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

Tong Tiangen
2022-06-18T09:29:13Z

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.
>
> .