linux-kernel - Re: [PATCH v5 14/16] x86/cpu: Make LAM depend on LASS

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Message-ID: <9e0645cd-9162-44fa-8fa6-971bb08fa755@intel.com>
Date: Wed, 30 Oct 2024 17:06:25 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Andy Lutomirski
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Subject: Re: [PATCH v5 14/16] x86/cpu: Make LAM depend on LASS

On 10/28/2024 9:08 AM, Alexander Shishkin wrote:
> 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>
> ---
>  arch/x86/kernel/cpu/cpuid-deps.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
> index 3f73c4b03348..d9fb2423605e 100644
> --- a/arch/x86/kernel/cpu/cpuid-deps.c
> +++ b/arch/x86/kernel/cpu/cpuid-deps.c
> @@ -84,6 +84,7 @@ static const struct cpuid_dep cpuid_deps[] = {
>  	{ X86_FEATURE_SHSTK,			X86_FEATURE_XSAVES    },
>  	{ X86_FEATURE_FRED,			X86_FEATURE_LKGS      },
>  	{ X86_FEATURE_LASS,			X86_FEATURE_SMAP      },
> +	{ X86_FEATURE_LAM,			X86_FEATURE_LASS      },

The dependencies listed in cpuid_deps[] are only enforced when a feature
such as LASS is explicitly disabled. If the system is missing LASS at
boot then LAM would still be enabled.

We would need this patch to enforce it:
https://lore.kernel.org/lkml/20241030233118.615493-1-sohil.mehta@intel.com/

Sohil