linux-kernel - Re: [PATCHv4, REBASED 3/4] x86/64/kexec: Map original relocate_kernel() in init_transition

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Message-ID: <20241031142324.GJZyOS3G62E3pn9ZJ-@fat_crate.local>
Date: Thu, 31 Oct 2024 15:23:24 +0100
From: Borislav Petkov <bp@...en8.de>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, Baoquan He <bhe@...hat.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Tom Lendacky <thomas.lendacky@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Sean Christopherson <seanjc@...gle.com>,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCHv4, REBASED 3/4] x86/64/kexec: Map original
 relocate_kernel() in init_transition_pgtable()

On Wed, Oct 16, 2024 at 02:14:57PM +0300, Kirill A. Shutemov wrote:
> The init_transition_pgtable() function sets up transitional page tables.
> It ensures that the relocate_kernel() function is present in the
> identity mapping at the same location as in the kernel page tables.
> relocate_kernel() switches to the identity mapping, and the function
> must be present at the same location in the virtual address space before
> and after switching page tables.
> 
> init_transition_pgtable() maps a copy of relocate_kernel() in
> image->control_code_page at the relocate_kernel() virtual address, but
> the original physical address of relocate_kernel() would also work.
> 
> It is safe to use original relocate_kernel() physical address cannot be
					^^^^^^^^^^^^^^^

something went missing here in that sentence. Reads weird.

> overwritten until swap_pages() is called, and the relocate_kernel()
> virtual address will not be used by then.

...

> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 9c9ac606893e..645690e81c2d 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -157,7 +157,7 @@ static int init_transition_pgtable(struct kimage *image, pgd_t *pgd)
>  	pte_t *pte;
>  
>  	vaddr = (unsigned long)relocate_kernel;
> -	paddr = __pa(page_address(image->control_code_page)+PAGE_SIZE);
> +	paddr = __pa(relocate_kernel);
>  	pgd += pgd_index(vaddr);
>  	if (!pgd_present(*pgd)) {
>  		p4d = (p4d_t *)get_zeroed_page(GFP_KERNEL);

Such changes always make me nervous so I'd queue them only after this merge
window is over so that they can get maximal testing in next. Unless someone
objects...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette