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Date: Mon, 4 Sep 2017 14:15:02 -0400 (EDT)
From: Nicolas Pitre <nicolas.pitre@...aro.org>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
cc: linux-arm-kernel@...ts.infradead.org, kernel-hardening@...ts.openwall.com,
Arnd Bergmann <arnd@...db.de>, Russell King <linux@...linux.org.uk>,
Kees Cook <keescook@...omium.org>, Thomas Garnier <thgarnie@...gle.com>,
Marc Zyngier <marc.zyngier@....com>, Mark Rutland <mark.rutland@....com>,
Tony Lindgren <tony@...mide.com>, Matt Fleming <matt@...eblueprint.co.uk>,
Dave Martin <dave.martin@....com>
Subject: Re: [PATCH v2 20/29] ARM: kernel: use PC-relative symbol references
in MMU switch code
On Sun, 3 Sep 2017, Ard Biesheuvel wrote:
> To prepare for adding support for KASLR, which relocates all absolute
> symbol references at runtime after the caches have been enabled,
> update the MMU switch code to avoid using absolute symbol references
> where possible. This ensures these quantities are invariant under
> runtime relocation.
>
> Cc: Russell King <linux@...linux.org.uk>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
> arch/arm/kernel/head-common.S | 39 ++++++++------------
> 1 file changed, 15 insertions(+), 24 deletions(-)
>
> diff --git a/arch/arm/kernel/head-common.S b/arch/arm/kernel/head-common.S
> index 06035488130c..b74477507a12 100644
> --- a/arch/arm/kernel/head-common.S
> +++ b/arch/arm/kernel/head-common.S
> @@ -79,9 +79,10 @@ ENDPROC(__vet_atags)
> */
> __INIT
> __mmap_switched:
> - adr r3, __mmap_switched_data
> -
> - ldmia r3!, {r4, r5, r6, r7}
> + adr_l r4, __data_loc
> + adr_l r5, _sdata
> + adr_l r6, __bss_start
> + adr_l r7, _end
> cmp r4, r5 @ Copy data segment if needed
> 1: cmpne r5, r6
> ldrne fp, [r4], #4
> @@ -93,9 +94,17 @@ __mmap_switched:
> strcc fp, [r6],#4
> bcc 1b
>
> - ARM( ldmia r3, {r4, r5, r6, r7, sp})
> - THUMB( ldmia r3, {r4, r5, r6, r7} )
> - THUMB( ldr sp, [r3, #16] )
> + adr_l r3, init_thread_union + THREAD_START_SP
> + mov sp, r3
> + adr_l r4, processor_id
> + adr_l r5, __machine_arch_type
> + adr_l r6, __atags_pointer
> +#ifdef CONFIG_CPU_CP15
> + adr_l r7, cr_alignment
> +#else
> + mov r7, #0
> +#endif
The code that follows is testing for a non-zero r7 value to store r0 so
you could make that code conditional rather than loading 0 here.
> +
> str r9, [r4] @ Save processor ID
> str r1, [r5] @ Save machine type
> str r2, [r6] @ Save atags pointer
> @@ -104,24 +113,6 @@ __mmap_switched:
> b start_kernel
> ENDPROC(__mmap_switched)
>
> - .align 2
> - .type __mmap_switched_data, %object
> -__mmap_switched_data:
> - .long __data_loc @ r4
> - .long _sdata @ r5
> - .long __bss_start @ r6
> - .long _end @ r7
> - .long processor_id @ r4
> - .long __machine_arch_type @ r5
> - .long __atags_pointer @ r6
> -#ifdef CONFIG_CPU_CP15
> - .long cr_alignment @ r7
> -#else
> - .long 0 @ r7
> -#endif
> - .long init_thread_union + THREAD_START_SP @ sp
> - .size __mmap_switched_data, . - __mmap_switched_data
> -
> /*
> * This provides a C-API version of __lookup_processor_type
> */
> --
> 2.11.0
>
>
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