linux-kernel - Re: [PATCH 3/3] x86/retpoline: Ensure default return thunk isn't used at runtime

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Date:   Tue, 10 Oct 2023 12:36:43 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     David Kaplan <david.kaplan@....com>
Cc:     x86@...nel.org, luto@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] x86/retpoline: Ensure default return thunk isn't
 used at runtime

On Tue, Oct 10, 2023 at 12:10:20PM -0500, David Kaplan wrote:
> 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>
> ---
>  arch/x86/lib/retpoline.S | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
> index 3da768a71cf9..10212cf4a9af 100644
> --- a/arch/x86/lib/retpoline.S
> +++ b/arch/x86/lib/retpoline.S
> @@ -358,15 +358,17 @@ SYM_FUNC_END(call_depth_return_thunk)
>   * This function name is magical and is used by -mfunction-return=thunk-extern
>   * for the compiler to generate JMPs to it.
>   *
> - * This code is only used during kernel boot or module init.  All
> + * This code is only used during kernel boot.  All
>   * 'JMP __x86_return_thunk' sites are changed to something else by
>   * apply_returns().
> + *
> + * This thunk is turned into a ud2 to ensure it is never used at runtime.
> + * Alternative instructions are applied after apply_returns().
>   */
>  SYM_CODE_START(__x86_return_thunk)
>  	UNWIND_HINT_FUNC
>  	ANNOTATE_NOENDBR
> -	ANNOTATE_UNRET_SAFE
> -	ret
> +	ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE;ret),"ud2", X86_FEATURE_RETHUNK

If it's truly never used after boot (even for non-rethunk cases) then
can we use X86_FEATURE_ALWAYS?

-- 
Josh