Date: Wed, 17 Oct 2018 10:48:37 -0400 From: Rich Felker <dalias@...c.org> To: "Perry E. Metzger" <perry@...rmont.com> 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, Oct 17, 2018 at 09:21:54AM -0400, Perry E. Metzger wrote: > 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. I don't, but one further idea that might appeal to upstream if they want the fs bindings for the sake of executing ancient programs written in postscript that operate on files: rather than binding to actual fs operations on the host, implement a virtual filesystem within the interpreter, and require explicit command line options to import/export files from/to the real filesystem at entry/exit. Rich
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