linux-kernel - Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200613160040.GI23230@ZenIV.linux.org.uk>
Date:   Sat, 13 Jun 2020 17:00:40 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     afzal mohammed <afzal.mohd.ma@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        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 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...