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Date: Mon, 4 Sep 2017 20:09:27 +0100
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Nicolas Pitre <nicolas.pitre@...aro.org>
Cc: "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
Kernel Hardening <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 17/29] ARM: kernel: use relative phys-to-virt patch tables
On 4 September 2017 at 19:03, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> On Sun, 3 Sep 2017, Ard Biesheuvel wrote:
>
>> Replace the contents of the __pv_table entries with relative references
>> so that we don't have to relocate them at runtime when running the KASLR
>> kernel. This ensures these quantities are invariant under runtime
>> relocation, which makes any cache maintenance after runtime relocation
>> unnecessary.
>>
>> Cc: Russell King <linux@...linux.org.uk>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>> ---
>> arch/arm/include/asm/memory.h | 6 +++---
>> arch/arm/kernel/head.S | 21 ++++++++++----------
>> 2 files changed, 13 insertions(+), 14 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/memory.h b/arch/arm/include/asm/memory.h
>> index 1f54e4e98c1e..47a984e3a244 100644
>> --- a/arch/arm/include/asm/memory.h
>> +++ b/arch/arm/include/asm/memory.h
>> @@ -195,7 +195,7 @@ extern const void *__pv_table_begin, *__pv_table_end;
>> __asm__("@ __pv_stub\n" \
>> "1: " instr " %0, %1, %2\n" \
>> " .pushsection .pv_table,\"a\"\n" \
>> - " .long 1b\n" \
>> + " .long 1b - .\n" \
>> " .popsection\n" \
>
> [...]
>
>> --- a/arch/arm/kernel/head.S
>> +++ b/arch/arm/kernel/head.S
>> @@ -612,22 +611,22 @@ __fixup_a_pv_table:
>> orr r6, r6, r7, lsl #12
>> orr r6, #0x4000
>> b 2f
>> -1: add r7, r3
>> - ldrh ip, [r7, #2]
>> +1: add r7, r4
>> + ldrh ip, [r7, #-2]
>
> Please use "ldrh ip, [r7, #(-4 + 2)]" so the -4 offset is clearly
> spelled out. The #-2 gave me a pause of confusion.
>
>> ARM_BE8(rev16 ip, ip)
>> tst ip, #0x4000
>> and ip, #0x8f00
>> orrne ip, r6 @ mask in offset bits 31-24
>> orreq ip, r0 @ mask in offset bits 7-0
>> ARM_BE8(rev16 ip, ip)
>> - strh ip, [r7, #2]
>> + strh ip, [r7, #-2]
>
> Ditto here.
>
>> bne 2f
>> - ldrh ip, [r7]
>> + ldrh ip, [r7, #-4]
>
> And to be clear, here I'd use "#(-4 + 0)" to be explicit.
>
>> ARM_BE8(rev16 ip, ip)
>> bic ip, #0x20
>> orr ip, ip, r0, lsr #16
>> ARM_BE8(rev16 ip, ip)
>> - strh ip, [r7]
>> + strh ip, [r7, #-4]
>
> And so here.
>
OK
>
> With that:
>
> Reviewed-by: Nicolas Pitre <nico@...aro.org>
Thanks.
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