linux-kernel - Re: [PATCH 3/7] x86/cpu: Disable kernel LASS when patching kernel alternatives

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Date:   Wed, 1 Feb 2023 10:20:33 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sohil Mehta <sohil.mehta@...el.com>,
        "Chen, Yian" <yian.chen@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Paul Lai <paul.c.lai@...el.com>
Subject: Re: [PATCH 3/7] x86/cpu: Disable kernel LASS when patching kernel
 alternatives

On 1/31/23 18:25, Sohil Mehta wrote:
>> 	/*
>> 	 * Set cr4 to a known state:
>> 	 *  - physical address extension enabled
>> 	 *  - 5-level paging, if it was enabled before
>> 	 */
>> 	movl	$X86_CR4_PAE, %eax
>> 	testq	$X86_CR4_LA57, %r13
>> 	jz	1f
>> 	orl	$X86_CR4_LA57, %eax
>> 1:
>> 	movq	%rax, %cr4
>>
>> 	jmp 1f
>> 1:
> Dave, does this address your concern or were you looking for something
> else? Is there some path other than kexec that should also be audited
> for this scenario?

Yep, that addresses it.  I don't know of any other path that would
matter.  Couldn't hurt to poke around and look for other CR4
manipulation that might need to be LASS-aware, though.