linux-hardening - Re: [PATCH v4 02/26] arm64: mm: make vabits_actual a build time constant if possible

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Date:   Tue, 14 Jun 2022 13:55:41 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-hardening@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v4 02/26] arm64: mm: make vabits_actual a build time
 constant if possible


On 6/13/22 20:15, Ard Biesheuvel wrote:
> Currently, we only support 52-bit virtual addressing on 64k pages

But going forward, will support on 4K/16K pages as well via FEAT_LPA2.

> configurations, and in all other cases, vabits_actual is guaranteed to
> equal VA_BITS (== VA_BITS_MIN). So get rid of the variable entirely in
> that case.

The change here does not really get rid of vabit_actual in those cases
either, it just makes it a build time constant AFAICS.

--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -174,7 +174,11 @@
 #include <linux/types.h>
 #include <asm/bug.h>
 
+#if VA_BITS > 48
 extern u64                     vabits_actual;
+#else
+#define vabits_actual          ((u64)VA_BITS)
+#endif

> 
> While at it, move the assignment out of the asm entry code - it has no
> need to be there.

This also changes when vabits_actual gets evaluated ? Then how would it
know, that CPU needs to be stuck in kernel (CPU_STUCK_REASON_52_BIT_VA)
in case all secondary CPUs do not support large VA feature ? Looking at
the sequence...

secondary_entry
OR
secondary_holding_pen
	secondary_startup
		__cpu_secondary_check52bitva

primary_entry
	__create_page_tables			<--- original position
	__primary_switch			 
		start_kernel
			setup_arch
				paging_init	<--- new position

It might still be possible for the secondary cpu start up sequence to
validate LVA support across the platform, but still why even send
vabits_actual evaluation down the line until paging_init(). Ideally
should not it be evaluated as early as possible during boot. Hence,
wondering - what is the real benefit here ?

> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/arm64/include/asm/memory.h |  4 ++++
>  arch/arm64/kernel/head.S        | 15 +--------------
>  arch/arm64/mm/mmu.c             | 15 ++++++++++++++-
>  3 files changed, 19 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 0af70d9abede..c751cd9b94f8 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -174,7 +174,11 @@
>  #include <linux/types.h>
>  #include <asm/bug.h>
>  
> +#if VA_BITS > 48
>  extern u64			vabits_actual;
> +#else
> +#define vabits_actual		((u64)VA_BITS)
> +#endif
>  
>  extern s64			memstart_addr;
>  /* PHYS_OFFSET - the physical address of the start of memory. */
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 1cdecce552bb..dc07858eb673 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -293,19 +293,6 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
>  	adrp	x0, idmap_pg_dir
>  	adrp	x3, __idmap_text_start		// __pa(__idmap_text_start)
>  
> -#ifdef CONFIG_ARM64_VA_BITS_52
> -	mrs_s	x6, SYS_ID_AA64MMFR2_EL1
> -	and	x6, x6, #(0xf << ID_AA64MMFR2_LVA_SHIFT)
> -	mov	x5, #52
> -	cbnz	x6, 1f
> -#endif
> -	mov	x5, #VA_BITS_MIN
> -1:
> -	adr_l	x6, vabits_actual
> -	str	x5, [x6]
> -	dmb	sy
> -	dc	ivac, x6		// Invalidate potentially stale cache line
> -
>  	/*
>  	 * VA_BITS may be too small to allow for an ID mapping to be created
>  	 * that covers system RAM if that is located sufficiently high in the
> @@ -713,7 +700,7 @@ SYM_FUNC_START(__enable_mmu)
>  SYM_FUNC_END(__enable_mmu)
>  
>  SYM_FUNC_START(__cpu_secondary_check52bitva)
> -#ifdef CONFIG_ARM64_VA_BITS_52
> +#if VA_BITS > 48

Just curious - why this is any better ? Although both (VA_BITS > 48)
and CONFIG_ARM64_VA_BITS_52 are build time constants.

>  	ldr_l	x0, vabits_actual
>  	cmp	x0, #52
>  	b.ne	2f
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 7148928e3932..17b339c1a326 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -46,8 +46,10 @@
>  u64 idmap_t0sz = TCR_T0SZ(VA_BITS_MIN);
>  u64 idmap_ptrs_per_pgd = PTRS_PER_PGD;
>  
> -u64 __section(".mmuoff.data.write") vabits_actual;
> +#if VA_BITS > 48
> +u64 vabits_actual __ro_after_init = VA_BITS_MIN;
>  EXPORT_SYMBOL(vabits_actual);
> +#endif
>  
>  u64 kimage_vaddr __ro_after_init = (u64)&_text;
>  EXPORT_SYMBOL(kimage_vaddr);
> @@ -772,6 +774,17 @@ void __init paging_init(void)
>  {
>  	pgd_t *pgdp = pgd_set_fixmap(__pa_symbol(swapper_pg_dir));
>  
> +#if VA_BITS > 48
> +	if (cpuid_feature_extract_unsigned_field(
> +				read_sysreg_s(SYS_ID_AA64MMFR2_EL1),
> +				ID_AA64MMFR2_LVA_SHIFT))
> +		vabits_actual = VA_BITS;
> +
> +	/* make the variable visible to secondaries with the MMU off */
> +	dcache_clean_inval_poc((u64)&vabits_actual,
> +			       (u64)&vabits_actual + sizeof(vabits_actual));
> +#endif
> +
>  	map_kernel(pgdp);
>  	map_mem(pgdp);
>