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

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Date:   Thu, 3 May 2018 18:51:14 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Jonathan Corbet <corbet@....net>,
        Mark Rutland <mark.rutland@....com>,
        Robin Murphy <robin.murphy@....com>,
        Al Viro <viro@...iv.linux.org.uk>,
        James Morse <james.morse@....com>,
        Kees Cook <keescook@...omium.org>,
        Bart Van Assche <bart.vanassche@....com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Lee Smith <Lee.Smith@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Evgeniy Stepanov <eugenis@...gle.com>
Subject: Re: [PATCH 4/6] mm, arm64: untag user addresses in mm/gup.c

On Thu, May 3, 2018 at 5:24 PM, Kirill A. Shutemov <kirill@...temov.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@...gle.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.