Re: [PATCH -next v5 7/8] arm64: add uaccess to machine check safe

[Tong Tiangen <tongtiangen@huawei.com>]

在 2022/6/17 17:06, Mark Rutland 写道:
> On Sat, May 28, 2022 at 06:50:55AM +0000, Tong Tiangen wrote:
>> If user access fail due to hardware memory error, only the relevant
>> processes are affected, so killing the user process and isolate the
>> error page with hardware memory errors is a more reasonable choice
>> than kernel panic.
>>
>> Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
> 
>> ---
>>   arch/arm64/lib/copy_from_user.S | 8 ++++----
>>   arch/arm64/lib/copy_to_user.S   | 8 ++++----
> 
> All of these changes are to the *kernel* accesses performed as part of copy
> to/from user, and have nothing to do with userspace, so it does not make sense
> to mark these as UACCESS.

You have a point. so there is no need to modify copy_from/to_user.S in 
this patch set.

> 
> Do we *actually* need to recover from failues on these accesses? Looking at
> _copy_from_user(), the kernel will immediately follow this up with a memset()
> to the same address which will be fatal anyway, so this is only punting the
> failure for a few instructions.

If recovery success, The task will be killed and there will be no 
subsequent memset().

> 
> If we really need to recover from certain accesses to kernel memory we should
> add a new EX_TYPE_KACCESS_ERR_ZERO_MC or similar, but we need a strong
> rationale as to why that's useful. As things stand I do not beleive it makes
> sense for copy to/from user specifically.
> 
>>   arch/arm64/mm/extable.c         | 8 ++++----
>>   3 files changed, 12 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
>> index 34e317907524..402dd48a4f93 100644
>> --- a/arch/arm64/lib/copy_from_user.S
>> +++ b/arch/arm64/lib/copy_from_user.S
>> @@ -25,7 +25,7 @@
>>   	.endm
>>   
>>   	.macro strb1 reg, ptr, val
>> -	strb \reg, [\ptr], \val
>> +	USER(9998f, strb \reg, [\ptr], \val)
>>   	.endm
>>   
>>   	.macro ldrh1 reg, ptr, val
>> @@ -33,7 +33,7 @@
>>   	.endm
>>   
>>   	.macro strh1 reg, ptr, val
>> -	strh \reg, [\ptr], \val
>> +	USER(9998f, strh \reg, [\ptr], \val)
>>   	.endm
>>   
>>   	.macro ldr1 reg, ptr, val
>> @@ -41,7 +41,7 @@
>>   	.endm
>>   
>>   	.macro str1 reg, ptr, val
>> -	str \reg, [\ptr], \val
>> +	USER(9998f, str \reg, [\ptr], \val)
>>   	.endm
>>   
>>   	.macro ldp1 reg1, reg2, ptr, val
>> @@ -49,7 +49,7 @@
>>   	.endm
>>   
>>   	.macro stp1 reg1, reg2, ptr, val
>> -	stp \reg1, \reg2, [\ptr], \val
>> +	USER(9998f, stp \reg1, \reg2, [\ptr], \val)
>>   	.endm
>>   
>>   end	.req	x5
>> diff --git a/arch/arm64/lib/copy_to_user.S b/arch/arm64/lib/copy_to_user.S
>> index 802231772608..4134bdb3a8b0 100644
>> --- a/arch/arm64/lib/copy_to_user.S
>> +++ b/arch/arm64/lib/copy_to_user.S
>> @@ -20,7 +20,7 @@
>>    *	x0 - bytes not copied
>>    */
>>   	.macro ldrb1 reg, ptr, val
>> -	ldrb  \reg, [\ptr], \val
>> +	USER(9998f, ldrb  \reg, [\ptr], \val)
>>   	.endm
>>   
>>   	.macro strb1 reg, ptr, val
>> @@ -28,7 +28,7 @@
>>   	.endm
>>   
>>   	.macro ldrh1 reg, ptr, val
>> -	ldrh  \reg, [\ptr], \val
>> +	USER(9998f, ldrh  \reg, [\ptr], \val)
>>   	.endm
>>   
>>   	.macro strh1 reg, ptr, val
>> @@ -36,7 +36,7 @@
>>   	.endm
>>   
>>   	.macro ldr1 reg, ptr, val
>> -	ldr \reg, [\ptr], \val
>> +	USER(9998f, ldr \reg, [\ptr], \val)
>>   	.endm
>>   
>>   	.macro str1 reg, ptr, val
>> @@ -44,7 +44,7 @@
>>   	.endm
>>   
>>   	.macro ldp1 reg1, reg2, ptr, val
>> -	ldp \reg1, \reg2, [\ptr], \val
>> +	USER(9998f, ldp \reg1, \reg2, [\ptr], \val)
>>   	.endm
>>   
>>   	.macro stp1 reg1, reg2, ptr, val
>> diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
>> index c301dcf6335f..8ca8d9639f9f 100644
>> --- a/arch/arm64/mm/extable.c
>> +++ b/arch/arm64/mm/extable.c
>> @@ -86,10 +86,10 @@ bool fixup_exception_mc(struct pt_regs *regs)
>>   	if (!ex)
>>   		return false;
>>   
>> -	/*
>> -	 * This is not complete, More Machine check safe extable type can
>> -	 * be processed here.
>> -	 */
>> +	switch (ex->type) {
>> +	case EX_TYPE_UACCESS_ERR_ZERO:
>> +		return ex_handler_uaccess_err_zero(ex, regs);
>> +	}
> 
> This addition specifically makes sense to me, so can you split this into a separate patch?

According to my understanding of the above, only the modification of 
extable.c is retained.

So what do you mean which part is made into a separate patch?

Thanks,
Tong.
> 
> Thanks,
> Mark.
> 
> .