linux-kernel - Re: [PATCH -v2] x86/retpoline: Ensure default return thunk isn't used at runtime

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Message-ID: <20231012171144.wtzcheq7x3qwpym6@treble>
Date:   Thu, 12 Oct 2023 10:11:44 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "Kaplan, David" <David.Kaplan@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -v2] x86/retpoline: Ensure default return thunk isn't
 used at runtime

On Thu, Oct 12, 2023 at 04:10:31PM +0200, Borislav Petkov wrote:
> On Tue, Oct 10, 2023 at 01:41:19PM -0700, Josh Poimboeuf wrote:
> > Even if it's not a security hole, I'd still view it as a major BUG() as
> > it would directly contradict our understanding (and the comments above)
> > and could cause performance or other correctness issues that would
> > otherwise go unnoticed.
> > 
> > So I think an unconditional UD2 is warranted.
> 
> Before David's outlook mangles v2, lemme send it from a real mail
> client :-P.
> 
> v2 uses X86_FEATURE_ALWAYS as Josh requested.
> 
> ---
> From: David Kaplan <david.kaplan@....com>
> Date: Thu, 12 Oct 2023 08:52:32 -0500
> Subject: [PATCH] x86/retpoline: Ensure default return thunk isn't used at runtime
> 
> All CPU bugs that require a return thunk define a special return thunk
> to use (e.g., srso_return_thunk).  The default thunk,
> __x86_return_thunk, should never be used after apply_returns()
> completes.  Otherwise this could lead to potential speculation holes.
> 
> Enforce this by replacing this thunk with a ud2 when alternatives are
> applied.  Alternative instructions are applied after apply_returns().
> 
> The default thunk is only used during kernel boot, it is not used during
> module init since that occurs after apply_returns().
> 
> Signed-off-by: David Kaplan <david.kaplan@....com>
> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>

Reviewed-by: Josh Poimboeuf <jpoimboe@...nel.org>

-- 
Josh