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Message-ID: <edf4fb6f-0f27-44fd-8f2a-ab60a156b20c@leemhuis.info>
Date: Wed, 17 Apr 2024 10:38:56 +0200
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Subject: Re: [bisected] Kernel v6.9-rc3 fails to boot on a Thinkpad T60 with
 MITIGATION_RETHUNK=y (regression from v6.8.5)
To: Erhard Furtner <erhard_f@mailbox.org>, Klara Modin <klarasmodin@gmail.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>, x86@kernel.org,
 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
 jpoimboe@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
 Ingo Molnar <mingo@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>,
 Peter Zijlstra <peterz@infradead.org>, Breno Leitao <leitao@debian.org>,
 Borislav Petkov <bp@alien8.de>,
 Linux regressions mailing list <regressions@lists.linux.dev>
References: <20240413024956.488d474e@yea> <ZhpOIeVq1KQXzjBp@archie.me>
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 <20240414083626.GAZhuViviWKudQ5Apm@fat_crate.local>
 <20240414090810.GBZhuc-lN6tyKbF_-M@fat_crate.local>
From: "Linux regression tracking (Thorsten Leemhuis)"
 <regressions@leemhuis.info>
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On 14.04.24 11:08, Borislav Petkov wrote:
> On Sun, Apr 14, 2024 at 10:36:26AM +0200, Borislav Petkov wrote:
>>> There was an earlier report about this here:
>>> https://lore.kernel.org/all/78e0d19c-b77a-4169-a80f-2eef91f4a1d6@gmail.com/
>> Am looking at the whole thing. Stay tuned...
> 
> Something like this, I guess...
> 
> Execution goes off somewhere into the weeds during alternatives patching
> of the return thunk while it tries to warn about it in the alternatives
> code itself and it all ends up in an endless INT3 exceptions due to our
> speculation blockers everywhere...
> 
> I could chase it as to why exactly but the warning is there for all
> those mitigations which need a special return thunk and 32-bit doesn't
> need them (and at least the AMD untraining sequences are 64-bit only
> so...).

Erhard Furtner, did you try if this helps for a kernel with
MITIGATION_RETHUNK=y? Klara Modin, or could you give it a try?

Without a check this is unlikely to be merged and then more people might
run into problems like you two did.

Ciao, Thorsten
> IOW:
> 
> 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)
>