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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery
Date:   Mon, 11 Jan 2021 14:11:56 -0800
Message-Id: <E1FCB534-9149-437A-971E-F93C009F99C3@amacapital.net>
References: <20210111214452.1826-2-tony.luck@intel.com>
Cc:     Borislav Petkov <bp@alien8.de>, 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
In-Reply-To: <20210111214452.1826-2-tony.luck@intel.com>
To:     Tony Luck <tony.luck@intel.com>
Precedence: bulk


> On Jan 11, 2021, at 1:45 PM, Tony Luck <tony.luck@intel.com> wrote:
>=20
> =EF=BB=BFRecovery 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.
>=20
> 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.
>=20
> 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 con=
text?  If the latter, maybe we should reconsider finding a way for the machi=
ne check code to do its work inline instead of deferring it.

Yes, I realize this is messy, but maybe it=E2=80=99s not that messy. Concept=
ually, we just (famous last words) need to arrange for an MCE with IF=3D1 to=
 switch off the IST stack and run like a normal exception.=