linux-kernel - Re: [PATCH 1/3] x86/cpu: Actually turn off mitigations by default for SPECULATION

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Message-ID: <87edb9d33r.fsf@mail.lhotse>
Date: Sat, 13 Apr 2024 19:27:36 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Stephen Rothwell <sfr@...b.auug.org.au>, Sean Christopherson
 <seanjc@...gle.com>
Cc: Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave
 Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, Peter Zijlstra
 <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, Pawan Gupta
 <pawan.kumar.gupta@...ux.intel.com>, Daniel Sneddon
 <daniel.sneddon@...ux.intel.com>, linuxppc-dev@...ts.ozlabs.org,
 linux-arch@...r.kernel.org, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>, Heiko Carstens <hca@...ux.ibm.com>
Subject: Re: [PATCH 1/3] x86/cpu: Actually turn off mitigations by default
 for SPECULATION_MITIGATIONS=n

Stephen Rothwell <sfr@...b.auug.org.au> writes:
> Hi Sean,
>
> I noticed this commit in linux-next.
>
> On Tue,  9 Apr 2024 10:51:05 -0700 Sean Christopherson <seanjc@...gle.com> wrote:
>>
>> Initialize cpu_mitigations to CPU_MITIGATIONS_OFF if the kernel is built
>> with CONFIG_SPECULATION_MITIGATIONS=n, as the help text quite clearly
>> states that disabling SPECULATION_MITIGATIONS is supposed to turn off all
>> mitigations by default.
>> 
>>   │ If you say N, all mitigations will be disabled. You really
>>   │ should know what you are doing to say so.
>> 
>> As is, the kernel still defaults to CPU_MITIGATIONS_AUTO, which results in
>> some mitigations being enabled in spite of SPECULATION_MITIGATIONS=n.
>> 
>> Fixes: f43b9876e857 ("x86/retbleed: Add fine grained Kconfig knobs")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>> ---
>>  kernel/cpu.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/kernel/cpu.c b/kernel/cpu.c
>> index 8f6affd051f7..07ad53b7f119 100644
>> --- a/kernel/cpu.c
>> +++ b/kernel/cpu.c
>> @@ -3207,7 +3207,8 @@ enum cpu_mitigations {
>>  };
>>  
>>  static enum cpu_mitigations cpu_mitigations __ro_after_init =
>> -	CPU_MITIGATIONS_AUTO;
>> +	IS_ENABLED(CONFIG_SPECULATION_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
>> +						     CPU_MITIGATIONS_OFF;
>>  
>>  static int __init mitigations_parse_cmdline(char *arg)
>>  {
>> -- 
>> 2.44.0.478.gd926399ef9-goog
>> 
>
> I noticed because it turned off all mitigations for my PowerPC qemu
> boot tests - probably because CONFIG_SPECULATION_MITIGATIONS only
> exists in arch/x86/Kconfig ... thus for other architectures that have
> cpu mitigations, this will always default them to off, right?

Yep.

The patch has the effect of changing the default for non-x86 arches from
auto to off.

I see at least powerpc, arm64 and s390 use cpu_mitigations_off() and
will be affected.

cheers