linux-kernel - Re: [PATCH v2 07/39] x86/entry: Sprinkle ENDBR dust

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Date:   Thu, 24 Feb 2022 16:42:51 -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, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 07/39] x86/entry: Sprinkle ENDBR dust

On Thu, Feb 24, 2022 at 03:51:45PM +0100, Peter Zijlstra wrote:
> The SYSCALL entry points are found through taking their respective
> address in order to program them in the MSRs, while the exception
> entry points are found through UNWIND_HINT_IRET_REGS.

Stupid question: does CET consider exception and syscall entry points to
be indirect calls? (I would expect so, but they're ever so slightly
differently executed...)

> [...]
>  0 :
> +	ENDBR
>  	.byte	0x6a, vector
>  	jmp	asm_common_interrupt
> -	nop
> -	/* Ensure that the above is 8 bytes max */
> -	. = 0b + 8
> +	/* Ensure that the above is IDT_ALIGN bytes max */
> +	.fill 0b + IDT_ALIGN - ., 1, 0x90

IIUC, these are just padding -- let's use 0xcc instead of 0x90 as we do
in other places (e.g. vmlinux.lds.S).

-- 
Kees Cook