linux-kernel - Re: [RFC][PATCH 3/6] x86: Add ENDBR to IRET-to-Self

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Date:   Tue, 8 Feb 2022 15:33:44 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, ndesaulniers@...gle.com,
        samitolvanen@...gle.com
Subject: Re: [RFC][PATCH 3/6] x86: Add ENDBR to IRET-to-Self

On Mon, Nov 22, 2021 at 07:09:47PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 22, 2021 at 06:03:04PM +0100, Peter Zijlstra wrote:
> > The IRET-to-Self chunks trigger forward code references without ENDBR,
> > fix that.
> 
> Andy corrected me, IRET doesn't take ENBR, the alternative is the below.

Okay, good. I was scratching my head for a second there. :)

-Kees

> 
> ---
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1316,7 +1316,6 @@ SYM_CODE_START(asm_exc_nmi)
>  	iretq			/* continues at repeat_nmi below */
>  	UNWIND_HINT_IRET_REGS entry=0
>  1:
> -	ENDBR
>  #endif
>  
>  repeat_nmi:
> --- a/arch/x86/include/asm/sync_core.h
> +++ b/arch/x86/include/asm/sync_core.h
> @@ -6,7 +6,6 @@
>  #include <asm/processor.h>
>  #include <asm/cpufeature.h>
>  #include <asm/special_insns.h>
> -#include <asm/ibt.h>
>  
>  #ifdef CONFIG_X86_32
>  static inline void iret_to_self(void)
> @@ -35,7 +34,6 @@ static inline void iret_to_self(void)
>  		"pushq $1f\n\t"
>  		"iretq\n\t"
>  		"1:"
> -		ASM_ENDBR
>  		: "=&r" (tmp), ASM_CALL_CONSTRAINT : : "cc", "memory");
>  }
>  #endif /* CONFIG_X86_32 */
> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -598,6 +598,7 @@ int arch_decode_instruction(struct objto
>  				op->dest.type = OP_DEST_REG;
>  				op->dest.reg = CFI_SP;
>  			}
> +			*type = INSN_IRET;
>  			break;
>  		}
>  
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -3587,7 +3587,7 @@ static int validate_ibt_reloc(struct obj
>  static void validate_ibt_insn(struct objtool_file *file, struct instruction *insn)
>  {
>  	struct reloc *reloc = insn_reloc(file, insn);
> -	struct instruction *target;
> +	struct instruction *target, *n;
>  	unsigned long offset;
>  
>  	if (!reloc)
> @@ -3599,8 +3599,16 @@ static void validate_ibt_insn(struct obj
>  	offset = reloc->sym->offset + reloc->addend;
>  
>  	target = find_insn(file, reloc->sym->sec, offset);
> -	if (target && insn->func == target->func && target->this_ip)
> -		return;
> +	if (target && insn->func == target->func) {
> +		if (target->this_ip)
> +			return;
> +
> +		for (n = insn; n->offset <= target->offset;
> +		     n = next_insn_same_func(file, n)) {
> +			if (n->type == INSN_IRET)
> +				return;
> +		}
> +	}
>  
>  	WARN_FUNC("relocation to !ENDBR: %s+0x%lx",
>  		  insn->sec, insn->offset,
> --- a/tools/objtool/include/objtool/arch.h
> +++ b/tools/objtool/include/objtool/arch.h
> @@ -27,6 +27,7 @@ enum insn_type {
>  	INSN_STD,
>  	INSN_CLD,
>  	INSN_ENDBR,
> +	INSN_IRET,
>  	INSN_OTHER,
>  };
>  

-- 
Kees Cook