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From: "Luck, Tony" <tony.luck@intel.com>
To: Borislav Petkov <bp@alien8.de>
Cc: 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
Subject: Re: [PATCH v3] x86/mce: Avoid infinite loop for copy from user recovery
Date: Fri, 15 Jan 2021 11:34:35 -0800	[thread overview]
Message-ID: <20210115193435.GA4663@agluck-desk2.amr.corp.intel.com> (raw)
In-Reply-To: <20210115152754.GC9138@zn.tnic>

On Fri, Jan 15, 2021 at 04:27:54PM +0100, Borislav Petkov wrote:
> On Thu, Jan 14, 2021 at 04:38:17PM -0800, Tony Luck wrote:
> > Add a "mce_busy" counter so that task_work_add() is only called once
> > per faulty page in this task.
> 
> Yeah, that sentence can be removed now too.

I will update with new name "mce_count" and some details.

> > -static void queue_task_work(struct mce *m, int kill_current_task)
> > +static void queue_task_work(struct mce *m, char *msg, int kill_current_task)
> 
> So this function gets called in the user mode MCE case too:
> 
> 	if ((m.cs & 3) == 3) {
> 
> 		queue_task_work(&m, msg, kill_current_task);
> 	}
> 
> Do we want to panic for multiple MCEs to different addresses in user
> mode?

In the user mode case we should only bump mce_count to "1" and
before task_work() gets called. It shouldn't hurt to do the
same checks. Maybe it will catch something weird - like an NMI
handler on return from the machine check doing a get_user() that
hits another machine check during the return from this machine check.

AndyL has made me extra paranoid. :-)

> > -	current->mce_addr = m->addr;
> > -	current->mce_kflags = m->kflags;
> > -	current->mce_ripv = !!(m->mcgstatus & MCG_STATUS_RIPV);
> > -	current->mce_whole_page = whole_page(m);
> > +	if (current->mce_count++ == 0) {
> > +		current->mce_addr = m->addr;
> > +		current->mce_kflags = m->kflags;
> > +		current->mce_ripv = !!(m->mcgstatus & MCG_STATUS_RIPV);
> > +		current->mce_whole_page = whole_page(m);
> > +	}
> > +
> 
> 	/* Magic number should be large enough */
>
> > +	if (current->mce_count > 10)

Will add similar comment here ... and to other tests in this function
since it may not be obvious to me next year what I was thinking now :-)

> > +	if (current->mce_count > 10)
> > +		mce_panic("Too many machine checks while accessing user data", m, msg);
> > +
> > +	if (current->mce_count > 1 || (current->mce_addr >> PAGE_SHIFT) != (m->addr >> PAGE_SHIFT))
> > +		mce_panic("Machine checks to different user pages", m, msg);
> 
> Will this second part of the test expression, after the "||" ever hit?

No :-( This code is wrong. Should be "&&" not "||". Then it makes more sense.
Will fix for v4.

> In any case, what are you trying to catch with this? Two get_user() to
> different pages both catching MCEs?

Yes. Trying to catch two accesses to different pages. Need to do this
because kill_me_maybe() is only going to offline one page.

I'm not expecting that this would ever hit.  It means that calling code
took a machine check on one page and get_user() said -EFAULT. The the
code decided to access a different page *and* that other page also triggered
a machine check.

> > +	/* Do not call task_work_add() more than once */
> > +	if (current->mce_count > 1)
> > +		return;
> 
> That won't happen either, AFAICT. It'll panic above.

With the s/||/&&/ above, we can get here.
> 
> Regardless, I like how this is all confined to the MCE code and there's
> no need to touch stuff outside...

Thanks for the review.

-Tony

  reply	other threads:[~2021-01-15 19:37 UTC|newest]

Thread overview: 52+ 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
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 [this message]
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
2021-09-13  9:24     ` Borislav Petkov
2021-09-13 21:52       ` [PATCH v3] " Luck, Tony
2021-09-14  8:28         ` Borislav Petkov

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