Re: [PATCH 4/6] mm, arm64: untag user addresses in mm/gup.c

From: Andrey Konovalov <andreyknvl@google.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Mark Rutland <mark.rutland@arm.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	James Morse <james.morse@arm.com>,
	Kees Cook <keescook@chromium.org>,
	Bart Van Assche <bart.vanassche@wdc.com>,
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	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Philippe Ombredanne <pombredanne@nexb.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>,
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	Lee Smith <Lee.Smith@arm.com>, Kostya Serebryany <kcc@google.com>,
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	Ramana Radhakrishnan <Ramana.Radhakrishnan@arm.com>,
	Evgeniy Stepanov <eugenis@google.com>
Subject: Re: [PATCH 4/6] mm, arm64: untag user addresses in mm/gup.c
Date: Thu, 3 May 2018 18:51:14 +0200	[thread overview]
Message-ID: <CAAeHK+xzcjVm+E+nHLNcZ1jDOMM3ha2fH+Y0G26RU7aO81BSdw@mail.gmail.com> (raw)
In-Reply-To: <20180503152432.q742zvdbv6xtvo34@kshutemo-mobl1>

On Thu, May 3, 2018 at 5:24 PM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> On Thu, May 03, 2018 at 04:09:56PM +0200, Andrey Konovalov wrote:
>> On Wed, May 2, 2018 at 7:25 PM, Andrey Konovalov <andreyknvl@google.com> wrote:

>> I wasn't able to find anything that calls follow_page with pointers
>> passed from userspace except for the memory subsystem syscalls, and we
>> deliberately don't add untagging in those.
>
> I guess I missed this part, but could you elaborate on this? Why?
> Not yet or not ever?

Check out the discussion here:
https://www.spinics.net/lists/arm-kernel/msg640936.html

>
> Also I wounder if we can find (with sparse?) all places where we cast out
> __user. This would give a nice list of places where to pay attention.

The way I tested this is I added BUG_ON(top byte tag is set) to
find_vma and find_extend_vma and ran a modified version of syzkaller
that embeds tags into pointers overnight. The only crashes that I saw
were coming from memory subsystem syscalls. I then temporarily added
untagging to suppress those crashes
(https://gist.github.com/xairy/3aa1f57798fa62522c8ac53fad9b74ca), and
didn't see any crashes after that.