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

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Message-Id: <E1FCB534-9149-437A-971E-F93C009F99C3@amacapital.net>
Date:   Mon, 11 Jan 2021 14:11:56 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Tony Luck <tony.luck@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery


> On Jan 11, 2021, at 1:45 PM, Tony Luck <tony.luck@...el.com> wrote:
> 
> Recovery action when get_user() triggers a machine check uses the fixup
> path to make get_user() return -EFAULT.  Also queue_task_work() sets up
> so that kill_me_maybe() will be called on return to user mode to send a
> SIGBUS to the current process.
> 
> But there are places in the kernel where the code assumes that this
> EFAULT return was simply because of a page fault. The code takes some
> action to fix that, and then retries the access. This results in a second
> machine check.
> 
> While processing this second machine check queue_task_work() is called
> again. But since this uses the same callback_head structure that
> was used in the first call, the net result is an entry on the
> current->task_works list that points to itself.

Is this happening in pagefault_disable context or normal sleepable fault context?  If the latter, maybe we should reconsider finding a way for the machine check code to do its work inline instead of deferring it.

Yes, I realize this is messy, but maybe it’s not that messy. Conceptually, we just (famous last words) need to arrange for an MCE with IF=1 to switch off the IST stack and run like a normal exception.