linux-kernel - Re: [PATCH v2 01/17] arm64: mm: don't use x18 in idmap_kpti_install_ng

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Date:   Fri, 25 Oct 2019 10:24:22 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     samitolvanen@...gle.com
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/17] arm64: mm: don't use x18 in
 idmap_kpti_install_ng_mappings

On Thu, Oct 24, 2019 at 03:51:16PM -0700, samitolvanen@...gle.com wrote:
> idmap_kpti_install_ng_mappings uses x18 as a temporary register, which
> will result in a conflict when x18 is reserved. Use x16 and x17 instead
> where needed.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

AFAICT the new register assignment is sound, so FWIW:

Reviewed-by: Mark Rutland <mark.rutland@....com>

I was going to suggest adding menmonics for the remamining raw register
names, but after having a go locally I think it's cleaner as-is given
the registers are used in different widths for multiple purposes.

Thanks,
Mark.

> ---
>  arch/arm64/mm/proc.S | 63 ++++++++++++++++++++++----------------------
>  1 file changed, 32 insertions(+), 31 deletions(-)
> 
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index a1e0592d1fbc..fdabf40a83c8 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -250,15 +250,15 @@ ENTRY(idmap_kpti_install_ng_mappings)
>  	/* We're the boot CPU. Wait for the others to catch up */
>  	sevl
>  1:	wfe
> -	ldaxr	w18, [flag_ptr]
> -	eor	w18, w18, num_cpus
> -	cbnz	w18, 1b
> +	ldaxr	w17, [flag_ptr]
> +	eor	w17, w17, num_cpus
> +	cbnz	w17, 1b
>  
>  	/* We need to walk swapper, so turn off the MMU. */
>  	pre_disable_mmu_workaround
> -	mrs	x18, sctlr_el1
> -	bic	x18, x18, #SCTLR_ELx_M
> -	msr	sctlr_el1, x18
> +	mrs	x17, sctlr_el1
> +	bic	x17, x17, #SCTLR_ELx_M
> +	msr	sctlr_el1, x17
>  	isb
>  
>  	/* Everybody is enjoying the idmap, so we can rewrite swapper. */
> @@ -281,9 +281,9 @@ skip_pgd:
>  	isb
>  
>  	/* We're done: fire up the MMU again */
> -	mrs	x18, sctlr_el1
> -	orr	x18, x18, #SCTLR_ELx_M
> -	msr	sctlr_el1, x18
> +	mrs	x17, sctlr_el1
> +	orr	x17, x17, #SCTLR_ELx_M
> +	msr	sctlr_el1, x17
>  	isb
>  
>  	/*
> @@ -353,46 +353,47 @@ skip_pte:
>  	b.ne	do_pte
>  	b	next_pmd
>  
> +	.unreq	cpu
> +	.unreq	num_cpus
> +	.unreq	swapper_pa
> +	.unreq	cur_pgdp
> +	.unreq	end_pgdp
> +	.unreq	pgd
> +	.unreq	cur_pudp
> +	.unreq	end_pudp
> +	.unreq	pud
> +	.unreq	cur_pmdp
> +	.unreq	end_pmdp
> +	.unreq	pmd
> +	.unreq	cur_ptep
> +	.unreq	end_ptep
> +	.unreq	pte
> +
>  	/* Secondary CPUs end up here */
>  __idmap_kpti_secondary:
>  	/* Uninstall swapper before surgery begins */
> -	__idmap_cpu_set_reserved_ttbr1 x18, x17
> +	__idmap_cpu_set_reserved_ttbr1 x16, x17
>  
>  	/* Increment the flag to let the boot CPU we're ready */
> -1:	ldxr	w18, [flag_ptr]
> -	add	w18, w18, #1
> -	stxr	w17, w18, [flag_ptr]
> +1:	ldxr	w16, [flag_ptr]
> +	add	w16, w16, #1
> +	stxr	w17, w16, [flag_ptr]
>  	cbnz	w17, 1b
>  
>  	/* Wait for the boot CPU to finish messing around with swapper */
>  	sevl
>  1:	wfe
> -	ldxr	w18, [flag_ptr]
> -	cbnz	w18, 1b
> +	ldxr	w16, [flag_ptr]
> +	cbnz	w16, 1b
>  
>  	/* All done, act like nothing happened */
> -	offset_ttbr1 swapper_ttb, x18
> +	offset_ttbr1 swapper_ttb, x16
>  	msr	ttbr1_el1, swapper_ttb
>  	isb
>  	ret
>  
> -	.unreq	cpu
> -	.unreq	num_cpus
> -	.unreq	swapper_pa
>  	.unreq	swapper_ttb
>  	.unreq	flag_ptr
> -	.unreq	cur_pgdp
> -	.unreq	end_pgdp
> -	.unreq	pgd
> -	.unreq	cur_pudp
> -	.unreq	end_pudp
> -	.unreq	pud
> -	.unreq	cur_pmdp
> -	.unreq	end_pmdp
> -	.unreq	pmd
> -	.unreq	cur_ptep
> -	.unreq	end_ptep
> -	.unreq	pte
>  ENDPROC(idmap_kpti_install_ng_mappings)
>  	.popsection
>  #endif
> -- 
> 2.24.0.rc0.303.g954a862665-goog
>