Re: [PATCH 02/21] x86/mm/asi: add X86_FEATURE_ASI and asi=

[Borislav Petkov]

On Wed, Sep 24, 2025 at 02:59:37PM +0000, Brendan Jackman wrote:
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 6c42061ca20e581b5192b66c6f25aba38d4f8ff8..9b8330fc1fe31721af39b08b58b729ced78ba803 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5324,6 +5324,14 @@
>  
>  			Not specifying this option is equivalent to pti=auto.
>  
> +	asi=		[X86-64] Control Address Space Isolation (ASI), a
> +			technology for mitigating CPU vulnerabilities.

> 			ASI is
> +			not yet ready to provide security guarantees but can be
> +			enabled for evaluation.

Yeah, no need for such "temporary" statements in the help text since you're
going to have to touch it again once it becomes a full-fledged feature.

> +			on   - unconditionally enable
> +			off  - unconditionally disable

"unconditionally" as opposed to some other setting which is conditional?

> +
>  	pty.legacy_count=
>  			[KNL] Number of legacy pty's. Overwrites compiled-in
>  			default number.
> diff --git a/arch/x86/include/asm/asi.h b/arch/x86/include/asm/asi.h
> index 53acdf22fe33efc6ccedbae52b262a904868459a..32a4c04c4be0f6f425c7cbcff4c58f1827a4b4c4 100644
> --- a/arch/x86/include/asm/asi.h
> +++ b/arch/x86/include/asm/asi.h
> @@ -2,4 +2,14 @@
>  #ifndef _ASM_X86_ASI_H
>  #define _ASM_X86_ASI_H
>  
> +#include <asm/cpufeature.h>
> +
> +void asi_check_boottime_disable(void);
> +
> +/* Helper for generic code. Arch code just uses cpu_feature_enabled(). */
> +static inline bool asi_enabled_static(void)

"static" because? There will be a dynamic one too?

> +{
> +	return cpu_feature_enabled(X86_FEATURE_ASI);
> +}
> +
>  #endif /* _ASM_X86_ASI_H */
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 4091a776e37aaed67ca93b0a0cd23cc25dbc33d4..3eee24a4cabf3b2131c34596236d8bc8eec05b3b 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -499,6 +499,7 @@
>  #define X86_FEATURE_IBPB_EXIT_TO_USER	(21*32+14) /* Use IBPB on exit-to-userspace, see VMSCAPE bug */
>  #define X86_FEATURE_ABMC		(21*32+15) /* Assignable Bandwidth Monitoring Counters */
>  #define X86_FEATURE_MSR_IMM		(21*32+16) /* MSR immediate form instructions */
> +#define X86_FEATURE_ASI			(21*32+17) /* Kernel Address Space Isolation */

I think we really will need to show this in /proc/cpuinfo as it is a real, big
feature which gets proper kernel glue vs some silly CPUID bit.

IOW,

#define X86_FEATURE_ASI			(21*32+17) /* "asi" Kernel Address Space Isolation */
						      ^^^^

Not sure, though, when we should make it an ABI - perhaps once the whole pile
has landed...

>  /*
>   * BUG word(s)
> diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
> index 5b9908f13dcfd092897f3778ee56ea4d45bdb868..5ecbff70964f61a903ac96cec3736a7cec1221fd 100644
> --- a/arch/x86/mm/Makefile
> +++ b/arch/x86/mm/Makefile
> @@ -52,6 +52,7 @@ obj-$(CONFIG_ACPI_NUMA)		+= srat.o
>  obj-$(CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS)	+= pkeys.o
>  obj-$(CONFIG_RANDOMIZE_MEMORY)			+= kaslr.o
>  obj-$(CONFIG_MITIGATION_PAGE_TABLE_ISOLATION)	+= pti.o
> +obj-$(CONFIG_MITIGATION_ADDRESS_SPACE_ISOLATION)	+= asi.o
>  
>  obj-$(CONFIG_X86_MEM_ENCRYPT)	+= mem_encrypt.o
>  obj-$(CONFIG_AMD_MEM_ENCRYPT)	+= mem_encrypt_amd.o
> diff --git a/arch/x86/mm/asi.c b/arch/x86/mm/asi.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..8c907f3c84f43f66e412ecbfa99e67390d31a66f
> --- /dev/null
> +++ b/arch/x86/mm/asi.c
> @@ -0,0 +1,28 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/asi.h>
> +#include <linux/init.h>
> +#include <linux/string.h>
> +
> +#include <asm/cmdline.h>
> +#include <asm/cpufeature.h>
> +
> +void __init asi_check_boottime_disable(void)
> +{
> +	bool enabled = false;
> +	char arg[4];
> +	int ret;
> +
> +	ret = cmdline_find_option(boot_command_line, "asi", arg, sizeof(arg));
> +	if (ret == 3 && !strncmp(arg, "off", 3)) {
> +		enabled = false;
> +		pr_info("ASI explicitly disabled by kernel cmdline.\n");
> +	} else if (ret == 2 && !strncmp(arg, "on", 2)) {
> +		enabled = true;
> +		pr_info("ASI enabled.\n");

I'm not sure about those pr_info()s. When it is disabled, you can clear
X86_FEATURE_ASI so you won't see it in /proc/cpuinfo and then it is disabled.
And the same when it is enabled.

> +	} else if (ret) {
> +		pr_err("Unknown asi= flag '%s', try 'off' or 'on'\n", arg);
> +	}
> +
> +	if (enabled)
> +		setup_force_cpu_cap(X86_FEATURE_ASI);
> +}

Not an early_param() ?

-- 
Regards/Gruss,
    Boris.

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