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

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Date:   Wed, 25 Oct 2023 14:49:09 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     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>,
        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,
        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 Wed, Oct 25, 2023 at 10:30:52PM +0100, Andrew Cooper wrote:
> On 25/10/2023 10:28 pm, Josh Poimboeuf wrote:
> > On Wed, Oct 25, 2023 at 10:10:41PM +0100, Andrew Cooper wrote:
> >> 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.
> > +1 to putting it in .rodata or so.
> 
> It's necessarily in .entry.text so it doesn't explode with KPTI active.

Ah, right.  In general tooling doesn't take too kindly to putting data
in a text section.  But it might be ok.

-- 
Josh