linux-kernel - Re: [PATCH v4 02/11] x86/startup_64: Replace pointer fixups with RIP-relative references

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Date: Sat, 17 Feb 2024 13:51:02 +0100
From: Borislav Petkov <bp@...en8.de>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
	Kevin Loughlin <kevinloughlin@...gle.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	Dionna Glaze <dionnaglaze@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Kees Cook <keescook@...omium.org>, Brian Gerst <brgerst@...il.com>,
	linux-arch@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v4 02/11] x86/startup_64: Replace pointer fixups with
 RIP-relative references

On Tue, Feb 13, 2024 at 01:41:46PM +0100, Ard Biesheuvel wrote:
> @@ -201,25 +201,19 @@ unsigned long __head __startup_64(unsigned long physaddr,
>  	load_delta += sme_get_me_mask();
>  
>  	/* Fixup the physical addresses in the page table */
> -
> -	pgd = fixup_pointer(early_top_pgt, physaddr);
> -	p = pgd + pgd_index(__START_KERNEL_map);
> -	if (la57)
> -		*p = (unsigned long)level4_kernel_pgt;
> -	else
> -		*p = (unsigned long)level3_kernel_pgt;
> -	*p += _PAGE_TABLE_NOENC - __START_KERNEL_map + load_delta;
> -
>  	if (la57) {
> -		p4d = fixup_pointer(level4_kernel_pgt, physaddr);
> -		p4d[511] += load_delta;
> +		p4d = (p4dval_t *)&RIP_REL_REF(level4_kernel_pgt);
> +		p4d[MAX_PTRS_PER_P4D - 1] += load_delta;
>  	}
>  
> -	pud = fixup_pointer(level3_kernel_pgt, physaddr);
> -	pud[510] += load_delta;
> -	pud[511] += load_delta;
> +	pud = &RIP_REL_REF(level3_kernel_pgt)->pud;
> +	pud[PTRS_PER_PUD - 2] += load_delta;
> +	pud[PTRS_PER_PUD - 1] += load_delta;
> +
> +	pgd = &RIP_REL_REF(early_top_pgt)->pgd;

Let's do the pgd assignment above, where it was so that we have that
natural order of p4d -> pgd -> pud ->pmd etc manipulations.

> +	pgd[PTRS_PER_PGD - 1] = (pgdval_t)(la57 ? p4d : pud) | _PAGE_TABLE_NOENC;

I see what you mean with pgd_index(__START_KERNEL_map) always being 511
but this:

	pgd[pgd_index(__START_KERNEL_map)] = (pgdval_t)(la57 ? p4d : pud) | _PAGE_TABLE_NOENC;

says exactly what gets mapped there in the pagetable while

	PTRS_PER_PGD - 1

makes me wonder what's that last pud supposed to map.

Other than that, my gut feeling right now is, this would need extensive
testing so that we make sure there's no fallout from it.

Thx.

-- 
Regards/Gruss,
    Boris.

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