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

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Message-ID: <20190626162149.GB4144@fieldses.org>
Date:   Wed, 26 Jun 2019 12:21:49 -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 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.

Of those 57 were in drivers/net/wireless, and from a quick check seemed
mostly to be for SSIDs in debug messages.  I *think* SSIDs can be
arbitrary bytes?  If they really want them escaped then I suspect they
want more than just nonprintable characters escaped.

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.

--b.