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Date:   Mon, 20 Jun 2022 10:04:42 +0800
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Subject: Re: [PATCH -next v5 7/8] arm64: add uaccess to machine check safe
To:     Mark Rutland <mark.rutland@arm.com>
CC:     James Morse <james.morse@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Ingo Molnar" <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Robin Murphy <robin.murphy@arm.com>,
        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>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>, <linuxppc-dev@lists.ozlabs.org>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Xie XiuQi <xiexiuqi@huawei.com>,
        Guohanjun <guohanjun@huawei.com>
References: <20220528065056.1034168-1-tongtiangen@huawei.com>
 <20220528065056.1034168-8-tongtiangen@huawei.com>
 <YqxELtYkqQNibHaX@FVFF77S0Q05N>
 <a26c74eb-76c2-570a-2f82-503c812dc0f0@huawei.com>
 <Yq24TSpZK+3/86Pj@FVFF77S0Q05N>
From:   Tong Tiangen <tongtiangen@huawei.com>
In-Reply-To: <Yq24TSpZK+3/86Pj@FVFF77S0Q05N>
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在 2022/6/18 19:35, Mark Rutland 写道:
> On Sat, Jun 18, 2022 at 05:27:45PM +0800, Tong Tiangen wrote:
>>
>>
>> 在 2022/6/17 17:06, Mark Rutland 写道:
>>> 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@huawei.com>
>>>
>>>> ---
>>>>    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.
>>
>> You have a point. so there is no need to modify copy_from/to_user.S in this
>> patch set.
> 
> Cool, thanks. If this patch just has the extable change, that's fine by me.
> 
>>> 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 recovery success, The task will be killed and there will be no subsequent
>> memset().
> 
> I don't think that's true.
> 
> IIUC per the last patch, in the exception handler we'll apply the fixup then
> force a signal. That doesn't kill the task immediately, and we'll return from
> the exception handler back into the original context (with the fixup applied).
> 

correct.

> The structure of copy_from_user() is
> 
> 	copy_from_user(to, from, n) {
> 		_copy_from_user(to, from, n) {
> 			res = n;
> 			res = raw_copy_from_user(to, from, n);
> 			if (res)
> 				memset(to + (n - res), 0, res);
> 		}
> 	}
> 
> So when the fixup is applied and res indicates that the copy terminated early,
> there is an unconditinal memset() before the fatal signal is handled in the
> return to userspace path.

correct in this scenario.

My idea is also valuable in many other scenarios.

> 
>>> 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.
> 
> [...]
> 
>>>> 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?
>>
>> According to my understanding of the above, only the modification of
>> extable.c is retained.
>>
>> So what do you mean which part is made into a separate patch?
> 
> As above, if you just retain the extable.c changes, that's fine by me.

Thanks,
Tong.
> 
> Thanks,
> Mark.
> .