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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 09:21:21 -0800	[thread overview]
Message-ID: <CALCETrWijyKoopqAHjohMbvfX191GhmMVQCQjKWx1s3+SA+-uA@mail.gmail.com> (raw)
In-Reply-To: <20210112171628.GA15664@agluck-desk2.amr.corp.intel.com>

On Tue, Jan 12, 2021 at 9:16 AM Luck, Tony <tony.luck@intel.com> wrote:
>
> On Tue, Jan 12, 2021 at 09:00:14AM -0800, Andy Lutomirski wrote:
> > > On Jan 11, 2021, at 2:21 PM, Luck, Tony <tony.luck@intel.com> wrote:
> > >
> > > On Mon, Jan 11, 2021 at 02:11:56PM -0800, Andy Lutomirski wrote:
> > >>
> > >>>> On Jan 11, 2021, at 1:45 PM, Tony Luck <tony.luck@intel.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.
> > >
> > > The first machine check is in pagefault_disable() context.
> > >
> > > static int get_futex_value_locked(u32 *dest, u32 __user *from)
> > > {
> > >        int ret;
> > >
> > >        pagefault_disable();
> > >        ret = __get_user(*dest, from);
> >
> > I have very mixed feelings as to whether we should even try to recover
> > from the first failure like this.  If we actually want to try to
> > recover, perhaps we should instead arrange for the second MCE to
> > recover successfully instead of panicking.
>
> Well we obviously have to "recover" from the first machine check
> in order to get to the second. Are you saying you don't like the
> different return value from get_user()?
>
> In my initial playing around with this I just had the second machine
> check simply skip the task_work_add(). This worked for this case, but
> only because there wasn't a third, fourth, etc. access to the poisoned
> data. If the caller keeps peeking, then we'll keep taking more machine
> checks - possibly forever.
>
> Even if we do recover with just one extra machine check ... that's one
> more than was necessary.

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.

  reply	other threads:[~2021-01-12 17:22 UTC|newest]

Thread overview: 62+ 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 17:16           ` Luck, Tony
2021-01-12 17:21             ` Andy Lutomirski [this message]
2021-01-12 18:23               ` Luck, Tony
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
2021-01-15  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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