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From: Andy Lutomirski <luto@kernel.org>
To: "Luck, Tony" <tony.luck@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	X86 ML <x86@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Darren Hart <dvhart@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-edac <linux-edac@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery
Date: Tue, 12 Jan 2021 10:57:07 -0800	[thread overview]
Message-ID: <CALCETrXw59WDTwfoZHVtDrveMpFF=Eh4jaOF7vFsF02Zk54mDw@mail.gmail.com> (raw)
In-Reply-To: <20210112182357.GA16390@agluck-desk2.amr.corp.intel.com>

On Tue, Jan 12, 2021 at 10:24 AM Luck, Tony <tony.luck@intel.com> wrote:
>
> On Tue, Jan 12, 2021 at 09:21:21AM -0800, Andy Lutomirski wrote:
> > Well, we need to do *something* when the first __get_user() trips the
> > #MC.  It would be nice if we could actually fix up the page tables
> > inside the #MC handler, but, if we're in a pagefault_disable() context
> > we might have locks held.  Heck, we could have the pagetable lock
> > held, be inside NMI, etc.  Skipping the task_work_add() might actually
> > make sense if we get a second one.
> >
> > We won't actually infinite loop in pagefault_disable() context -- if
> > we would, then we would also infinite loop just from a regular page
> > fault, too.
>
> Fixing the page tables inside the #MC handler to unmap the poison
> page would indeed be a good solution. But, as you point out, not possible
> because of locks.
>
> Could we take a more drastic approach? We know that this case the kernel
> is accessing a user address for the current process. Could the machine
> check handler just re-write %cr3 to point to a kernel-only page table[1].
> I.e. unmap the entire current user process.

That seems scary, especially if we're in the middle of a context
switch when this happens.  We *could* make it work, but I'm not at all
convinced it's wise.

>
> Then any subsequent access to user space will page fault. Maybe have a
> flag in the task structure to help the #PF handler understand what just
> happened.
>
> The code we execute in the task_work handler can restore %cr3

This would need to be integrated with something much more local IMO.
Maybe it could be scoped to pagefault_disable()/pagefault_enable()?

--Andy

  reply	other threads:[~2021-01-12 18:58 UTC|newest]

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-08 22:22 [PATCH 0/2] Fix infinite machine check loop in futex_wait_setup() Tony Luck
2021-01-08 22:22 ` [PATCH 1/2] x86/mce: Avoid infinite loop for copy from user recovery Tony Luck
2021-01-08 22:22 ` [PATCH 2/2] futex, x86/mce: Avoid double machine checks Tony Luck
2021-01-08 22:47   ` Peter Zijlstra
2021-01-08 23:08     ` Luck, Tony
2021-01-08 23:14       ` Peter Zijlstra
2021-01-08 23:20         ` Luck, Tony
2021-01-11 21:44 ` [PATCH v2 0/3] Fix infinite machine check loop in futex_wait_setup() Tony Luck
2021-01-11 21:44   ` [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery Tony Luck
2021-01-11 22:11     ` Andy Lutomirski
2021-01-11 22:20       ` Luck, Tony
2021-01-12 17:00         ` Andy Lutomirski
2021-01-12 17:00           ` Andy Lutomirski
2021-01-12 17:16           ` Luck, Tony
2021-01-12 17:21             ` Andy Lutomirski
2021-01-12 17:21               ` Andy Lutomirski
2021-01-12 18:23               ` Luck, Tony
2021-01-12 18:57                 ` Andy Lutomirski [this message]
2021-01-12 18:57                   ` Andy Lutomirski
2021-01-12 20:52                   ` Luck, Tony
2021-01-12 22:04                     ` Andy Lutomirski
2021-01-13  1:50                       ` Luck, Tony
2021-01-13  4:15                         ` Andy Lutomirski
2021-01-13 10:00                           ` Borislav Petkov
2021-01-13 16:06                             ` Luck, Tony
2021-01-13 16:19                               ` Borislav Petkov
2021-01-13 16:32                                 ` Luck, Tony
2021-01-13 17:35                                   ` Borislav Petkov
2021-01-14 20:22     ` Borislav Petkov
2021-01-14 21:05       ` Luck, Tony
2021-01-11 21:44   ` [PATCH v2 2/3] x86/mce: Add new return value to get_user() for machine check Tony Luck
2021-01-11 21:44   ` [PATCH v2 3/3] futex, x86/mce: Avoid double machine checks Tony Luck
2021-01-14 17:22   ` [PATCH v2 0/3] Fix infinite machine check loop in futex_wait_setup() Andy Lutomirski
2021-01-14 17:22     ` Andy Lutomirski
2021-01-15  0:38   ` [PATCH v3] x86/mce: Avoid infinite loop for copy from user recovery Tony Luck
2021-01-15 15:27     ` Borislav Petkov
2021-01-15 19:34       ` Luck, Tony
2021-01-15 20:51         ` [PATCH v4] " Luck, Tony
2021-01-15 23:23           ` Luck, Tony
2021-01-19 10:56             ` Borislav Petkov
2021-01-19 23:57               ` Luck, Tony
2021-01-20 12:18                 ` Borislav Petkov
2021-01-20 17:17                   ` Luck, Tony
2021-01-21 21:09                   ` Luck, Tony
2021-01-25 22:55                     ` [PATCH v5] " Luck, Tony
2021-01-26 11:03                       ` Borislav Petkov
2021-01-26 22:36                         ` Luck, Tony
2021-01-28 17:57                           ` Borislav Petkov
2021-02-01 18:58                             ` Luck, Tony
2021-02-02 11:01                               ` Borislav Petkov
2021-02-02 16:04                                 ` Luck, Tony
2021-02-02 21:06                                   ` Borislav Petkov
2021-02-02 22:12                                     ` Luck, Tony
2021-01-18 15:39         ` [PATCH v3] " Borislav Petkov
2021-07-06 19:06 [PATCH 0/3] More machine check recovery fixes Tony Luck
2021-08-18  0:29 ` [PATCH v2 " Tony Luck
2021-08-18  0:29   ` [PATCH v2 1/3] x86/mce: Avoid infinite loop for copy from user recovery Tony Luck
2021-08-20 17:31     ` Borislav Petkov
2021-08-20 18:59       ` Luck, Tony
2021-08-20 19:27         ` Borislav Petkov
2021-08-20 20:23           ` Luck, Tony
2021-08-21  4:51             ` Tony Luck
2021-08-21 21:51               ` Al Viro
2021-08-22 14:36             ` Borislav Petkov
2021-08-20 20:33           ` Luck, Tony
2021-08-22 14:46             ` Borislav Petkov
2021-08-23 15:24               ` Luck, Tony
2021-09-13  9:24     ` Borislav Petkov

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