	 *   je      .Ltmp1		; 2 bytes
> +	 *   ud2			; <- addr
> +	 *   .Ltmp1:
> +	 *
> +	 * Therefore, the target address is in a register that we can
> +	 * decode from the cmpl instruction.
> +	 */
> +	if (copy_from_kernel_nofault(buffer, (void *)addr - 9, MAX_INSN_SIZE))
> +		return NULL;
> +	if (insn_decode(&insn, buffer, MAX_INSN_SIZE, INSN_MODE_64))
> +		return NULL;

insn_decode_kernel()

> +	if (insn.opcode.value != 0x81)
> +		return NULL;

That's not sufficient to uniquely identify cmp, you also need to look at
the modrm to find r==7 I think.

> +
> +	offset = insn_get_modrm_rm_off(&insn, regs);
> +	if (offset < 0)
> +		return NULL;
> +
> +	target = (void *)regs + offset;
> +
> +	return (void *)*target;
> +}
> +#endif
> +
>  static noinstr bool handle_bug(struct pt_regs *regs)
>  {
>  	bool handled = false;
> @@ -312,7 +348,8 @@ static noinstr bool handle_bug(struct pt_regs *regs)
>  	 */
>  	if (regs->flags & X86_EFLAGS_IF)
>  		raw_local_irq_enable();
> -	if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
> +	if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
> +	    report_cfi(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {

This way you'll first get a BUG splat and then tack on the CFI thing.
Seems a bit daft to have two splats.

>  		regs->ip += LEN_UD2;
>  		handled = true;
>  	}
> -- 
> 2.36.0.464.gb9c8b46e94-goog
>