Re: [PATCH] x86/cpu: Avoid writing MSR_IA32_TSX_CTRL when writing it is not supported

[Andrew Cooper]

On 08/09/2022 02:03, Pawan Gupta wrote:
> On Tue, Sep 06, 2022 at 11:00:08PM +0000, Andrew Cooper wrote:
>> On 06/09/2022 22:00, Peter Zijlstra wrote:
>>> On Tue, Sep 06, 2022 at 10:56:47PM +0200, Hans de Goede wrote:
>>>> Hi,
>>>>
>>>> On 9/6/22 22:43, Peter Zijlstra wrote:
>>>>> On Tue, Sep 06, 2022 at 10:17:43PM +0200, Hans de Goede wrote:
>>>>>> On an Intel Atom N2600 (and presumable other Cedar Trail models)
>>>>>> MSR_IA32_TSX_CTRL can be read, causing saved_msr.valid to be set for it
>>>>>> by msr_build_context().
>>>>>>
>>>>>> This causes restore_processor_state() to try and restore it, but writing
>>>>>> this MSR is not allowed on the Intel Atom N2600 leading to:
>>>>> FWIW, virt tends to do this same thing a lot. They'll allow reading
>>>>> random MSRs and only fail on write.
>>>> Right. So I guess I should send a v2 with an updated commit
>>>> message mentioning this ?
>>> Nah, just saying this is a somewhat common pattern with MSRs.
>>>
>>> The best ones are the one where writing the value read is invalid :/ or
>>> those who also silently eat a 0 write just for giggles. Luckily that
>>> doesn't happen often.
>> Several comments.  First of all, MSR_TSX_CTRL is a fully read/write
>> MSR.  If virt is doing this wrong, fix the hypervisor.  But this doesn't
>> look virt related?
>>
>> More importantly, MSR_TSX_CTRL does not plausibly exist on an Atom
>> N2600, as it is more than a decade old.
>>
>> MSR_TSX_CTRL was retrofitted in microcode to the MDS_NO, TAA-vulnerable
>> CPUs which is a very narrow range from about 1 quarter of 2019 which
>> includes Cascade Lake, and then included architecturally on subsequent
>> parts which support TSX.
>>
>> pm_save_spec_msr() is totally broken.  It's poking MSRs blindly without
>> checking the enumeration of the capability first.
> pm_save_spec_msr() relies on valid-msr-check in build_msr_context(), but
> obviously it is not working in this particular case.
>
> Does adding the enumeration check as below looks okay:
>
> (I am not sure if I got the enumeration right for MSR_AMD64_LS_CFG).

family >= 0x10 && family <= 0x18

>
> ---
> diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
> index 8cbf623f0ecf..a750c1a1964b 100644
> --- a/arch/x86/include/asm/cpu.h
> +++ b/arch/x86/include/asm/cpu.h
> @@ -76,6 +76,8 @@ static inline void init_ia32_feat_ctl(struct cpuinfo_x86 *c) {}
>  
>  extern __noendbr void cet_disable(void);
>  
> +extern bool spec_msr_valid(u32 msr_id);
> +
>  struct ucode_cpu_info;
>  
>  int intel_cpu_collect_info(struct ucode_cpu_info *uci);
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 3e508f239098..7430a36fd7ae 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1278,6 +1278,26 @@ static bool __init cpu_matches(const struct x86_cpu_id *table, unsigned long whi
>  	return m && !!(m->driver_data & which);
>  }
>  
> +bool spec_msr_valid(u32 msr_id)
> +{
> +	u64 ia32_cap = x86_read_arch_cap_msr();
> +
> +	switch (msr_id) {
> +	case MSR_IA32_SPEC_CTRL:
> +		return boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL);
> +	case MSR_IA32_TSX_CTRL:
> +		return !!(ia32_cap & ARCH_CAP_TSX_CTRL_MSR);
> +	case MSR_TSX_FORCE_ABORT:
> +		return boot_cpu_has(X86_FEATURE_TSX_FORCE_ABORT);
> +	case MSR_IA32_MCU_OPT_CTRL:
> +		return boot_cpu_has(X86_FEATURE_SRBDS_CTRL);
> +	case MSR_AMD64_LS_CFG:
> +		return boot_cpu_has(X86_FEATURE_LS_CFG_SSBD);
> +	}
> +
> +	return false;
> +}
> +
>  u64 x86_read_arch_cap_msr(void)
>  {
>  	u64 ia32_cap = 0;
> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> index bb176c72891c..8db73f7982c7 100644
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -520,8 +520,12 @@ static void pm_save_spec_msr(void)
>  		MSR_IA32_MCU_OPT_CTRL,
>  		MSR_AMD64_LS_CFG,

Checking the enumerations is definitely an improvement, but this wants
to become a tuple list of { msr, flag } so it can't get out of sync.

Except two of the options aren't simple bits.  The contents of
MSR_ARCH_CAPS ought to become feature bits because it's a CPUID feature
leaf in disguise.

AMD LS_CFG is more complicated, because the dispatch serialising bit
needs setting unilaterally (families 0x10, 0x12 thru 0x18), but the SSBD
control ought to resolve on the next context switch.

~Andrew