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Date:   Mon, 18 Jan 2021 16:39:39 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     "Luck, Tony" <tony.luck@intel.com>
Cc:     x86@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        linux-kernel@vger.kernel.org, linux-edac@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH v3] x86/mce: Avoid infinite loop for copy from user
 recovery
Message-ID: <20210118153939.GC30090@zn.tnic>
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 <20210115152754.GC9138@zn.tnic>
 <20210115193435.GA4663@agluck-desk2.amr.corp.intel.com>
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On Fri, Jan 15, 2021 at 11:34:35AM -0800, Luck, Tony wrote:
> In the user mode case we should only bump mce_count to "1" and
> before task_work() gets called.

Ok, right, it should not be possible to trigger a second MCE while
queue_task_work() runs when it is a user MCE. The handler itself won't
touch the page with the hw error so our assumption is that it'll get
poisoned.

If it doesn't, I guess the memory failure code will kill the process
yadda yadda...

> It shouldn't hurt to do the same checks. Maybe it will catch something
> weird - like an NMI handler on return from the machine check doing a
> get_user() that hits another machine check during the return from this
> machine check.

Eww.

> AndyL has made me extra paranoid. :-)

Yeah, he comes up with the nuttiest scenarios. :-)

-- 
Regards/Gruss,
    Boris.

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