linux-kernel - Re: [PATCHv8 15/17] x86/cpu: Make LAM depend on LASS

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Message-ID: <121a34a7-518c-4f2c-8e89-c6cacd02a9df@intel.com>
Date: Tue, 1 Jul 2025 16:03:45 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Andy Lutomirski
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	<brijesh.singh@....com>, Michael Roth <michael.roth@....com>, Tony Luck
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Subject: Re: [PATCHv8 15/17] x86/cpu: Make LAM depend on LASS

On 7/1/2025 2:58 AM, Kirill A. Shutemov wrote:
> From: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> 
> To prevent exploits for Spectre based on LAM as demonstrated by the
> whitepaper [1], make LAM depend on LASS, which avoids this type of
> vulnerability.
> 
> [1] https://download.vusec.net/papers/slam_sp24.pdf
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/kernel/cpu/cpuid-deps.c | 1 +
>  1 file changed, 1 insertion(+)
> 

In terms of patch ordering, wouldn't it make more sense to introduce LAM
related changes after LASS has been fully enabled? This patch should
probably be after Patch #16 which enables LASS.

Logically, the LAM re-enabling stuff can be a separate series, but since
it's only a few changed lines having it at the end seems okay.

Patch 1-15  => Enable LASS
Patch 16-17 => Re-enable LAM

Other than that,

Reviewed-by: Sohil Mehta <sohil.mehta@...el.com>

> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
> index 98d0cdd82574..11bb9ed40140 100644
> --- a/arch/x86/kernel/cpu/cpuid-deps.c
> +++ b/arch/x86/kernel/cpu/cpuid-deps.c
> @@ -90,6 +90,7 @@ static const struct cpuid_dep cpuid_deps[] = {
>  	{ X86_FEATURE_FRED,			X86_FEATURE_LKGS      },
>  	{ X86_FEATURE_SPEC_CTRL_SSBD,		X86_FEATURE_SPEC_CTRL },
>  	{ X86_FEATURE_LASS,			X86_FEATURE_SMAP      },
> +	{ X86_FEATURE_LAM,			X86_FEATURE_LASS      },
>  	{}
>  };
>