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Date: Mon, 14 Aug 2017 14:37:51 -0400 (EDT)
From: Nicolas Pitre <nicolas.pitre@...aro.org>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
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 20/30] ARM: kernel: use PC relative symbol references in
suspend/resume code
On Mon, 14 Aug 2017, Ard Biesheuvel wrote:
> On 14 August 2017 at 17:02, Nicolas Pitre <nicolas.pitre@...aro.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@...linux.org.uk>
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.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
>
Yeah... I see no way around it.
And if you make this particular case into a commit of its own, then the
commit log may carry the above reasoning.
Nicolas
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