linux-hardening - Re: [PATCH v4 17/26] arm64: head: populate kernel page tables with MMU and caches on

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Date:   Fri, 24 Jun 2022 13:56:31 +0100
From:   Will Deacon <will@...nel.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        linux-hardening@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Brown <broonie@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v4 17/26] arm64: head: populate kernel page tables with
 MMU and caches on

On Mon, Jun 13, 2022 at 04:45:41PM +0200, Ard Biesheuvel wrote:
> Now that we can access the entire kernel image via the ID map, we can
> execute the page table population code with the MMU and caches enabled.
> The only thing we need to ensure is that translations via TTBR1 remain
> disabled while we are updating the page tables the second time around,
> in case KASLR wants them to be randomized.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/arm64/kernel/head.S | 62 +++++---------------
>  1 file changed, 16 insertions(+), 46 deletions(-)
> 
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index d704d0bd8ffc..583cbea865e1 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -85,8 +85,6 @@
>  	 *  x21        primary_entry() .. start_kernel()        FDT pointer passed at boot in x0
>  	 *  x22        create_idmap() .. start_kernel()         ID map VA of the DT blob
>  	 *  x23        primary_entry() .. start_kernel()        physical misalignment/KASLR offset
> -	 *  x28        clear_page_tables()                      callee preserved temp register
> -	 *  x19/x20    __primary_switch()                       callee preserved temp registers
>  	 *  x24        __primary_switch() .. relocate_kernel()  current RELR displacement
>  	 *  x28        create_idmap()                           callee preserved temp register
>  	 */
> @@ -96,9 +94,7 @@ SYM_CODE_START(primary_entry)
>  	adrp	x23, __PHYS_OFFSET
>  	and	x23, x23, MIN_KIMG_ALIGN - 1	// KASLR offset, defaults to 0
>  	bl	set_cpu_boot_mode_flag
> -	bl	clear_page_tables
>  	bl	create_idmap
> -	bl	create_kernel_mapping
>  
>  	/*
>  	 * The following calls CPU setup code, see arch/arm64/mm/proc.S for
> @@ -128,32 +124,14 @@ SYM_CODE_START_LOCAL(preserve_boot_args)
>  SYM_CODE_END(preserve_boot_args)
>  
>  SYM_FUNC_START_LOCAL(clear_page_tables)
> -	mov	x28, lr
> -
> -	/*
> -	 * Invalidate the init page tables to avoid potential dirty cache lines
> -	 * being evicted. Other page tables are allocated in rodata as part of
> -	 * the kernel image, and thus are clean to the PoC per the boot
> -	 * protocol.
> -	 */
> -	adrp	x0, init_pg_dir
> -	adrp	x1, init_pg_end
> -	bl	dcache_inval_poc
> -
>  	/*
>  	 * Clear the init page tables.
>  	 */
>  	adrp	x0, init_pg_dir
>  	adrp	x1, init_pg_end
> -	sub	x1, x1, x0
> -1:	stp	xzr, xzr, [x0], #16
> -	stp	xzr, xzr, [x0], #16
> -	stp	xzr, xzr, [x0], #16
> -	stp	xzr, xzr, [x0], #16
> -	subs	x1, x1, #64
> -	b.ne	1b
> -
> -	ret	x28
> +	sub	x2, x1, x0
> +	mov	x1, xzr
> +	b	__pi_memset			// tail call
>  SYM_FUNC_END(clear_page_tables)
>  
>  /*
> @@ -399,16 +377,8 @@ SYM_FUNC_START_LOCAL(create_kernel_mapping)
>  
>  	map_memory x0, x1, x5, x6, x7, x3, (VA_BITS - PGDIR_SHIFT), x10, x11, x12, x13, x14
>  
> -	/*
> -	 * Since the page tables have been populated with non-cacheable
> -	 * accesses (MMU disabled), invalidate those tables again to
> -	 * remove any speculatively loaded cache lines.
> -	 */
> -	dmb	sy
> -
> -	adrp	x0, init_pg_dir
> -	adrp	x1, init_pg_end
> -	b	dcache_inval_poc		// tail call
> +	dsb	ishst				// sync with page table walker
> +	ret
>  SYM_FUNC_END(create_kernel_mapping)
>  
>  	/*
> @@ -863,14 +833,15 @@ SYM_FUNC_END(__relocate_kernel)
>  #endif
>  
>  SYM_FUNC_START_LOCAL(__primary_switch)
> -#ifdef CONFIG_RANDOMIZE_BASE
> -	mov	x19, x0				// preserve new SCTLR_EL1 value
> -	mrs	x20, sctlr_el1			// preserve old SCTLR_EL1 value
> -#endif
> -
> -	adrp	x1, init_pg_dir
> +	adrp	x1, reserved_pg_dir
>  	adrp	x2, init_idmap_pg_dir
>  	bl	__enable_mmu
> +
> +	bl	clear_page_tables
> +	bl	create_kernel_mapping
> +
> +	adrp	x1, init_pg_dir
> +	load_ttbr1 x1, x1, x2
>  #ifdef CONFIG_RELOCATABLE
>  #ifdef CONFIG_RELR
>  	mov	x24, #0				// no RELR displacement yet
> @@ -886,9 +857,8 @@ SYM_FUNC_START_LOCAL(__primary_switch)
>  	 * to take into account by discarding the current kernel mapping and
>  	 * creating a new one.
>  	 */
> -	pre_disable_mmu_workaround
> -	msr	sctlr_el1, x20			// disable the MMU
> -	isb
> +	adrp	x1, reserved_pg_dir		// Disable translations via TTBR1
> +	load_ttbr1 x1, x1, x2

I'd have thought we'd need some TLB maintenance here... is that not the
case?

Also, it might be a tiny bit easier to clear EPD1 instead of using the
reserved_pg_dir.

Will