Re: [bisected] Kernel v6.9-rc3 fails to boot on a Thinkpad T60 with MITIGATION_RETHUNK=y (regression from v6.8.5)

[Thorsten Leemhuis]

On 17.04.24 10:38, Linux regression tracking (Thorsten Leemhuis) wrote:
> 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@xxxxxxxxx/
>>> 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.

Ignore that, I only not noticed the discussion continued in the other
thread and Klara Modin already provided a tested-by. Sorry for the noise.

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)
>>