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Date: Wed, 17 Oct 2018 09:21:54 -0400
From: "Perry E. Metzger" <perry@...rmont.com>
To: Rich Felker <dalias@...c.org>
Cc: oss-security@...ts.openwall.com, Tavis Ormandy <taviso@...gle.com>, Bob
 Friesenhahn <bfriesen@...ple.dallas.tx.us>
Subject: Re: ghostscript: bypassing executeonly to escape
 -dSAFER sandbox (CVE-2018-17961)

On Wed, 17 Oct 2018 02:09:28 -0400 Rich Felker <dalias@...c.org>
wrote:
> > > > I keep wondering if there isn't a way to fully remove the
> > > > dangerous bits from a postscript interpreter so it can _only_
> > > > be used to view the document and literally has no file system
> > > > access compiled in at all, so there's no way to touch the fs
> > > > etc. regardless of what flags the interpreter is invoked with.
> > > >
> > > > (I, too, find removing the ability to look at historical
> > > > postscript documents a bit more draconian than I like.)
> > > >
> > > >    
> > > I've discussed it with upstream, it's a hard no because they
> > > feel it would make ghostscript non-conforming (i.e.
> > > non-conforming with the Adobe PostScript Language Reference
> > > Manual)
> > > 
> > > We probably have similar thoughts on this, but that is the final
> > > word from upstream.  
> > 
> > They wouldn't even support a compilation mode where if you #define
> > the right thing those syscalls are cut out?
> > 
> > I don't care much about upstream's desires on this if they oppose
> > that. I'd be happy to have patches that simply cut out the
> > dangerous syscalls entirely. It's open source, that should be
> > feasible.  
> 
> This. It's utterly ridiculous that the interpreter even has bindings
> for accessing the filesystem and such. But I wonder if some of its
> library routines (e.g. font loading) are implemented in Postscript,
> using these bindings, rather than being implemented in C outside of
> the language interpreter. If so it might be harder to extricate.
> But I still think it's worthwhile to try. Once there are patches I
> would expect all reasonable distros to start shipping with them,
> and if upstream tries to make it hard, I would expect one of the
> big distros to just fork and abandon upstream.

Does anyone other than Tavis know their way around the inside of the
codebase? Perhaps we can collaborate on patches.

Perry
-- 
Perry E. Metzger		perry@...rmont.com

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