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

[Mark Rutland]

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@xxxxxxxxxx>

> ---
>  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.

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 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?

Thanks,
Mark.