Re: [PATCH] x86: enable Data Operand Independent Timing Mode

[Roxana Bradescu]

> On Feb 3, 2023, at 10:25 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> 
> BTW, I'm basically moving forward assuming that we're going to apply
> this patch in _some_ form.  I'm going to make some changes, but I'll
> discuss them in this thread to make sure we're all on the same page first.

Just checking in on the changes mentioned here.

> On 1/24/23 17:28, Eric Biggers wrote:
>> +Affected CPUs
>> +-------------
>> +
>> +This vulnerability affects Intel Core family processors based on the Ice Lake
>> +and later microarchitectures, and Intel Atom family processors based on the
>> +Gracemont and later microarchitectures.  For more information, see Intel's
>> +documentation [1]_.
> 
> I had a hard time following the docs in this area.
> 
> But I'm not sure this statement is correct.  The docs actually say:
> 
> 	For Intel® Core™ family processors based on microarchitectures
> 	before Ice Lake and Intel Atom® family processors based on
> 	microarchitectures before Gracemont that do not enumerate
> 	IA32_UARCH_MISC_CTL, developers may assume that the instructions
> 	listed here operate as if DOITM is enabled.

Have we been able to clarify if this assumption is guaranteed?

> 
> A processor needs to be before "Ice Lake" and friends *AND* not
> enumerate IA32_UARCH_MISC_CTL to be unaffected.
> 
> There's also another tweak that's needed because:
> 
> 	Processors that do not enumerate IA32_ARCH_CAPABILITIES[DOITM]
> 	when the latest microcode is applied do not need to set
> 	IA32_UARCH_MISC_CTL [DOITM] in order to have the behavior
> 	described in this document...
> 
> First, we need to mention the "latest microcode" thing in the kernel
> docs.  I also _think_ the whole "microarchitectures before" stuff is
> rather irrelevant and we can simplify this down to:
> 
> 	This vulnerability affects all Intel processors that support
> 	MSR_IA32_ARCH_CAPABILITIES and set MSR_IA32_ARCH_CAPABILITIES[DOITM]
> 	when the latest microcode is applied.
> 

Certainly a lot cleaner. Would be great if the Intel docs reflected this.

—
Regards, Roxana


> Which reminds me.  This:
> 
>> +void update_doitm_msr(void)
>> +{
>> +	u64 msr;
>> +
>> +	if (doitm_off)
>> +		return;
>> +
>> +	rdmsrl(MSR_IA32_UARCH_MISC_CTL, msr);
>> +	wrmsrl(MSR_IA32_UARCH_MISC_CTL, msr | UARCH_MISC_DOITM);
>> +}
> 
> should probably be:
> 
> void update_doitm_msr(void)
> {
> 	u64 msr;
> 
> 	/*
> 	 * All processors that enumerate support for DOIT
> 	 * are affected *and* have the mitigation available.
> 	 */
> 	if (!boot_cpu_has_bug(X86_BUG_DODT))
> 		return;
> 
> 	rdmsrl(MSR_IA32_UARCH_MISC_CTL, msr);
> 	if (doitm_off)
> 		msr &= ~UARCH_MISC_DOITM;
> 	else
> 		msr |= UARCH_MISC_DOITM;
> 	wrmsrl(MSR_IA32_UARCH_MISC_CTL, msr);
> }
> 
> in case the CPU isn't actually coming out of reset, like if kexec() left
> DOITM=1.
>

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