Re: [PATCH -next v4 6/7] arm64: add {get, put}_user to machine check safe

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Apr 20, 2022 at 03:04:17AM +0000, Tong Tiangen wrote:
> Add {get, put}_user() to machine check safe.
> 
> If get/put fail due to hardware memory error, only the relevant processes
> are affected, so killing the user process and isolate the user page with
> hardware memory errors is a more reasonable choice than kernel panic.
> 
> Add new extable type EX_TYPE_UACCESS_MC_ERR_ZERO which can be used for
> uaccess that can be recovered from hardware memory errors. The difference
> from EX_TYPE_UACCESS_MC is that this type also sets additional two target
> register which save error code and value needs to be set zero.

Why does this need to be in any way distinct from the existing
EX_TYPE_UACCESS_ERR_ZERO ?

Other than the case where we currently (ab)use that for
copy_{to,from}_kernel_nofault(), where do we *not* want to use
EX_TYPE_UACCESS_ERR_ZERO and *not* recover from a memory error?

Thanks,
Mark.

> 
> Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
> ---
>  arch/arm64/include/asm/asm-extable.h | 14 ++++++++++++++
>  arch/arm64/include/asm/uaccess.h     |  4 ++--
>  arch/arm64/mm/extable.c              |  4 ++++
>  3 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/asm-extable.h b/arch/arm64/include/asm/asm-extable.h
> index 75b2c00e9523..80410899a9ad 100644
> --- a/arch/arm64/include/asm/asm-extable.h
> +++ b/arch/arm64/include/asm/asm-extable.h
> @@ -13,6 +13,7 @@
>  
>  /* _MC indicates that can fixup from machine check errors */
>  #define EX_TYPE_UACCESS_MC		5
> +#define EX_TYPE_UACCESS_MC_ERR_ZERO	6
>  
>  #ifdef __ASSEMBLY__
>  
> @@ -78,6 +79,15 @@
>  #define EX_DATA_REG(reg, gpr)						\
>  	"((.L__gpr_num_" #gpr ") << " __stringify(EX_DATA_REG_##reg##_SHIFT) ")"
>  
> +#define _ASM_EXTABLE_UACCESS_MC_ERR_ZERO(insn, fixup, err, zero)		\
> +	__DEFINE_ASM_GPR_NUMS							\
> +	__ASM_EXTABLE_RAW(#insn, #fixup,					\
> +			  __stringify(EX_TYPE_UACCESS_MC_ERR_ZERO),		\
> +			  "("							\
> +			    EX_DATA_REG(ERR, err) " | "				\
> +			    EX_DATA_REG(ZERO, zero)				\
> +			  ")")
> +
>  #define _ASM_EXTABLE_UACCESS_ERR_ZERO(insn, fixup, err, zero)		\
>  	__DEFINE_ASM_GPR_NUMS						\
>  	__ASM_EXTABLE_RAW(#insn, #fixup, 				\
> @@ -90,6 +100,10 @@
>  #define _ASM_EXTABLE_UACCESS_ERR(insn, fixup, err)			\
>  	_ASM_EXTABLE_UACCESS_ERR_ZERO(insn, fixup, err, wzr)
>  
> +
> +#define _ASM_EXTABLE_UACCESS_MC_ERR(insn, fixup, err)			\
> +	_ASM_EXTABLE_UACCESS_MC_ERR_ZERO(insn, fixup, err, wzr)
> +
>  #define EX_DATA_REG_DATA_SHIFT	0
>  #define EX_DATA_REG_DATA	GENMASK(4, 0)
>  #define EX_DATA_REG_ADDR_SHIFT	5
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index e8dce0cc5eaa..e41b47df48b0 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -236,7 +236,7 @@ static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
>  	asm volatile(							\
>  	"1:	" load "	" reg "1, [%2]\n"			\
>  	"2:\n"								\
> -	_ASM_EXTABLE_UACCESS_ERR_ZERO(1b, 2b, %w0, %w1)			\
> +	_ASM_EXTABLE_UACCESS_MC_ERR_ZERO(1b, 2b, %w0, %w1)		\
>  	: "+r" (err), "=&r" (x)						\
>  	: "r" (addr))
>  
> @@ -325,7 +325,7 @@ do {									\
>  	asm volatile(							\
>  	"1:	" store "	" reg "1, [%2]\n"			\
>  	"2:\n"								\
> -	_ASM_EXTABLE_UACCESS_ERR(1b, 2b, %w0)				\
> +	_ASM_EXTABLE_UACCESS_MC_ERR(1b, 2b, %w0)			\
>  	: "+r" (err)							\
>  	: "r" (x), "r" (addr))
>  
> diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
> index 525876c3ebf4..1023ccdb2f89 100644
> --- a/arch/arm64/mm/extable.c
> +++ b/arch/arm64/mm/extable.c
> @@ -88,6 +88,7 @@ bool fixup_exception(struct pt_regs *regs)
>  	case EX_TYPE_BPF:
>  		return ex_handler_bpf(ex, regs);
>  	case EX_TYPE_UACCESS_ERR_ZERO:
> +	case EX_TYPE_UACCESS_MC_ERR_ZERO:
>  		return ex_handler_uaccess_err_zero(ex, regs);
>  	case EX_TYPE_LOAD_UNALIGNED_ZEROPAD:
>  		return ex_handler_load_unaligned_zeropad(ex, regs);
> @@ -107,6 +108,9 @@ bool fixup_exception_mc(struct pt_regs *regs)
>  	switch (ex->type) {
>  	case EX_TYPE_UACCESS_MC:
>  		return ex_handler_uaccess_type(ex, regs, FIXUP_TYPE_MC);
> +	case EX_TYPE_UACCESS_MC_ERR_ZERO:
> +		return ex_handler_uaccess_err_zero(ex, regs);
> +
>  	}
>  
>  	return false;
> -- 
> 2.25.1
>