linux-kernel - Re: [PATCH v4 1/3] x86/mce: Use is_copy_from_user() to determine copy-from-user context

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Message-ID: <20250307204018.GAZ8tZstt11Y4KFprC@fat_crate.local>
Date: Fri, 7 Mar 2025 21:40:18 +0100
From: Borislav Petkov <bp@...en8.de>
To: Shuai Xue <xueshuai@...ux.alibaba.com>
Cc: tony.luck@...el.com, peterz@...radead.org, catalin.marinas@....com,
	yazen.ghannam@....com, akpm@...ux-foundation.org,
	linmiaohe@...wei.com, nao.horiguchi@...il.com, tglx@...utronix.de,
	mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org,
	hpa@...or.com, jpoimboe@...nel.org, linux-edac@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	baolin.wang@...ux.alibaba.com, tianruidong@...ux.alibaba.com
Subject: Re: [PATCH v4 1/3] x86/mce: Use is_copy_from_user() to determine
 copy-from-user context

On Fri, Mar 07, 2025 at 01:44:02PM +0800, Shuai Xue wrote:
> Commit 4c132d1d844a ("x86/futex: Remove .fixup usage") introduced a new
> extable fixup type, EX_TYPE_EFAULT_REG, and commit 4c132d1d844a
> ("x86/futex: Remove .fixup usage") updated the extable fixup type for
> copy-from-user operations, changing it from EX_TYPE_UACCESS to
> EX_TYPE_EFAULT_REG. The error context for copy-from-user operations no
> longer functions as an in-kernel recovery context. Consequently, the error
> context for copy-from-user operations no longer functions as an in-kernel
> recovery context, resulting in kernel panics with the message: "Machine
> check: Data load in unrecoverable area of kernel."
> 
> The critical aspect is identifying whether the error context involves a
> read from user memory. We do not care about the ex-type if we know its a

Please use passive voice in your commit message: no "we" or "I", etc,
and describe your changes in imperative mood.

Also, pls read section "2) Describe your changes" in
Documentation/process/submitting-patches.rst for more details.

Also, see section "Changelog" in
Documentation/process/maintainer-tip.rst

Bottom line is: personal pronouns are ambiguous in text, especially with
so many parties/companies/etc developing the kernel so let's avoid them
please.

"ex-type"?

Please write in plain English - not in a programming language.

> MOV reading from userspace. is_copy_from_user() return true when both of
> the following conditions are met:
> 
>     - the current instruction is copy

There is no "copy instruction". You mean the "current operation".

>     - source address is user memory

So you can simply say "when reading user memory". Simple.
> 
> So, use is_copy_from_user() to determin if a context is copy user directly.

Unknown word [determin] in commit message.
Suggestions: ['determine',

Please introduce a spellchecker into your patch creation workflow.

Also, run your commit messages through AI to correct the grammar and
formulations in them.

The more important part which I asked for already is, is is_copy_from_user()
exhaustive in determining the that the operation really is a copy from user?

The EX_TYPE_UACCESS things *explicitly* marked such places in the code. Does
is_copy_from_user() guarantee the same, without false positives?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette