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

Mark Rutland
2024-08-19T17:29:29Z

Hi Tong,

On Tue, May 28, 2024 at 04:59:11PM +0800, Tong Tiangen 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 <tongtiangen@huawei.com>

Generally this looks ok, but I have a couple of comments below.

> ---
> arch/arm64/Kconfig | 1 +
> arch/arm64/include/asm/asm-extable.h | 31 +++++++++++++++++++++++-----
> arch/arm64/include/asm/asm-uaccess.h | 4 ++++
> arch/arm64/include/asm/extable.h | 1 +
> arch/arm64/lib/copy_to_user.S | 10 ++++-----
> arch/arm64/mm/extable.c | 19 +++++++++++++++++
> arch/arm64/mm/fault.c | 27 +++++++++++++++++-------
> 7 files changed, 75 insertions(+), 18 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 5d91259ee7b5..13ca06ddf3dd 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -20,6 +20,7 @@ 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 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

Could we please use 'MEM_ERR', and likewise for the macros below? That's
more obvious than 'ME_SAFE', and we wouldn't need the comment here.
Likewise elsewhere in this patch and the series.

To Jonathan's comment, I do prefer these numbers are aligned, so aside
from the naming, the diff above looks good.

>
> /* Data fields for EX_TYPE_UACCESS_ERR_ZERO */
> #define EX_DATA_REG_ERR_SHIFT 0
> @@ -51,6 +53,17 @@
> #define _ASM_EXTABLE_UACCESS(insn, fixup) \
> _ASM_EXTABLE_UACCESS_ERR_ZERO(insn, fixup, wzr, wzr)
>
> +#define _ASM_EXTABLE_KACCESS_ERR_ZERO_ME_SAFE(insn, fixup, err, zero) \
> + __ASM_EXTABLE_RAW(insn, fixup, \
> + EX_TYPE_KACCESS_ERR_ZERO_ME_SAFE, \
> + ( \
> + EX_DATA_REG(ERR, err) | \
> + EX_DATA_REG(ZERO, zero) \
> + ))
> +
> +#define _ASM_EXTABLE_KACCESS_ME_SAFE(insn, fixup) \
> + _ASM_EXTABLE_KACCESS_ERR_ZERO_ME_SAFE(insn, fixup, wzr, wzr)
> +
> /*
> * Create an exception table entry for uaccess `insn`, which will branch to `fixup`
> * when an unhandled fault is taken.
> @@ -69,6 +82,14 @@
> .endif
> .endm
>
> +/*
> + * Create an exception table entry for kaccess me(memory error) safe `insn`, which
> + * will branch to `fixup` when an unhandled fault is taken.
> + */
> + .macro _asm_extable_kaccess_me_safe, insn, fixup
> + _ASM_EXTABLE_KACCESS_ME_SAFE(\insn, \fixup)
> + .endm
> +

With the naming above, I think this can be:

| /*
| * Create an exception table entry for kaccess `insn`, which will branch to
| * `fixup` when a memory error is taken
| */
| .macro _asm_extable_kaccess_mem_err, insn, fixup
| _ASM_EXTABLE_KACCESS_MEM_ERR(\insn, \fixup)
| .endm

> #else /* __ASSEMBLY__ */
>
> #include <linux/stringify.h>
> diff --git a/arch/arm64/include/asm/asm-uaccess.h b/arch/arm64/include/asm/asm-uaccess.h
> index 5b6efe8abeeb..7bbebfa5b710 100644
> --- a/arch/arm64/include/asm/asm-uaccess.h
> +++ b/arch/arm64/include/asm/asm-uaccess.h
> @@ -57,6 +57,10 @@ alternative_else_nop_endif
> .endm
> #endif
>
> +#define KERNEL_ME_SAFE(l, x...) \
> +9999: x; \
> + _asm_extable_kaccess_me_safe 9999b, l
> +
> #define USER(l, x...) \
> 9999: x; \
> _asm_extable_uaccess 9999b, l
> diff --git a/arch/arm64/include/asm/extable.h b/arch/arm64/include/asm/extable.h
> index 72b0e71cc3de..bc49443bc502 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_me(struct pt_regs *regs);
> #endif
> 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

These changes mean that regular copy_to_user() will handle kernel memory
errors, rather than only doing that in copy_mc_to_user(). If that's
intentional, please call that out explicitly in the commit message.

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

Same comment as above.

> diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
> index 228d681a8715..8c690ae61944 100644
> --- a/arch/arm64/mm/extable.c
> +++ b/arch/arm64/mm/extable.c
> @@ -72,7 +72,26 @@ bool fixup_exception(struct pt_regs *regs)
> return ex_handler_uaccess_err_zero(ex, regs);
> case EX_TYPE_LOAD_UNALIGNED_ZEROPAD:
> return ex_handler_load_unaligned_zeropad(ex, regs);
> + case EX_TYPE_KACCESS_ERR_ZERO_ME_SAFE:
> + return false;
> }
>
> BUG();
> }
> +
> +bool fixup_exception_me(struct pt_regs *regs)
> +{
> + const struct exception_table_entry *ex;
> +
> + ex = search_exception_tables(instruction_pointer(regs));
> + if (!ex)
> + return false;
> +
> + switch (ex->type) {
> + case EX_TYPE_UACCESS_ERR_ZERO:
> + case EX_TYPE_KACCESS_ERR_ZERO_ME_SAFE:
> + return ex_handler_uaccess_err_zero(ex, regs);
> + }
> +
> + return false;
> +}
> 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))
> + return true;
> + } else if (IS_ENABLED(CONFIG_ARCH_HAS_COPY_MC)) {
> + if (fixup_exception_me(regs) && !apei_claim_sea(regs))
> + return true;
> + }
> +
> + return false;
> +}

Hmm... that'll fixup the exception even if we don't manage to claim a
the SEA. I suspect this should probably be:

static bool do_apei_claim_sea(struct pt_regs *regs)
{
if (apei_claim_sea(regs))
return false;
if (user_mode(regs))
return true;
if (IS_ENABLED(CONFIG_ARCH_HAS_COPY_MC))
return !fixup_excepton_mem_err(regs);

return false;
}

... unless we *don't* want to claim the SEA in the case we don't have a
fixup?

Mark.

> +
> 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))
> return 0;
> - }
>
> + inf = esr_to_fault_info(esr);
> if (esr & ESR_ELx_FnV) {
> siaddr = 0;
> } else {
> --
> 2.25.1
>
>