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

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Date: Wed, 17 Apr 2024 05:59:42 +0200
From: Klara Modin <klarasmodin@...il.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, "Kaplan, David"
 <David.Kaplan@....com>, Ingo Molnar <mingo@...nel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-tip-commits@...r.kernel.org" <linux-tip-commits@...r.kernel.org>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 "x86@...nel.org" <x86@...nel.org>, David Howells <dhowells@...hat.com>
Subject: Re: [PATCH -v2] x86/retpoline: Ensure default return thunk isn't used
 at runtime

On 2024-04-16 11:27, Borislav Petkov wrote:
> On Wed, Apr 03, 2024 at 07:10:17PM +0200, Klara Modin wrote:
>> With this patch/commit, one of my machines (older P4 Xeon, 32-bit only)
>> hangs on boot with CONFIG_RETHUNK=y / CONFIG_MITIGATION_RETHUNK=y.
> 
> Ok, this should fix it:
> 
> ---
> From: "Borislav Petkov (AMD)" <bp@...en8.de>
> Date: Mon, 15 Apr 2024 18:15:43 +0200
> Subject: [PATCH] x86/retpolines: Enable the default thunk warning only on relevant configs
> 
> The using-default-thunk warning check makes sense only with
> configurations which actually enable the special return thunks.
> 
> Otherwise, it fires on unrelated 32-bit configs on which the special
> return thunks won't even work (they're 64-bit only) and, what is more,
> those configs even go off into the weeds when booting in the
> alternatives patching code, leading to a dead machine.
> 
> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
> Link: https://lore.kernel.org/r/78e0d19c-b77a-4169-a80f-2eef91f4a1d6@gmail.com
> Link: https://lore.kernel.org/r/20240413024956.488d474e@yea
> ---
>   arch/x86/lib/retpoline.S | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
> index e674ccf720b9..391059b2c6fb 100644
> --- a/arch/x86/lib/retpoline.S
> +++ b/arch/x86/lib/retpoline.S
> @@ -382,8 +382,15 @@ SYM_FUNC_END(call_depth_return_thunk)
>   SYM_CODE_START(__x86_return_thunk)
>   	UNWIND_HINT_FUNC
>   	ANNOTATE_NOENDBR
> +#if defined(CONFIG_MITIGATION_UNRET_ENTRY) || \
> +    defined(CONFIG_MITIGATION_SRSO) || \
> +    defined(CONFIG_MITIGATION_CALL_DEPTH_TRACKING)
>   	ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \
>   		   "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS
> +#else
> +	ANNOTATE_UNRET_SAFE
> +	ret
> +#endif
>   	int3
>   SYM_CODE_END(__x86_return_thunk)
>   EXPORT_SYMBOL(__x86_return_thunk)

Thanks,
Tested-by: Klara Modin <klarasmodin@...il.com>