linux-kernel - Re: [PATCH 04/22] x86/srso: Fix SBPB enablement for spec_rstack

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Message-ID: <20230821141619.GCZONxs5OdbbXFYSq2@fat_crate.local>
Date:   Mon, 21 Aug 2023 16:16:19 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Babu Moger <babu.moger@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, David.Kaplan@....com,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Nikolay Borisov <nik.borisov@...e.com>,
        gregkh@...uxfoundation.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 04/22] x86/srso: Fix SBPB enablement for
 spec_rstack_overflow=off

On Sun, Aug 20, 2023 at 06:19:01PM -0700, Josh Poimboeuf wrote:
> If the user has requested no SRSO mitigation, other mitigations can use
> the lighter-weight SBPB instead of IBPB.
> 
> Fixes: fb3bd914b3ec ("x86/srso: Add a Speculative RAS Overflow mitigation")
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  arch/x86/kernel/cpu/bugs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index b0ae985aa6a4..10499bcd4e39 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -2433,7 +2433,7 @@ static void __init srso_select_mitigation(void)
>  
>  	switch (srso_cmd) {
>  	case SRSO_CMD_OFF:
> -		return;
> +		goto pred_cmd;

Can't do that - you need to synchronize it with retbleed. If retbleed
has selected IBPB mitigation you must not override it.

It's a whole different story whether it makes sense.

Thx.

-- 
Regards/Gruss,
    Boris.

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