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From: Andy Lutomirski <luto@kernel.org>
To: Tony Luck <tony.luck@intel.com>
Cc: 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>,
	Andy Lutomirski <luto@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-edac <linux-edac@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH v2 0/3] Fix infinite machine check loop in futex_wait_setup()
Date: Thu, 14 Jan 2021 09:22:44 -0800	[thread overview]
Message-ID: <CALCETrUvSWM5yOviJRD4Y+HZxDschAuTHfyffTVc8qifvh1AiA@mail.gmail.com> (raw)
In-Reply-To: <20210111214452.1826-1-tony.luck@intel.com>

On Mon, Jan 11, 2021 at 1:45 PM Tony Luck <tony.luck@intel.com> wrote:
>
> Linux can now recover from machine checks where kernel code is
> doing get_user() to access application memory. But there isn't
> a way to distinguish whether get_user() failed because of a page
> fault or a machine check.
>
> Thus there is a problem if any kernel code thinks it can retry
> an access after doing something that would fix the page fault.
>
> One such example (I'm sure there are more) is in futex_wait_setup()
> where an attempt to read the futex with page faults disabled. Then
> a retry (after dropping a lock so page faults are safe):
>
>
>         ret = get_futex_value_locked(&uval, uaddr);
>
>         if (ret) {
>                 queue_unlock(*hb);
>
>                 ret = get_user(uval, uaddr);
>
> It would be good to avoid deliberately taking a second machine
> check (especially as the recovery code does really bad things
> and ends up in an infinite loop!).
>
> V2 (thanks to feedback from PeterZ) fixes this by changing get_user() to
> return -ENXIO ("No such device or address") for the case where a machine
> check occurred. Peter left it open which error code to use (suggesting
> "-EMEMERR or whatever name we come up with"). I think the existing ENXIO
> error code seems appropriate (the address being accessed has effectively
> gone away). But I don't have a strong attachment if anyone thinks we
> need a new code.
>
> Callers can check for ENXIO in paths where the access would be
> retried so they can avoid a second machine check.
>

I don't love this -- I'm concerned that there will be some code path
that expects a failing get_user() to return -EFAULT, not -ENXIO.
Also, get_user() *can* return -EFAULT when it hits bad memory even
with your patch if the recovery code manages to yank the PTE before
get_user().

So I tend to think that the machine check code should arrange to
survive some reasonable number of duplicate machine checks.

  parent reply	other threads:[~2021-01-14 17:23 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-08 22:22 [PATCH 0/2] " 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   ` Andy Lutomirski [this message]
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

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