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

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Date:   Thu, 27 Jun 2019 16:21:24 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     Kees Cook <keescook@...omium.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 Wed, Jun 26, 2019 at 12:21:49PM -0400, J. Bruce Fields wrote:
> On Mon, Jun 24, 2019 at 05:05:12PM -0400, J. Bruce Fields wrote:
> > On Sat, Jun 22, 2019 at 01:22:56PM -0700, Kees Cook wrote:
> > > On Sat, Jun 22, 2019 at 03:00:58PM -0400, J. Bruce Fields wrote:
> > > > The logic around ESCAPE_NP and the "only" string is really confusing.  I
> > > > started assuming I could just add an ESCAPE_NONASCII flag and stick "
> > > > and \ into the "only" string, but it doesn't work that way.
> > > 
> > > Yeah, if ESCAPE_NP isn't specified, the "only" characters are passed
> > > through. It'd be nice to have an "add" or a clearer way to do actual
> > > ctype subsets, etc. If there isn't an obviously clear way to refactor
> > > it, just skip it for now and I'm happy to ack your original patch. :)
> > 
> > There may well be some simplification possible here....  There aren't
> > really many users of "only", for example.  I'll look into it some more.
> 
> The printk users are kind of mysterious to me.  I did a grep for
> 
> 	git grep '%[0-9.*]pE'
> 
> which got 75 hits.  All of them for pE.  I couldn't find any of the
> other pE[achnops] variants.  pE is equivalent to ESCAPE_ANY|ESCAPE_NP.
> Confusingly, ESCAPE_NP doesn't mean "escape non-printable", it means
> "don't escape printable".  So things like carriage returns aren't
> escaped.

No, I was confused: "\n" is non-printable according to isprint(), so
ESCAPE_ANY_NP *will* escape it.  So this isn't quite so bad.  SSIDs are
usually printed as '%*pE', so arguably we should be escaping the single
quote character too, but at least we're not allowing line breaks
through.  I don't know about non-ascii.

> One of the hits outside wireless code was in drm_dp_cec_adap_status,
> which was printing some device ID into a debugfs file with "ID: %*pE\n".
> If the ID actually needs escaping, then I suspect the meant to escape \n
> too to prevent misparsing that output.

And same here, this is OK.

--b.