linux-kernel - Re: [PATCH v5 12/15] binfmt_flat: allow compressed flat binary format to work on MMU systems

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Date:	Mon, 25 Jul 2016 10:18:52 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Alan Cox <gnomes@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
	linux-m68k@...ts.linux-m68k.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, Greg Ungerer <gerg@...ux-m68k.org>
Subject: Re: [PATCH v5 12/15] binfmt_flat: allow compressed flat binary format to work on MMU systems

On Sunday, July 24, 2016 4:25:16 PM CEST Nicolas Pitre wrote:
> On Sun, 24 Jul 2016, Arnd Bergmann wrote:
> 
> > On Sunday, July 24, 2016 11:30:26 AM CEST Nicolas Pitre wrote:
> > > +#else
> > > +                       /*
> > > +                        * This is used on MMU systems mainly for testing.
> > > +                        * Let's use a kernel buffer to simplify things.
> > > +                        */
> > > +                       long unz_text_len = text_len - sizeof(struct flat_hdr);
> > > +                       long unz_len = unz_text_len + full_data;
> > > +                       char *unz_data = vmalloc(unz_len);
> > > +                       if (!unz_data) {
> > > +                               result = -ENOMEM;
> > > 
> > 
> > Is there a risk of a malicious user exhausting vmalloc space with a
> > binary that has forged headers? If there is, maybe put an upper bound on
> > the size of allocation.
> 
> Patch #3 enforces a cap on all parameters to avoid overflows and 
> unreasonable section sizes.
> 
> Then vmalloc space is used here only for decompressing the binary into, 
> after which the whole thing is copied to user space and the vmalloc area 
> is freed right away.
> 
> > More broadly speaking, are there any other attacks that may get enabled
> > through forged binaries? We've had a couple of vulnerabilities in
> > binfmt_elf over the years, and I wonder how dangerous it might be
> > if distros turn on binfmt_flat support by default.
> 
> That was Alan's concern too which prompted patch #3. But with a clamp on 
> all parameters, everything else is done via user accessors.  So an 
> executable still can crap onto itself or generate a segfault but I doubt 
> we really care at that point.
> 

Ok, sounds good.

	Arnd