linux-hardening - Re: [PATCH] [RFC] aarch64: scs: reload shadow call stack in user exception entry

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Date:   Tue, 4 Jan 2022 06:32:16 -0800
From:   Dan Li <ashimida@...ux.alibaba.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     catalin.marinas@....com, will@...nel.org, samitolvanen@...gle.com,
        maz@...nel.org, joey.gouly@....com, pcc@...gle.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] [RFC] aarch64: scs: reload shadow call stack in user
 exception entry



On 1/4/22 2:40 AM, Mark Rutland wrote:
> Hi,
> 
> On Mon, Dec 27, 2021 at 05:06:04PM -0800, Dan Li wrote:
>> When el0 exception occurs, kernel_entry/exit will load/save tsk->scs_sp
>> to ensure scs working properly. AFAIK, the SCS offset should always be
>> 0 at this time.
>>
>> Is it reasonable to reload x18 to scs_base directly in kernel_entry
>> here, or am I missing something?
>>
>> Signed-off-by: Dan Li <ashimida@...ux.alibaba.com>
> 
> AFAICT the saving here is just that we avoid storing the SCS SP upon return to
> EL0, and I suspect that in practice that's not measureable.
> 
> So without numbers to show otherwise, I'd prefer to leave this as-is, though I
> agree that this patch should work.  If we do want to change this, I think we
> need a comment in the `kernel_exit` path to pair with the `scs_load` in
> `kernel_entry`.
> 
> Thanks,
> Mark.
>
Yes, there is only one instruction saved here, which should have a
minimal impact on performance.
The current code is also good for me :)

Thanks,
Dan.

>> ---
>>   arch/arm64/include/asm/scs.h | 7 +++++++
>>   arch/arm64/kernel/entry.S    | 3 +--
>>   2 files changed, 8 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/scs.h b/arch/arm64/include/asm/scs.h
>> index 8297bccf0784..2bc0d0575e75 100644
>> --- a/arch/arm64/include/asm/scs.h
>> +++ b/arch/arm64/include/asm/scs.h
>> @@ -9,6 +9,10 @@
>>   #ifdef CONFIG_SHADOW_CALL_STACK
>>   	scs_sp	.req	x18
>>   
>> +	.macro scs_reload tsk
>> +	ldr	scs_sp, [\tsk, #TSK_TI_SCS_BASE]
>> +	.endm
>> +
>>   	.macro scs_load tsk
>>   	ldr	scs_sp, [\tsk, #TSK_TI_SCS_SP]
>>   	.endm
>> @@ -17,6 +21,9 @@
>>   	str	scs_sp, [\tsk, #TSK_TI_SCS_SP]
>>   	.endm
>>   #else
>> +	.macro scs_reload tsk
>> +	.endm
>> +
>>   	.macro scs_load tsk
>>   	.endm
>>   
>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>> index bc6d5a970a13..57547a3e4f7c 100644
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
>> @@ -265,7 +265,7 @@ alternative_if ARM64_HAS_ADDRESS_AUTH
>>   alternative_else_nop_endif
>>   1:
>>   
>> -	scs_load tsk
>> +	scs_reload tsk
>>   	.else
>>   	add	x21, sp, #PT_REGS_SIZE
>>   	get_current_task tsk
>> @@ -365,7 +365,6 @@ alternative_if ARM64_WORKAROUND_845719
>>   alternative_else_nop_endif
>>   #endif
>>   3:
>> -	scs_save tsk
>>   
>>   	/* Ignore asynchronous tag check faults in the uaccess routines */
>>   	ldr	x0, [tsk, THREAD_SCTLR_USER]
>> -- 
>> 2.17.1
>>