Re: [PATCH v2] x86/bugs: Explicitly clear speculative MSR bits

[Borislav Petkov]

On Thu, Nov 24, 2022 at 02:46:50AM -0800, Breno Leitao wrote:
> Currently x86_spec_ctrl_base is read at boot time, and speculative bits
> are set if configs are enable, such as MSR[SPEC_CTRL_IBRS] is enabled if
> CONFIG_CPU_IBRS_ENTRY is configured. These MSR bits are not cleared if
> the mitigations are disabled.
> 
> This is a problem when kexec-ing a kernel that has the mitigation
> disabled, from a kernel that has the mitigation enabled. In this case,
> the MSR bits are carried forward and not cleared at the boot of the new
> kernel. This might have some performance degradation that is hard to
> find.
> 
> This problem does not happen if the machine is (hard) rebooted, because
> the bit will be cleared by default.
> 
> This patch also defines a SPEC_CTRL_MASK macro, so, we can easily track
> and clear if eventually some new mitigation shows up.

Just remove that sentence - the macro's function is kinda obvious from
the diff itself.

> Suggested-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/msr-index.h | 3 +++
>  arch/x86/kernel/cpu/bugs.c       | 9 ++++++++-
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 4a2af82553e4..704f49580ee1 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -54,6 +54,9 @@
>  #define SPEC_CTRL_RRSBA_DIS_S_SHIFT	6	   /* Disable RRSBA behavior */
>  #define SPEC_CTRL_RRSBA_DIS_S		BIT(SPEC_CTRL_RRSBA_DIS_S_SHIFT)
>  
> +#define SPEC_CTRL_MASK			(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD \
> +							| SPEC_CTRL_RRSBA_DIS_S)

Call that SPEC_CTRL_MITIGATIONS_MASK or so to denote what it is - a
mask of the SPEC_CTRL bits which the kernel toggles when controlling
mitigations.

A comment above it wouldn't hurt either.

> +
>  #define MSR_IA32_PRED_CMD		0x00000049 /* Prediction Command */
>  #define PRED_CMD_IBPB			BIT(0)	   /* Indirect Branch Prediction Barrier */
>  
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 3e3230cccaa7..88957da1029b 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -137,8 +137,15 @@ void __init check_bugs(void)
>  	 * have unknown values. AMD64_LS_CFG MSR is cached in the early AMD
>  	 * init code as it is not enumerated and depends on the family.
>  	 */
> -	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
> +	if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
>  		rdmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> +		/*
> +		 * Previously running software may have some controls turned ON.

"Previously running software, like kexec for example, ..."

> +		 * Clear them and let kernel decide which controls to use.

"Clear them and let the mitigations setup below set them based on configuration."

> +		 */
> +		x86_spec_ctrl_base &= ~SPEC_CTRL_MASK;
> +		wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);

So this WRMSR will happen on the BSP only but the SPEC_CTRL MSR is
per-CPU. As is x86_spec_ctrl_current which tracks it.

So I'd say you don't need that WRMSR here - the proper value will get
replicated eventually everywhere...

Thx.

-- 
Regards/Gruss,
    Boris.

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