linux-kernel - Re: [RFC PATCH v0 3/6] x86: Enable Upper Address Ignore(UAI) feature

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Date:   Thu, 10 Mar 2022 19:46:59 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Bharata B Rao <bharata@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "ananth.narayan@....com" <ananth.narayan@....com>,
        "Andrew Cooper" <Andrew.Cooper3@...rix.com>
Subject: Re: [RFC PATCH v0 3/6] x86: Enable Upper Address Ignore(UAI) feature

On 10/03/2022 11:15, Bharata B Rao wrote:
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index f7a132eb794d..12615b1b4af5 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -740,6 +740,12 @@ dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
>  	return 0;
>  }
>  
> +static inline void __init uai_enable(void)
> +{
> +	if (boot_cpu_has(X86_FEATURE_UAI))
> +		msr_set_bit(MSR_EFER, _EFER_UAI);
> +}
> +
>  /*
>   * Determine if we were loaded by an EFI loader.  If so, then we have also been
>   * passed the efi memmap, systab, etc., so we should use these data structures
> @@ -1146,6 +1152,8 @@ void __init setup_arch(char **cmdline_p)
>  
>  	x86_init.paging.pagetable_init();
>  
> +	uai_enable();

I would think incredibly carefully before enabling UAI by default.

Suffice it to say that Intel were talked down from 7 bits to 6, and
apparently AMD didn't get the same memo from the original requesters.

The problem is that UAI + LA57 means that all the poison pointers cease
functioning as a defence-in-depth mechanism, and become legal pointers
pointing at random positions in user or kernel space.

~Andrew