From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: kstewart@linuxfoundation.org, gustavo@embeddedor.com,
gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
Jing Xiangfeng <jingxiangfeng@huawei.com>,
linux-mm@kvack.org, sakari.ailus@linux.intel.com,
bhelgaas@google.com, tglx@linutronix.de,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm: fix page faults in do_alignment
Date: Fri, 30 Aug 2019 23:29:06 +0100 [thread overview]
Message-ID: <20190830222906.GH13294@shell.armlinux.org.uk> (raw)
In-Reply-To: <87y2zav01z.fsf@x220.int.ebiederm.org>
On Fri, Aug 30, 2019 at 04:02:48PM -0500, Eric W. Biederman wrote:
> Russell King - ARM Linux admin <linux@armlinux.org.uk> writes:
>
> > On Fri, Aug 30, 2019 at 02:45:36PM -0500, Eric W. Biederman wrote:
> >> Russell King - ARM Linux admin <linux@armlinux.org.uk> writes:
> >>
> >> > On Fri, Aug 30, 2019 at 09:31:17PM +0800, Jing Xiangfeng wrote:
> >> >> The function do_alignment can handle misaligned address for user and
> >> >> kernel space. If it is a userspace access, do_alignment may fail on
> >> >> a low-memory situation, because page faults are disabled in
> >> >> probe_kernel_address.
> >> >>
> >> >> Fix this by using __copy_from_user stead of probe_kernel_address.
> >> >>
> >> >> Fixes: b255188 ("ARM: fix scheduling while atomic warning in alignment handling code")
> >> >> Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
> >> >
> >> > NAK.
> >> >
> >> > The "scheduling while atomic warning in alignment handling code" is
> >> > caused by fixing up the page fault while trying to handle the
> >> > mis-alignment fault generated from an instruction in atomic context.
> >> >
> >> > Your patch re-introduces that bug.
> >>
> >> And the patch that fixed scheduling while atomic apparently introduced a
> >> regression. Admittedly a regression that took 6 years to track down but
> >> still.
> >
> > Right, and given the number of years, we are trading one regression for
> > a different regression. If we revert to the original code where we
> > fix up, we will end up with people complaining about a "new" regression
> > caused by reverting the previous fix. Follow this policy and we just
> > end up constantly reverting the previous revert.
> >
> > The window is very small - the page in question will have had to have
> > instructions read from it immediately prior to the handler being entered,
> > and would have had to be made "old" before subsequently being unmapped.
>
> > Rather than excessively complicating the code and making it even more
> > inefficient (as in your patch), we could instead retry executing the
> > instruction when we discover that the page is unavailable, which should
> > cause the page to be paged back in.
>
> My patch does not introduce any inefficiencies. It onlys moves the
> check for user_mode up a bit. My patch did duplicate the code.
>
> > If the page really is unavailable, the prefetch abort should cause a
> > SEGV to be raised, otherwise the re-execution should replace the page.
> >
> > The danger to that approach is we page it back in, and it gets paged
> > back out before we're able to read the instruction indefinitely.
>
> I would think either a little code duplication or a function that looks
> at user_mode(regs) and picks the appropriate kind of copy to do would be
> the best way to go. Because what needs to happen in the two cases for
> reading the instruction are almost completely different.
That is what I mean. I'd prefer to avoid that with the large chunk of
code. How about instead adding a local replacement for
probe_kernel_address() that just sorts out the reading, rather than
duplicating all the code to deal with thumb fixup.
> > However, as it's impossible for me to contact the submitter, anything
> > I do will be poking about in the dark and without any way to validate
> > that it does fix the problem, so I think apart from reviewing of any
> > patches, there's not much I can do.
>
> I didn't realize your emails to him were bouncing. That is odd. Mine
> don't appear to be.
Hmm, so the fact I posted publically in reply to my reply with the MTA
bounce message didn't give you a clue?
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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next prev parent reply other threads:[~2019-08-30 22:29 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-30 13:31 [PATCH] arm: fix page faults in do_alignment Jing Xiangfeng
2019-08-30 13:35 ` Russell King - ARM Linux admin
2019-08-30 13:48 ` Russell King - ARM Linux admin
2019-08-30 19:45 ` Eric W. Biederman
2019-08-30 20:30 ` Russell King - ARM Linux admin
2019-08-30 21:02 ` Eric W. Biederman
2019-08-30 22:29 ` Russell King - ARM Linux admin [this message]
2019-09-02 17:36 ` Eric W. Biederman
2019-09-04 2:17 ` Jing Xiangfeng
2019-09-06 15:17 ` Russell King - ARM Linux admin
2019-09-15 18:34 ` Russell King - ARM Linux admin
2019-09-16 14:31 ` Eric W. Biederman
2019-08-31 1:49 ` Jing Xiangfeng
2019-08-31 7:55 ` Russell King - ARM Linux admin
2019-08-31 9:16 ` Jing Xiangfeng
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