Re: [PATCH -next v4 3/7] arm64: add support for machine check error safe

[Tong Tiangen <tongtiangen@huawei.com>]

在 2022/5/13 23:26, Mark Rutland 写道:
> On Wed, Apr 20, 2022 at 03:04:14AM +0000, Tong Tiangen wrote:
>> During the processing of arm64 kernel hardware memory errors(do_sea()), if
>> the errors is consumed in the kernel, the current processing is panic.
>> However, it is not optimal.
>>
>> Take uaccess for example, if the uaccess operation fails due to memory
>> error, only the user process will be affected, kill the user process
>> and isolate the user page with hardware memory errors is a better choice.
> 
> Conceptually, I'm fine with the idea of constraining what we do for a
> true uaccess, but I don't like the implementation of this at all, and I
> think we first need to clean up the arm64 extable usage to clearly
> distinguish a uaccess from another access.

OK,using EX_TYPE_UACCESS and this extable type could be recover, this is 
more reasonable.

For EX_TYPE_UACCESS_ERR_ZERO, today we use it for kernel accesses in a 
couple of cases, such as 
get_user/futex/__user_cache_maint()/__user_swpX_asm(), your suggestion is:
get_user continues to use EX_TYPE_UACCESS_ERR_ZERO and the other cases 
use new type EX_TYPE_FIXUP_ERR_ZERO?

Thanks,
Tong.

> 
>> This patch only enable machine error check framework, it add exception
>> fixup before kernel panic in do_sea() and only limit the consumption of
>> hardware memory errors in kernel mode triggered by user mode processes.
>> If fixup successful, panic can be avoided.
>>
>> Consistent with PPC/x86, it is implemented by CONFIG_ARCH_HAS_COPY_MC.
>>
>> Also add copy_mc_to_user() in include/linux/uaccess.h, this helper is
>> called when CONFIG_ARCH_HAS_COPOY_MC is open.
>>
>> Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
>> ---
>>   arch/arm64/Kconfig               |  1 +
>>   arch/arm64/include/asm/extable.h |  1 +
>>   arch/arm64/mm/extable.c          | 17 +++++++++++++++++
>>   arch/arm64/mm/fault.c            | 27 ++++++++++++++++++++++++++-
>>   include/linux/uaccess.h          |  9 +++++++++
>>   5 files changed, 54 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index d9325dd95eba..012e38309955 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -19,6 +19,7 @@ config ARM64
>>   	select ARCH_ENABLE_SPLIT_PMD_PTLOCK if PGTABLE_LEVELS > 2
>>   	select ARCH_ENABLE_THP_MIGRATION if TRANSPARENT_HUGEPAGE
>>   	select ARCH_HAS_CACHE_LINE_SIZE
>> +	select ARCH_HAS_COPY_MC if ACPI_APEI_GHES
>>   	select ARCH_HAS_CURRENT_STACK_POINTER
>>   	select ARCH_HAS_DEBUG_VIRTUAL
>>   	select ARCH_HAS_DEBUG_VM_PGTABLE
>> diff --git a/arch/arm64/include/asm/extable.h b/arch/arm64/include/asm/extable.h
>> index 72b0e71cc3de..f80ebd0addfd 100644
>> --- a/arch/arm64/include/asm/extable.h
>> +++ b/arch/arm64/include/asm/extable.h
>> @@ -46,4 +46,5 @@ bool ex_handler_bpf(const struct exception_table_entry *ex,
>>   #endif /* !CONFIG_BPF_JIT */
>>   
>>   bool fixup_exception(struct pt_regs *regs);
>> +bool fixup_exception_mc(struct pt_regs *regs);
>>   #endif
>> diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
>> index 489455309695..4f0083a550d4 100644
>> --- a/arch/arm64/mm/extable.c
>> +++ b/arch/arm64/mm/extable.c
>> @@ -9,6 +9,7 @@
>>   
>>   #include <asm/asm-extable.h>
>>   #include <asm/ptrace.h>
>> +#include <asm/esr.h>
>>   
>>   static inline unsigned long
>>   get_ex_fixup(const struct exception_table_entry *ex)
>> @@ -84,3 +85,19 @@ bool fixup_exception(struct pt_regs *regs)
>>   
>>   	BUG();
>>   }
>> +
>> +bool fixup_exception_mc(struct pt_regs *regs)
>> +{
>> +	const struct exception_table_entry *ex;
>> +
>> +	ex = search_exception_tables(instruction_pointer(regs));
>> +	if (!ex)
>> +		return false;
>> +
>> +	/*
>> +	 * This is not complete, More Machine check safe extable type can
>> +	 * be processed here.
>> +	 */
>> +
>> +	return false;
>> +}
> 
> This is at best misnamed; It doesn't actually apply the fixup, it just
> searches for one.

Yeah, you're right about the current logic, so i added notes to explain 
the scenarios that will be added later.

> 
>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>> index 77341b160aca..a9e6fb1999d1 100644
>> --- a/arch/arm64/mm/fault.c
>> +++ b/arch/arm64/mm/fault.c
>> @@ -695,6 +695,29 @@ static int do_bad(unsigned long far, unsigned int esr, struct pt_regs *regs)
>>   	return 1; /* "fault" */
>>   }
>>   
>> +static bool arm64_do_kernel_sea(unsigned long addr, unsigned int esr,
>> +				     struct pt_regs *regs, int sig, int code)
>> +{
>> +	if (!IS_ENABLED(CONFIG_ARCH_HAS_COPY_MC))
>> +		return false;
>> +
>> +	if (user_mode(regs) || !current->mm)
>> +		return false;
>> +
>> +	if (apei_claim_sea(regs) < 0)
>> +		return false;
>> +
>> +	if (!fixup_exception_mc(regs))
>> +		return false;
>> +
>> +	set_thread_esr(0, esr);
>> +
>> +	arm64_force_sig_fault(sig, code, addr,
>> +		"Uncorrected hardware memory error in kernel-access\n");
>> +
>> +	return true;
>> +}
>> +
>>   static int do_sea(unsigned long far, unsigned int esr, struct pt_regs *regs)
>>   {
>>   	const struct fault_info *inf;
>> @@ -720,7 +743,9 @@ static int do_sea(unsigned long far, unsigned int esr, struct pt_regs *regs)
>>   		 */
>>   		siaddr  = untagged_addr(far);
>>   	}
>> -	arm64_notify_die(inf->name, regs, inf->sig, inf->code, siaddr, esr);
>> +
>> +	if (!arm64_do_kernel_sea(siaddr, esr, regs, inf->sig, inf->code))
>> +		arm64_notify_die(inf->name, regs, inf->sig, inf->code, siaddr, esr);
>>   
>>   	return 0;
>>   }
>> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
>> index 546179418ffa..884661b29c17 100644
>> --- a/include/linux/uaccess.h
>> +++ b/include/linux/uaccess.h
>> @@ -174,6 +174,15 @@ copy_mc_to_kernel(void *dst, const void *src, size_t cnt)
>>   }
>>   #endif
>>   
>> +#ifndef copy_mc_to_user
>> +static inline unsigned long __must_check
>> +copy_mc_to_user(void *dst, const void *src, size_t cnt)
>> +{
>> +	check_object_size(src, cnt, true);
>> +	return raw_copy_to_user(dst, src, cnt);
>> +}
>> +#endif
> 
> Why do we need a special copy_mc_to_user() ?
> 
> Why are we not making *every* true uaccess recoverable? That way the
> regular copy_to_user() would just work.

Agreed, will fixed next version.

Thanks,
Tong.

> 
> Thanks,
> Mark.
> .