Re: [RFC PATCH -next V2 5/7] arm64: add get_user to machine check safe

From: Tong Tiangen <tongtiangen@huawei.com>
To: Mark Rutland <mark.rutland@arm.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>
Subject: Re: [RFC PATCH -next V2 5/7] arm64: add get_user to machine check safe
Date: Sat, 9 Apr 2022 17:17:40 +0800	[thread overview]
Message-ID: <8ef7a71b-fd0d-f86e-98bc-0a9cffc79207@huawei.com> (raw)
In-Reply-To: <YlBTLn1cf7+gJmiM@lakrids>

在 2022/4/8 23:22, Mark Rutland 写道:
> On Thu, Apr 07, 2022 at 10:38:04PM +0800, Tong Tiangen wrote:
>> 在 2022/4/6 19:22, Mark Rutland 写道:
>>> On Wed, Apr 06, 2022 at 09:13:09AM +0000, Tong Tiangen wrote:
>>>> Add scenarios get_user to machine check safe. The processing of
>>>> EX_TYPE_UACCESS_ERR_ZERO and EX_TYPE_UACCESS_ERR_ZERO_UCE_RECOVERY is same
>>>> and both return -EFAULT.
>>>
>>> Which uaccess cases do we expect to *not* be recoverable?
>>>
>>> Naively I would assume that if we're going to treat a memory error on a uaccess
>>> as fatal to userspace we should be able to do that for *any* uacesses.
>>>
>>> The commit message should explain why we need the distinction between a
>>> recoverable uaccess and a non-recoverable uaccess.
>>>
>>> Thanks,
>>> Mark.
>>
>> Currently, any memory error consumed in kernel mode will lead to panic
>> (do_sea()).
>>
>> My idea is that not all memory errors consumed in kernel mode are fatal,
>> such as copy_ from_ user/get_ user is a memory error consumed when
>> reading user data in the process context. In this case, we can not let the
>> kernel panic, just kill the process without affecting the operation
>> of the system.
> 
> I understood this part.
> 
>> However, not all uaccess can be recovered without affecting the normal
>> operation of the system. The key is not whether it is uaccess, but whether
>> there are key data affecting the normal operation of the system in the read
>> page.
> 
> Ok. Can you give an example of such a case where the a uaccess that hits
> a memory error must be fatal?
> 
> I think you might be trying to say that for copy_{to,from}_user() we can
> make that judgement, but those are combined user+kernel access
> primitives, and the *uaccess* part should never be reading from a page
> with "key data affecting the normal operation of the system", since
> that's userspace memory.
> 
> Is there any *userspace access* (e.g. where we use LDTR/STTR today)
> where we must treat a memory error as fatal to the system?
> 
> Thanks,
> Mark.
> .

I seem to understand what you mean.
Take copy_to_user()/put_user() as an example. If it encounters memory 
error, only related processes will be affected. According to this 
understanding, it seems that all uaccess can be recovered.

Thanks,
Tong.