linux-kernel - Re: [PATCH v3 2/6] x86/entry_64: Add VERW just before userspace transition

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Message-ID: <20231026192950.ylzc66f3f5naqvjv@desk>
Date:   Thu, 26 Oct 2023 12:29:50 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Nikolay Borisov <nik.borisov@...e.com>
Cc:     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,
        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,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v3 2/6] x86/entry_64: Add VERW just before userspace
 transition

On Thu, Oct 26, 2023 at 07:25:27PM +0300, Nikolay Borisov wrote:
> 
> 
> On 25.10.23 г. 23:52 ч., Pawan Gupta wrote:
> 
> <snip>
> 
> > @@ -1520,6 +1530,7 @@ SYM_CODE_START(ignore_sysret)
> >   	UNWIND_HINT_END_OF_STACK
> >   	ENDBR
> >   	mov	$-ENOSYS, %eax
> > +	CLEAR_CPU_BUFFERS
> 
> nit: Just out of curiosity is it really needed in this case or it's doesn
> for the sake of uniformity so that all ring3 transitions are indeed
> covered??

Interrupts returning to kernel don't clear the CPU buffers. I believe
interrupts will be enabled here, and getting an interrupt here could
leak the data that interrupt touched.