linux-kernel - Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB

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Message-ID: <31c52131-5f7a-8af0-3092-5fc9e322a734@amazon.com>
Date:   Sun, 21 Jan 2018 19:29:43 +0100
From:   KarimAllah Ahmed <karahmed@...zon.com>
To:     Borislav Petkov <bp@...en8.de>, David Woodhouse <dwmw@...zon.co.uk>
CC:     <arjan@...ux.intel.com>, <tglx@...utronix.de>,
        <karahmed@...zon.de>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>, <tim.c.chen@...ux.intel.com>,
        <peterz@...radead.org>, <pbonzini@...hat.com>,
        <ak@...ux.intel.com>, <torvalds@...ux-foundation.org>,
        <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB

On 01/21/2018 07:06 PM, Borislav Petkov wrote:

> On Sun, Jan 21, 2018 at 09:49:06AM +0000, David Woodhouse wrote:
>> From: Thomas Gleixner <tglx@...utronix.de>
>>
>> Expose indirect_branch_prediction_barrier() for use in subsequent patches.
>>
>> [karahmed: remove the special-casing of skylake for using IBPB (wtf?),
>>             switch to using ALTERNATIVES instead of static_cpu_has]
>> [dwmw2:    set up ax/cx/dx in the asm too so it gets NOP'd out]
>>
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>> Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>
>> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
>> ---
>>   arch/x86/include/asm/cpufeatures.h   |  1 +
>>   arch/x86/include/asm/nospec-branch.h | 16 ++++++++++++++++
>>   arch/x86/kernel/cpu/bugs.c           |  7 +++++++
>>   3 files changed, 24 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
>> index 8c9e5c0..cf28399 100644
>> --- a/arch/x86/include/asm/cpufeatures.h
>> +++ b/arch/x86/include/asm/cpufeatures.h
>> @@ -207,6 +207,7 @@
>>   #define X86_FEATURE_RETPOLINE_AMD	( 7*32+13) /* AMD Retpoline mitigation for Spectre variant 2 */
>>   #define X86_FEATURE_INTEL_PPIN		( 7*32+14) /* Intel Processor Inventory Number */
>>   
>> +#define X86_FEATURE_IBPB		( 7*32+16) /* Using Indirect Branch Prediction Barrier */
> Right, and as AMD has a separate bit for this in CPUID_80000008_EBX[12],
> we probably don't really need the synthetic bit here but simply use the
> one at (13*32+12) - word 13.
>
>>   #define X86_FEATURE_AMD_PRED_CMD	( 7*32+17) /* Prediction Command MSR (AMD) */
>>   #define X86_FEATURE_MBA			( 7*32+18) /* Memory Bandwidth Allocation */
>>   #define X86_FEATURE_RSB_CTXSW		( 7*32+19) /* Fill RSB on context switches */
>> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
>> index 4ad4108..c333c95 100644
>> --- a/arch/x86/include/asm/nospec-branch.h
>> +++ b/arch/x86/include/asm/nospec-branch.h
>> @@ -218,5 +218,21 @@ static inline void vmexit_fill_RSB(void)
>>   #endif
>>   }
>>   
>> +static inline void indirect_branch_prediction_barrier(void)
> I like ibp_barrier() better.
>
>> +{
>> +	unsigned long ax, cx, dx;
>> +
>> +	asm volatile(ALTERNATIVE("",
>> +				 "movl %[msr], %%ecx\n\t"
>> +				 "movl %[val], %%eax\n\t"
>> +				 "movl $0, %%edx\n\t"
>> +				 "wrmsr",
>> +				 X86_FEATURE_IBPB)
>> +		     : "=a" (ax), "=c" (cx), "=d" (dx)
>> +		     : [msr] "i" (MSR_IA32_PRED_CMD),
>> +		       [val] "i" (PRED_CMD_IBPB)
>> +		     : "memory");
>> +}
> Btw, we can simplify this a bit by dropping the inputs and marking the 3
> GPRs as clobbered:
>
>          alternative_input("",
>                            "mov $0x49, %%ecx\n\t"
>                            "mov $1, %%eax\n\t"
>                            "xor %%edx, %%edx\n\t"
>                            "wrmsr\n\t",
>                            X86_FEATURE_IBPB,
>                            ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
>
>
> The "memory" clobber is probably not really needed but it wouldn't
> hurt...
>
> Also, above says:
>
>> switch to using ALTERNATIVES instead of static_cpu_has]
> Why?
>
> 	if (static_cpu_has(X86_FEATURE_IBPB))
> 		wrmsr(MSR_IA32_PRED_CMD, PRED_CMD_IBPB, 0);
>
> It can't get any more readable than this. Why even f*ck with
> alternatives?

Because static_cpu_has is an indirect branch which will cause 
speculation and
we have to avoid that.

David told me that Peter was working on a fix for static_cpu_has to 
avoid the
speculation but I do not know what is the status of this.

>
>> +
>>   #endif /* __ASSEMBLY__ */
>>   #endif /* __NOSPEC_BRANCH_H__ */
>> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>> index 390b3dc..96548ff 100644
>> --- a/arch/x86/kernel/cpu/bugs.c
>> +++ b/arch/x86/kernel/cpu/bugs.c
>> @@ -249,6 +249,13 @@ static void __init spectre_v2_select_mitigation(void)
>>   		setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
>>   		pr_info("Filling RSB on context switch\n");
>>   	}
>> +
>> +	/* Initialize Indirect Branch Prediction Barrier if supported */
>> +	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL) ||
>> +	    boot_cpu_has(X86_FEATURE_AMD_PRED_CMD)) {
>> +		setup_force_cpu_cap(X86_FEATURE_IBPB);
>> +		pr_info("Enabling Indirect Branch Prediction Barrier\n");
> We don't really need the pr_info as "ibpb" will appear in /proc/cpuinfo.
>

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