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

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Date:   Tue, 31 Jan 2023 18:25:04 -0800
From:   Sohil Mehta <sohil.mehta@...el.com>
To:     Dave Hansen <dave.hansen@...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 1/12/2023 10:48 AM, Dave Hansen wrote:
> Also, Andy Cooper made a very good point: when the kernel enables
> paging, it's running with a low address so that the instruction pointer
> stays valid as paging becomes enabled.
> 
> But, if LASS were enabled and enforced at that point, you'd get a LASS
> fault and go kablooey.  Can you see what simics does in that case, and
> also make sure we're clearing CR4.LASS when enabling paging?  That would
> obviate the need to do it later in C code too.

CR4 and CR0 are always restored to a known state during kexec. So,
running with LASS enabled should not happen during early boot.

machine_kexec()
-> relocate_kernel()
   -> identity_mapped()

> 	/*
> 	 * Set cr0 to a known state:
> 	 *  - Paging enabled
> 	 *  - Alignment check disabled
> 	 *  - Write protect disabled
> 	 *  - No task switch
> 	 *  - Don't do FP software emulation.
> 	 *  - Protected mode enabled
> 	 */
> 	movq	%cr0, %rax
> 	andq	$~(X86_CR0_AM | X86_CR0_WP | X86_CR0_TS | X86_CR0_EM), %rax
> 	orl	$(X86_CR0_PG | X86_CR0_PE), %eax
> 	movq	%rax, %cr0
> 
> 	/*
> 	 * 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?