linux-kernel - Re: [PATCH v2] kernel: escape non-ASCII and control characters in printk()

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Message-ID: <20110703221031.15cc2278@lxorguk.ukuu.org.uk>
Date:	Sun, 3 Jul 2011 22:10:31 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Vasiliy Kulikov <segoon@...nwall.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Morris <jmorris@...ei.org>,
	Namhyung Kim <namhyung@...il.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kernel: escape non-ASCII and control characters in
 printk()

> That said, if we really want to do this, I think doing it with %s
> filtering is the only way, and it would make the default case where
> people really don't think about possible user-supplied strings be
> safe.
> 
> So saying "%s is for pure 7-bit ASCII with no control codes" is
> annoying, but would really fix it.

It's fairly easy for the console in question to provide a filter but 7bit
ascii wouldn't be a bad default for a printing console, definitely a
wrong default for the log file though.

> That said, I think it should be unconditional. None of this "safe vs
> unsafe" flags, and none of this "printk format strings are different
> from other vsprintf format strings". If special characters are a
> potential security problem for printk(), then they are a potential
> security problem for other things (eg /proc filenames or content etc).

In which case we can't do it because we need \n in proc content so that's
a complete and utter non starter. In addition we have things with
filenames in it and filenames are unicode so we'd break apps that look
filenames up via /proc for things like monitoring.

It depends how much you care as well - any idiot can figure out how to
simply use spaces and/or tabs to build multiple lines of fake looking
output like say a spoofed Oops.

As Lars can no doubt remind people spoofing oopses can be fun ;)

Alan
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