linux-kernel - Re: [PATCH 08/16] nfsd: escape high characters in binary data

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Date:   Fri, 21 Jun 2019 15:26:00 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/16] nfsd: escape high characters in binary data

On Fri, Jun 21, 2019 at 01:45:44PM -0400, J. Bruce Fields wrote:
> I'm not sure who to get review from for this kind of thing.
> 
> Kees, you seem to be one of the only people to touch string_helpers.c
> at all recently, any ideas?

Hi! Yeah, I'm happy to take a look. Notes below...

> 
> --b.
> 
> On Thu, Jun 20, 2019 at 10:51:07AM -0400, J. Bruce Fields wrote:
> > From: "J. Bruce Fields" <bfields@...hat.com>
> > 
> > I'm exposing some information about NFS clients in pseudofiles.  I
> > expect to eventually have simple tools to help read those pseudofiles.
> > 
> > But it's also helpful if the raw files are human-readable to the extent
> > possible.  It aids debugging and makes them usable on systems that don't
> > have the latest nfs-utils.
> > 
> > A minor challenge there is opaque client-generated protocol objects like
> > state owners and client identifiers.  Some clients generate those to
> > include handy information in plain ascii.  But they may also include
> > arbitrary byte sequences.
> > 
> > I think the simplest approach is to limit to isprint(c) && isascii(c)
> > and escape everything else.

Can you get the same functionality out of sprintf's %pE (escaped
string)? If not, maybe we should expand the flags available?

 * - 'E[achnops]' For an escaped buffer, where rules are defined by
 * combination
 *                of the following flags (see string_escape_mem() for
 *                the
 *                details):
 *                  a - ESCAPE_ANY
 *                  c - ESCAPE_SPECIAL
 *                  h - ESCAPE_HEX
 *                  n - ESCAPE_NULL
 *                  o - ESCAPE_OCTAL
 *                  p - ESCAPE_NP
 *                  s - ESCAPE_SPACE
 *                By default ESCAPE_ANY_NP is used.

This doesn't cover escaping >0x7f and " and \

And perhaps I should rework kstrdup_quotable() to have that flag? It's
not currently escaping non-ascii and it probably should. Maybe
"ESCAPE_QUOTABLE" as "q"?

-- 
Kees Cook