linux-kernel - Re: [PATCH] [iov_iter] use memmove() when copying to/from user page

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Message-ID: <20170525212715.GA23446@amd>
Date:   Thu, 25 May 2017 23:27:15 +0200
From:   Pavel Machek <pavel@....cz>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, mtk.manpages@...il.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [iov_iter] use memmove() when copying to/from user page

On Thu 2017-05-25 10:15:11, Eric Dumazet wrote:
> On Thu, May 25, 2017 at 10:04 AM, Pavel Machek <pavel@....cz> wrote:
> > Hi!
> >
> > On Tue 2017-05-16 23:48:54, Al Viro wrote:
> >> On Tue, May 16, 2017 at 03:15:16PM -0700, Dmitry Vyukov wrote:
> >> > > Because it's not going to be *one* call of memcpy() or memmove().  It's
> >> > > one per page.
> >> >
> >> >
> >> > I missed that.
> >> >
> >> > I assumed that in the case of sendfile from memfd to memfd data will
> >> > be copied directly. But it goes through a pipe with multiple buffers.
> >> > Does not look easily fixable.
> >>
> >> Which leaves us only with "will nasal demons really fly there?".
> >
> > It seems so:
> >
> > Date: Tue, 16 May 2017 14:27:34 +0200
> > From: Alexander Potapenko <glider@...gle.com>
> > Subject: [PATCH] [iov_iter] use memmove() when copying to/from user
> > page
> >
> > BUG: memcpy-param-overlap in generic_perform_write+0x551/0xa20
> > __msan_memcpy(ffff88013c6e3001, ffff88013c6e3000, 105)
> > CPU: 0 PID: 1040 Comm: probe Not tainted 4.11.0-rc5+ #2562
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs
> > 01/01/2011
> >
> > At the very least, we do not want userland to trigger kernel
> > BUG()s... so this needs fixes beyond documentation.
> 
> To be fair, this BUG() only happens because Alexander added one in memcpy() ,
> testing for the cases where memmove() should have been used.
> 
> Kind of a debugging trap if you prefer.
> 
> This is not something that a pristine kernel would do.

Aha, so BUG() is not realy a problem. Problem is that memcpy() may not
be called on overlapping regions, and may do something stupid; but we
don't have evidence that it does.

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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