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

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Date:	Wed, 11 Mar 2015 09:09:38 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
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 03/10/2015, 11:41 PM, One Thousand Gnomes wrote:
> 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.

I don't know, I have to study and try this first, before I can take any
action.

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

Yes, that was actually the whole point of the exercise: move from
current_fs_time() (one nanosecond granularity (for devtmpfs)) to
get_seconds() & 7 (8 seconds).

thanks,
-- 
js
suse labs

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