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Date:   Sat, 13 Jun 2020 20:55:08 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     afzal mohammed <afzal.mohd.ma@...il.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Linus Walleij <linus.walleij@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nicolas Pitre <nico@...xnic.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()

On Sat, Jun 13, 2020 at 6:00 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> On Sat, Jun 13, 2020 at 04:41:18PM +0100, Al Viro wrote:
> > On Sat, Jun 13, 2020 at 04:31:02PM +0100, Al Viro wrote:
> > > On Sat, Jun 13, 2020 at 07:12:36PM +0530, afzal mohammed wrote:
> > > > Hi,
> > > >
> > > > On Sat, Jun 13, 2020 at 01:56:15PM +0100, Al Viro wrote:
> > > >
> > > > > Incidentally, what about get_user()/put_user()?  _That_ is where it's
> > > > > going to really hurt...
> > > >
> > > > All other uaccess routines are also planned to be added, posting only
> > > > copy_{from,to}_user() was to get early feedback (mentioned in the
> > > > cover letter)
> > >
> > > Sure, but what I mean is that I'd expect the performance loss to be
> > > dominated by that, not by copy_from_user/copy_to_user on large amounts
> > > of data.  Especially on the loads like kernel builds - a lot of stat()
> > > and getdents() calls there.
> >
> > To clarify: stat() means small copy_to_user(), getdents() - a mix of
> > put_user() and small copy_to_user().  I would be very surprised if it
> > does not hurt a lot.
>
> PS: there's another fun issue here:
>
> fill a file with zeroes
> mmap that file in two areas, MAP_SHARED
> thread 1:
> munmap() the first area
> fill the second one with 'X'
> thread 2:
> write() from the first area into pipe
>
> One could expect that nothing by zeroes gets written into
> pipe - it might be a short write() (or -EFAULT), but finding
> any 'X' there would be a bug.
>
> Your patches allow for a possibility of write() doing
> get_user_pages_fast(), getting the first page just as
> munmap() is about to remove it from page tables and bugger
> off.  Then thread 1 proceeds with the store (via the
> second area).  And then thread 2 does memcpy() from that
> thing via a kmap_atomic()-created alias, observing the
> effect of the store.
>
> That might or might not be a POSIX violation, but it does
> look like a QoI issue...

I assume this problem exists in arch/um/kernel/skas/uaccess.c
and in Ingo's old x86 VMSPLIT_4G_4G patch as well, right?

I guess holding mmap_read_lock() would prevent it but make
it even more expensive.

      Arnd

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