linux-kernel - Re: [PATCH 1/1] tty: fix up atime/mtime mess, take four

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Message-ID: <20150310224100.48b1adf6@lxorguk.ukuu.org.uk>
Date:	Tue, 10 Mar 2015 22:41:00 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Raymond Jennings <shentino@...il.com>, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, john_paul.perry@...atel-lucent.com,
	stable@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/1] tty: fix up atime/mtime mess, take four

On Mon, 09 Mar 2015 11:01:12 +0100
Jiri Slaby <jslaby@...e.cz> wrote:

> On 03/06/2015, 02:16 PM, Raymond Jennings wrote:
> > On Fri, 2015-02-27 at 18:40 +0100, Jiri Slaby wrote:
> >> So check the absolute difference of times and if it large than "8
> >> seconds or so", always update the time. That means we will update
> >> immediatelly when changing time. Ergo, CAP_SYS_TIME can foul the
> >> check, but it was always that way.
> > 
> > If I may ask, what is supposed to happen normally when you write to a
> > tty device?  I always thought the tty device was treated just like a
> > normal file wrt. timestamps.
> > 
> > Now I see a patch for 8 seconds something.
> 
> Yes, because you do not want to be given any clue when users are typing
> passwords. You could intercept the length of the password from the
> pauses between key strokes (tty timestamps).

On any vaguely idle box I can do the same and in fact probably far
better by measuring latencies via rdtsc and continually forcing a dword
out of cache in a tight loop.

It's a pointless change, second granularities are not useful for most
kinds of attack of this nature.

Alan
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