Subject: Re: [PATCH v12 2/6] arm64: add support for ARCH_HAS_COPY_MC

From: Jonathan Cameron <[email-masked]>
Date: Mon, 19 Aug 2024 11:30:32 +0100

On Tue, 28 May 2024 16:59:11 +0800
Tong Tiangen <[email-masked]> wrote:

> For the arm64 kernel, when it processes hardware memory errors for
> synchronize notifications(do_sea()), if the errors is consumed within the
> kernel, the current processing is panic. However, it is not optimal.
> 
> Take copy_from/to_user for example, If ld* triggers a memory error, even in
> kernel mode, only the associated process is affected. Killing the user
> process and isolating the corrupt page is a better choice.
> 
> New fixup type EX_TYPE_KACCESS_ERR_ZERO_ME_SAFE is added to identify insn
> that can recover from memory errors triggered by access to kernel memory.
> 
> Signed-off-by: Tong Tiangen <[email-masked]>

Hi - this is going slow :(

A few comments inline in the meantime but this really needs ARM maintainers
to take a (hopefully final) look.

Jonathan

> diff --git a/arch/arm64/include/asm/asm-extable.h b/arch/arm64/include/asm/asm-extable.h
> index 980d1dd8e1a3..9c0664fe1eb1 100644
> --- a/arch/arm64/include/asm/asm-extable.h
> +++ b/arch/arm64/include/asm/asm-extable.h
> @@ -5,11 +5,13 @@
>  #include <linux/bits.h>
>  #include <asm/gpr-num.h>
>  
> -#define EX_TYPE_NONE			0
> -#define EX_TYPE_BPF			1
> -#define EX_TYPE_UACCESS_ERR_ZERO	2
> -#define EX_TYPE_KACCESS_ERR_ZERO	3
> -#define EX_TYPE_LOAD_UNALIGNED_ZEROPAD	4
> +#define EX_TYPE_NONE				0
> +#define EX_TYPE_BPF				1
> +#define EX_TYPE_UACCESS_ERR_ZERO		2
> +#define EX_TYPE_KACCESS_ERR_ZERO		3
> +#define EX_TYPE_LOAD_UNALIGNED_ZEROPAD		4
> +/* kernel access memory error safe */
> +#define EX_TYPE_KACCESS_ERR_ZERO_ME_SAFE	5

Does anyone care enough about the alignment to bother realigning for one
long line? I'd be tempted not to bother, but up to maintainers.

> diff --git a/arch/arm64/lib/copy_to_user.S b/arch/arm64/lib/copy_to_user.S
> index 802231772608..2ac716c0d6d8 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
> +	KERNEL_ME_SAFE(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
> +	KERNEL_ME_SAFE(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
> +	KERNEL_ME_SAFE(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
> +	KERNEL_ME_SAFE(9998f, ldp \reg1, \reg2, [\ptr], \val)
>  	.endm
>  
>  	.macro stp1 reg1, reg2, ptr, val
> @@ -64,7 +64,7 @@ SYM_FUNC_START(__arch_copy_to_user)
>  9997:	cmp	dst, dstin
>  	b.ne	9998f
>  	// Before being absolutely sure we couldn't copy anything, try harder
> -	ldrb	tmp1w, [srcin]
> +KERNEL_ME_SAFE(9998f, ldrb	tmp1w, [srcin])

Alignment looks off?

>  USER(9998f, sttrb tmp1w, [dst])
>  	add	dst, dst, #1
>  9998:	sub	x0, end, dst			// bytes not copied

> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 451ba7cbd5ad..2dc65f99d389 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -708,21 +708,32 @@ static int do_bad(unsigned long far, unsigned long esr, struct pt_regs *regs)
>  	return 1; /* "fault" */
>  }
>  
> +/*
> + * APEI claimed this as a firmware-first notification.
> + * Some processing deferred to task_work before ret_to_user().
> + */
> +static bool do_apei_claim_sea(struct pt_regs *regs)
> +{
> +	if (user_mode(regs)) {
> +		if (!apei_claim_sea(regs))

I'd keep to the the (apei_claim_sea(regs) == 0)
used in the original code. That hints to the reader that we are
interested here in an 'error' code rather than apei_claim_sea() returning
a bool.   I initially wondered why we return true when the code
fails to claim it.

Also, perhaps if you return 0 for success and an error code if not
you could just make this

	if (user_mode(regs))
		return apei_claim_sea(regs);

	if (IS_ENABLED(CONFIG_ARCH_HAS_COPY_MC)) {
		if (fixup_exception_me(regs)) {
			return apei_claim_sea(regs);
		}
	}

	return false;

or maybe even (I may have messed this up, but I think this logic
works).

	if (!user_mode(regs) && IS_ENABLED(CONFIG_ARCH_HAS_COPY_MC)) {
		if (!fixup_exception_me(regs))
			return false;
	}
	return apei_claim_sea(regs);

> +			return true;
> +	} else if (IS_ENABLED(CONFIG_ARCH_HAS_COPY_MC)) {
> +		if (fixup_exception_me(regs) && !apei_claim_sea(regs))

Same here with using apei_claim_sea(regs) == 0 so it's obvious we
are checking for an error, not a boolean.

> +			return true;
> +	}
> +
> +	return false;
> +}
> +
>  static int do_sea(unsigned long far, unsigned long esr, struct pt_regs *regs)
>  {
>  	const struct fault_info *inf;
>  	unsigned long siaddr;
>  
> -	inf = esr_to_fault_info(esr);
> -
> -	if (user_mode(regs) && apei_claim_sea(regs) == 0) {
> -		/*
> -		 * APEI claimed this as a firmware-first notification.
> -		 * Some processing deferred to task_work before ret_to_user().
> -		 */
> +	if (do_apei_claim_sea(regs))

It might be made sense to factor this out first, then could be reviewed
as a noop before the new stuff is added.  Still it's not much code, so doesn't
really matter.
Might be worth keeping to returning 0 for success, error code
otherwise as per apei_claim_sea(regs)

The bool returning functions in the nearby code tend to be is_xxxx
not things that succeed or not.

If you change it to return int make this
	if (do_apei_claim_sea(regs) == 0)
so it's obvious this is the no error case.

>  		return 0;
> -	}
>  
> +	inf = esr_to_fault_info(esr);
>  	if (esr & ESR_ELx_FnV) {
>  		siaddr = 0;
>  	} else {