linux-kernel - RE: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery

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Date:   Wed, 13 Jan 2021 16:32:54 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-edac <linux-edac@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: RE: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user
 recovery

>> Maybe the other difference in approach between Andy and me is whether to
>> go for a solution that covers all the corner cases, or just make an incremental
>> improvement that allows for recover in some useful subset of remaining fatal
>> cases, but still dies in other cases.
>
> Does that mean more core code surgery?

Yes. I need to look at other user access inside pagefault_disable/enable()
as likely spots where the code may continue after a machine check and
retry the access.  So expect some more "if (ret == ENXIO) { do something to give up gracefully }"

>> I'm happy to replace error messages with ones that are more descriptive and
>> helpful to humans.
>
> Yap, that: "Multiple copyin" with something more understandable to users...

I'll work on it. We tend not to have essay length messages as panic() strings. But I can
add a comment in the code there so that people who grep whatever panic message
we choose can get more details on what happened and what to do.

-Tony