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From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date: Mon, 14 Aug 2017 19:14:40 +0100
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Subject: [kernel-hardening] Re: [PATCH 20/30] ARM: kernel: use PC relative symbol references in
 suspend/resume code
To: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, Arnd Bergmann <arnd@arndb.de>, Russell King <linux@armlinux.org.uk>, Kees Cook <keescook@chromium.org>, Thomas Garnier <thgarnie@google.com>, Marc Zyngier <marc.zyngier@arm.com>, Mark Rutland <mark.rutland@arm.com>, Tony Lindgren <tony@atomide.com>, Matt Fleming <matt@codeblueprint.co.uk>, Dave Martin <dave.martin@arm.com>
List-ID: <kernel-hardening.lists.openwall.com>

On 14 August 2017 at 17:02, Nicolas Pitre <nicolas.pitre@linaro.org> wrote:
> On Mon, 14 Aug 2017, Ard Biesheuvel wrote:
>
>> Replace some unnecessary absolute references with relative ones. Also,
>> to prepare for runtime relocation, which occurs with the caches on,
>> defer taking the absolute address of cpu_resume_after_mmu() until after
>> the MMU is enabled.
>>
>> Cc: Russell King <linux@armlinux.org.uk>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> ---
>>  arch/arm/kernel/sleep.S | 11 +++++------
>>  1 file changed, 5 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm/kernel/sleep.S b/arch/arm/kernel/sleep.S
>> index 3026b119d3ff..9efd1c7d3552 100644
>> --- a/arch/arm/kernel/sleep.S
>> +++ b/arch/arm/kernel/sleep.S
>> @@ -60,18 +60,17 @@
>>  ENTRY(__cpu_suspend)
>>       stmfd   sp!, {r4 - r11, lr}
>>  #ifdef MULTI_CPU
>> -     ldr     r10, =processor
>> -     ldr     r4, [r10, #CPU_SLEEP_SIZE] @ size of CPU sleep state
>> +     ldr_l   r4, processor + CPU_SLEEP_SIZE  @ size of CPU sleep state
>>  #else
>> -     ldr     r4, =cpu_suspend_size
>> +     adr_l   r4, cpu_suspend_size
>>  #endif
>>       mov     r5, sp                  @ current virtual SP
>>       add     r4, r4, #12             @ Space for pgd, virt sp, phys resume fn
>>       sub     sp, sp, r4              @ allocate CPU state on stack
>> -     ldr     r3, =sleep_save_sp
>> +     adr_l   r3, sleep_save_sp
>>       stmfd   sp!, {r0, r1}           @ save suspend func arg and pointer
>>       ldr     r3, [r3, #SLEEP_SAVE_SP_VIRT]
>> -     ALT_SMP(ldr r0, =mpidr_hash)
>> +     ALT_SMP(adr_l r0, mpidr_hash)
>>       ALT_UP_B(1f)
>
> The above is dangerous. adr_l expands to more than one instruction which
> is not what ALT_SMP() was designed for. Here it might happen to work
> anyway because it is combined with ALT_UP_B() but with ALT_UP() it
> wouldn't. This is a mistake waiting to happen.
>

OK. I will use the opencoded sequence instead in this case. I.e.,

-       ALT_SMP(ldr r0, =mpidr_hash)
+0:     ALT_SMP(adr r0, 2f)
        ALT_UP_B(1f)
+       ldr     r1, [r0]
+       add     r0, r0, r1

and

 ENDPROC(__cpu_suspend)
+       .align  2
+2:     .long   mpidr_hash - .
        .ltorg