Re: [RFC PATCH -next V2 3/7] arm64: add support for machine check error safe

[Tong Tiangen <tongtiangen@huawei.com>]

在 2022/4/6 18:58, Mark Rutland 写道:
> On Wed, Apr 06, 2022 at 09:13:07AM +0000, Tong Tiangen wrote:
>> In arm64 kernel hardware memory errors process(do_sea()), if the errors
>> is consumed in the kernel, the current processing is panic. However,
>> it is not optimal. In some case, the page accessed in kernel is a user
>> page (such as copy_from_user/get_user), kill the user process and
>> isolate the user page with hardware memory errors is a better choice.
>>
>> Consistent with PPC/x86, it is implemented by CONFIG_ARCH_HAS_COPY_MC.
> 
> Why do we need new helpers for this, rather than doing this for *any* uaccess >
> I understand this is consistent with PPC & X86, but *why* is it done that way
> today? e.g. are there cases where we access memroy where we do not expect the
> situation to be recoverable?

Here[1] i explain why not all uaccess needs to be recoverable.

[1]https://lore.kernel.org/lkml/bdf67ff6-9eb8-c2b4-2c2f-b160d4f879cd@huawei.com/

> 
>> 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, there is no need to panic.
>>
>> Also add _asm_extable_mc macro used for add extable entry to help
>> fixup.
>>
>> Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
>> ---
>>   arch/arm64/Kconfig                   |  1 +
>>   arch/arm64/include/asm/asm-extable.h | 13 ++++++++++++
>>   arch/arm64/include/asm/esr.h         |  5 +++++
>>   arch/arm64/include/asm/extable.h     |  2 +-
>>   arch/arm64/kernel/probes/kprobes.c   |  2 +-
>>   arch/arm64/mm/extable.c              | 20 ++++++++++++++++++-
>>   arch/arm64/mm/fault.c                | 30 +++++++++++++++++++++++++++-
>>   include/linux/uaccess.h              |  8 ++++++++
>>   8 files changed, 77 insertions(+), 4 deletions(-)
>>
>> 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/asm-extable.h b/arch/arm64/include/asm/asm-extable.h
>> index c39f2437e08e..74d1db74fd86 100644
>> --- a/arch/arm64/include/asm/asm-extable.h
>> +++ b/arch/arm64/include/asm/asm-extable.h
>> @@ -8,6 +8,11 @@
>>   #define EX_TYPE_UACCESS_ERR_ZERO	3
>>   #define EX_TYPE_LOAD_UNALIGNED_ZEROPAD	4
>>   
>> +/* _MC indicates that can fixup from machine check errors */
>> +#define EX_TYPE_FIXUP_MC		5
>> +
>> +#define IS_EX_TYPE_MC(type) (type == EX_TYPE_FIXUP_MC)
> 
> If we need this, I'd strongly prefer that we have a EX_TYPE_UACCESS_MC or
> EX_TYPE_UACCESS_MC_ERR_ZERO for the uaccess cases, so that we can clearly
> distinguish those from non-uaccess cases.

Agreed.

> 
> AFAICT the only remaining raw EX_TYPE_FIXUP cases we have today are in some
> cache maintenance routines, and we should be able to convert those to a new
> EX_TYPE_FIXUP_UACCESS, or EX_TYPE_UACCESS_ERR_ZERO.

I will redesign this part. The general idea is not to use x0 to save
exception information. At that time, it may be merged into raw
EX_TYPE_FIXUP processing.
> 
>> +
>>   #ifdef __ASSEMBLY__
>>   
>>   #define __ASM_EXTABLE_RAW(insn, fixup, type, data)	\
>> @@ -27,6 +32,14 @@
>>   	__ASM_EXTABLE_RAW(\insn, \fixup, EX_TYPE_FIXUP, 0)
>>   	.endm
>>   
>> +/*
>> + * Create an exception table entry for `insn`, which will branch to `fixup`
>> + * when an unhandled fault(include sea fault) is taken.
>> + */
>> +	.macro		_asm_extable_mc, insn, fixup
>> +	__ASM_EXTABLE_RAW(\insn, \fixup, EX_TYPE_FIXUP_MC, 0)
>> +	.endm
>> +
>>   /*
>>    * Create an exception table entry for `insn` if `fixup` is provided. Otherwise
>>    * do nothing.
>> diff --git a/arch/arm64/include/asm/esr.h b/arch/arm64/include/asm/esr.h
>> index d52a0b269ee8..11fcfc002654 100644
>> --- a/arch/arm64/include/asm/esr.h
>> +++ b/arch/arm64/include/asm/esr.h
>> @@ -330,6 +330,11 @@
>>   #ifndef __ASSEMBLY__
>>   #include <asm/types.h>
>>   
>> +static inline bool esr_is_sea(u32 esr)
>> +{
>> +	return (esr & ESR_ELx_FSC) == ESR_ELx_FSC_EXTABT;
>> +}
>> +
>>   static inline bool esr_is_data_abort(u32 esr)
>>   {
>>   	const u32 ec = ESR_ELx_EC(esr);
>> diff --git a/arch/arm64/include/asm/extable.h b/arch/arm64/include/asm/extable.h
>> index 72b0e71cc3de..f7835b0f473b 100644
>> --- a/arch/arm64/include/asm/extable.h
>> +++ b/arch/arm64/include/asm/extable.h
>> @@ -45,5 +45,5 @@ bool ex_handler_bpf(const struct exception_table_entry *ex,
>>   }
>>   #endif /* !CONFIG_BPF_JIT */
>>   
>> -bool fixup_exception(struct pt_regs *regs);
>> +bool fixup_exception(struct pt_regs *regs, unsigned int esr);
>>   #endif
>> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
>> index d9dfa82c1f18..16a069e8eec3 100644
>> --- a/arch/arm64/kernel/probes/kprobes.c
>> +++ b/arch/arm64/kernel/probes/kprobes.c
>> @@ -285,7 +285,7 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
>>   		 * In case the user-specified fault handler returned
>>   		 * zero, try to fix up.
>>   		 */
>> -		if (fixup_exception(regs))
>> +		if (fixup_exception(regs, fsr))
>>   			return 1;
>>   	}
>>   	return 0;
>> diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
>> index 489455309695..f1134c88e849 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)
>> @@ -23,6 +24,18 @@ static bool ex_handler_fixup(const struct exception_table_entry *ex,
>>   	return true;
>>   }
>>   
>> +static bool ex_handler_fixup_mc(const struct exception_table_entry *ex,
>> +				struct pt_regs *regs, unsigned int esr)
>> +{
>> +	if (esr_is_sea(esr))
>> +		regs->regs[0] = 0;
>> +	else
>> +		regs->regs[0] = 1;
> 
> This needs more explanation.
> 
> Why does this hard-code an assumption that we can alter x0?
> 
> Why is the x0 value distinct for SEA or non-SEA? What is this meant to
> represent specifically?
> 
> What if this SEA was taken for a reason other than a memory error?

There is a problem with the design of this place and it needs to be
modified. The value in x0 cannot be overwritten. will be fixed in next
version.

> 
>> +
>> +	regs->pc = get_ex_fixup(ex);
>> +	return true;
>> +}
>> +
>>   static bool ex_handler_uaccess_err_zero(const struct exception_table_entry *ex,
>>   					struct pt_regs *regs)
>>   {
>> @@ -63,7 +76,7 @@ ex_handler_load_unaligned_zeropad(const struct exception_table_entry *ex,
>>   	return true;
>>   }
>>   
>> -bool fixup_exception(struct pt_regs *regs)
>> +bool fixup_exception(struct pt_regs *regs, unsigned int esr)
>>   {
>>   	const struct exception_table_entry *ex;
>>   
>> @@ -71,9 +84,14 @@ bool fixup_exception(struct pt_regs *regs)
>>   	if (!ex)
>>   		return false;
>>   
>> +	if (esr_is_sea(esr) && !IS_EX_TYPE_MC(ex->type))
>> +		return false;
> 
> I don't think this check belongs here.
> 
> Either this should be folded into ex_handler_fixup_mc(), or we should make the
> judgement earlier in the fault handling path, and have a separate
> fixup_exception_mc() that we can call specifically in the case of a memory
> error.

Agreed, Maybe it's better to use a separate fixup_exception_mc().

> 
>> +
>>   	switch (ex->type) {
>>   	case EX_TYPE_FIXUP:
>>   		return ex_handler_fixup(ex, regs);
>> +	case EX_TYPE_FIXUP_MC:
>> +		return ex_handler_fixup_mc(ex, regs, esr);
>>   	case EX_TYPE_BPF:
>>   		return ex_handler_bpf(ex, regs);
>>   	case EX_TYPE_UACCESS_ERR_ZERO:
>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>> index 77341b160aca..ffdfab2fdd60 100644
>> --- a/arch/arm64/mm/fault.c
>> +++ b/arch/arm64/mm/fault.c
>> @@ -361,7 +361,7 @@ static void __do_kernel_fault(unsigned long addr, unsigned int esr,
>>   	 * Are we prepared to handle this kernel fault?
>>   	 * We are almost certainly not prepared to handle instruction faults.
>>   	 */
>> -	if (!is_el1_instruction_abort(esr) && fixup_exception(regs))
>> +	if (!is_el1_instruction_abort(esr) && fixup_exception(regs, esr))
>>   		return;
>>   
>>   	if (WARN_RATELIMIT(is_spurious_el1_translation_fault(addr, esr, regs),
>> @@ -695,6 +695,30 @@ static int do_bad(unsigned long far, unsigned int esr, struct pt_regs *regs)
>>   	return 1; /* "fault" */
>>   }
>>   
>> +static bool arm64_process_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;
>> +
>> +	current->thread.fault_address = 0;
>> +	current->thread.fault_code = esr;
>> +
>> +	if (!fixup_exception(regs, esr))
>> +		return false;
>> +
>> +	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;
>> @@ -720,6 +744,10 @@ static int do_sea(unsigned long far, unsigned int esr, struct pt_regs *regs)
>>   		 */
>>   		siaddr  = untagged_addr(far);
>>   	}
>> +
>> +	if (arm64_process_kernel_sea(siaddr, esr, regs, inf->sig, inf->code))
>> +		return 0;
>> +
>>   	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..dd952aeecdc1 100644
>> --- a/include/linux/uaccess.h
>> +++ b/include/linux/uaccess.h
>> @@ -174,6 +174,14 @@ 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)
>> +{
>> +	return raw_copy_to_user(dst, src, cnt);
>> +}
> 
> ... this isn't using the new EX_TYPE_FIXUP_MC type, so isn't this just broken
> as of this patch?
> 
> Thanks,
> Mark.

It is useful in the later patch (adding specific scenarios to use this
framework), here just add helper.

Great thanks,
Tong.
> 
>> +#endif
>> +
>>   static __always_inline void pagefault_disabled_inc(void)
>>   {
>>   	current->pagefault_disabled++;
>> -- 
>> 2.18.0.huawei.25
>>
>>
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> .