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Message-ID: <20110702081022.GA2755@albatros>
Date: Sat, 2 Jul 2011 12:10:22 +0400
From: Vasiliy Kulikov <segoon@...nwall.com>
To: Alan Cox <alan@...rguk.ukuu.org.uk>,
Greg Kroah-Hartman <gregkh@...e.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
James Morris <jmorris@...ei.org>, Namhyung Kim <namhyung@...il.com>,
kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kernel: escape non-ASCII and control characters in
printk()
On Fri, Jul 01, 2011 at 15:49 +0100, Alan Cox wrote:
> > of the multiline feature. Intoducing new "%S" format for single lines
> > makes little sense as there are tons of printk() calls that should be
> > already restricted to one line.
>
> You don't need a new format string surely. Your expectation for printk is
>
> "multiple new lines are cool providing they are in the format string"
>
> So that bit isn't hard to deal with,
>
> You make vprintk take an extra arg (trusted/untrusted args)
Not vprintk, but vscnprintf(), vsnprintf() and string() because
vprintk() is used in tens of places besides of printk(). Or better
implement _vscnprintf(.., bool untrusted) and
vscnprintf(...) { return _vscnprintf(..., false); }
to leave current users of it as is.
But yes, I got the idea.
> You make printk pass 'untrusted'
> You make %s quote the arguments for control codes
What to do with CSI? It is a valid byte inside of a UTF-8 string.
Parsing a supplied string assuming it is UTF-8 string and filtering CSI
iff it is not a part of UTF-8 symbol is something a bit ugly IMO.
Greg - do you know any devices supplying multibyte strings, but not in
UTF-8 encoding? If yes, then CSI filtering is a bad idea :\
> At which point your attacker has more work to do but given a long string
> yawns and stars using the right number of spaces for the likely 80 col
> screen :)
Yeah, but introducing some artificial limit for string length is IMO
more harmfull: there is no universal limit for all situations, somewhere
the resulting string is already 70 chars and even 20 bytes would
overflow the col; in rare cases a string of 50 bytes might be still
acceptable.
Thanks,
--
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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