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Date:   Wed, 6 Apr 2022 11:58:32 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Tong Tiangen <tongtiangen@huawei.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, james.morse@arm.com
Subject: Re: [RFC PATCH -next V2 3/7] arm64: add support for machine check
 error safe
Message-ID: <Yk1yWEKMYxUth93V@FVFF77S0Q05N>
References: <20220406091311.3354723-1-tongtiangen@huawei.com>
 <20220406091311.3354723-4-tongtiangen@huawei.com>
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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?

> 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.

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.

> +
>  #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?

> +
> +	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.

> +
>  	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.

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