linux-kernel - Re: strict copy_from

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Date:	Tue, 5 Jan 2010 05:31:17 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...e.hu>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: strict copy_from_user checks issues?

On Tue, 5 Jan 2010 14:19:11 +0100
Heiko Carstens <heiko.carstens@...ibm.com> wrote:

> On Tue, Jan 05, 2010 at 01:47:20PM +0100, Arnd Bergmann wrote:
> > On Tuesday 05 January 2010, Heiko Carstens wrote:
> > > On Mon, Jan 04, 2010 at 05:43:08PM -0800, Arjan van de Ven wrote:
> > > > On Mon, 4 Jan 2010 16:43:45 +0100
> > > > Heiko Carstens <heiko.carstens@...ibm.com> wrote:
> > > > > x86 and sparc return -EFAULT in copy_from_user instead of the
> > > > > number of not copied bytes as it should in case of a detected
> > > > > buffer overflow. That might have unwanted side effects. I
> > > > > would guess that is a bug.
> > > > 
> > > > killing the bad guy in case of a real buffer overflow is
> > > > appropriate.. this should never trigger for legitimate users.
> > > 
> > > The point I tried to make is that no caller of copy_from_user can
> > > assume that it would ever return -EFAULT. And if any caller does
> > > so it is broken. But then again it probably doesn't matter in
> > > this case as long as something != 0 is returned.
> > 
> > To quote simple_read_from_buffer():
> > 
> >         size_t ret;
> > 	...
> >         ret = copy_to_user(to, from + pos, count);
> >         if (ret == count)
> >                 return -EFAULT;
> >         count -= ret;
> >         *ppos = pos + count;
> >         return count;
> > 
> > If copy_from_user() returns a negative value, bad things happen to
> > f_pos and to the value returned from the syscall. Many read()
> > file_operations do this similarly, and I wouldn't be surprised if
> > this could be turned into a security exploit for one of them (not
> > simple_read_from_buffer probably).
> 
> Thanks Arnd. I was looking for such an example. That's why I was
> about to send the patch below (untested).


Acked-by: Arjan van de Ven <arjan@...ux.intel.com>


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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