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 <gli...@google.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
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