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

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Message-ID: <20170525170457.GA3085@amd>
Date:   Thu, 25 May 2017 19:04:57 +0200
From:   Pavel Machek <pavel@....cz>
To:     Al Viro <viro@...IV.linux.org.uk>, mtk.manpages@...il.com
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [iov_iter] use memmove() when copying to/from user page

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.

> Consider the case of write() from an mmapped piece of file to overlapping
> piece of the same file.  It is possible and not hard to trigger; all we
> can guarantee is the lack of infoleaks, filesystem corruption or memory
> corruption.  File *contents* in the affected area can't be sanely relied
> upon.
> 
> This case is not different.  BTW, neither SUS, nor our manpages for
> write(2) mention these issues with mmap()-created aliases between the
> source and destination.

Added people doing documentation; seems like we have some updates to
do there, too.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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