linux-kernel - Re: [PATCH v3 1/6] x86/bugs: Add asm helpers for executing VERW

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Message-ID: <8b6d857f-cbf6-4969-8285-f90254bdafc0@citrix.com>
Date:   Wed, 25 Oct 2023 22:10:41 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, tony.luck@...el.com,
        ak@...ux.intel.com, tim.c.chen@...ux.intel.com
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kvm@...r.kernel.org,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        antonio.gomez.iglesias@...ux.intel.com,
        Alyssa Milburn <alyssa.milburn@...el.com>
Subject: Re: [PATCH v3 1/6] x86/bugs: Add asm helpers for executing VERW

On 25/10/2023 9:52 pm, Pawan Gupta wrote:
> diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
> index bfb7bcb362bc..f8ba0c0b6e60 100644
> --- a/arch/x86/entry/entry.S
> +++ b/arch/x86/entry/entry.S
> @@ -20,3 +23,16 @@ SYM_FUNC_END(entry_ibpb)
>  EXPORT_SYMBOL_GPL(entry_ibpb);
>  
>  .popsection
> +
> +.pushsection .entry.text, "ax"
> +
> +.align L1_CACHE_BYTES, 0xcc
> +SYM_CODE_START_NOALIGN(mds_verw_sel)
> +	UNWIND_HINT_UNDEFINED
> +	ANNOTATE_NOENDBR
> +	.word __KERNEL_DS

You need another .align here.  Otherwise subsequent code will still
start in this cacheline and defeat the purpose of trying to keep it
separate.

> +SYM_CODE_END(mds_verw_sel);

Thinking about it, should this really be CODE and not a data entry?

It lives in .entry.text but it really is data and objtool shouldn't be
writing ORC data for it at all.

(Not to mention that if it's marked as STT_OBJECT, objdump -d will do
the sensible thing and not even try to disassemble it).

~Andrew

P.S. Please CC on the full series.  Far less effort than fishing the
rest off lore.