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

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Message-ID: <20180508151137.zguepljs3pa7xv5g@armageddon.cambridge.arm.com>
Date:   Tue, 8 May 2018 16:11:38 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        linux-doc@...r.kernel.org, Will Deacon <will.deacon@....com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Ingo Molnar <mingo@...nel.org>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Jonathan Corbet <corbet@....net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Bart Van Assche <bart.vanassche@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Lee Smith <Lee.Smith@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Kostya Serebryany <kcc@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        James Morse <james.morse@....com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Zi Yan <zi.yan@...rutgers.edu>
Subject: Re: [PATCH 4/6] mm, arm64: untag user addresses in mm/gup.c

On Wed, May 02, 2018 at 07:25:17PM +0200, Andrey Konovalov wrote:
> On Wed, May 2, 2018 at 5:36 PM, Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> > On Wed, May 02, 2018 at 02:38:42PM +0000, Andrey Konovalov wrote:
> >> > Does having a tagged address here makes any difference? I couldn't hit a
> >> > failure with my simple tests (LD_PRELOAD a library that randomly adds
> >> > tags to pointers returned by malloc).
> >>
> >> I think you're right, follow_page_mask is only called from
> >> __get_user_pages, which already untagged the address. I'll remove
> >> untagging here.
> >
> > It also called from follow_page(). Have you covered all its callers?
> 
> Oh, missed that, will take a look.
> 
> Thinking about that, would it make sense to add untagging to find_vma
> (and others) instead of trying to cover all find_vma callers?

I don't think adding the untagging to find_vma() is sufficient. In many
cases the caller does a subsequent check like 'start < vma->vm_start'
(see sys_msync() as an example, there are a few others as well). What I
did in my tests was a WARN_ON_ONCE() in find_vma() if the address is
tagged.

-- 
Catalin