Re: information leak in vga console scrollback buffer

[Kalin KOZHUHAROV]

Koblinger Egmont wrote:
> On Sat, 12 Jun 2004, Chris Wedgwood wrote:
>
>
> > > Rationale? (At least an rtfm-like pointer to that?)
> >
> > Maybe I didn't full understand you.  Generally I find it desirable to
> > be able to read things that scrolled off the screen a long time ago.
> > It's very useful for unattended machines if I need to 'look' back.
>
>
> Generally console's scrollback buffer disappears as soon as you switch to
> another console.
>
> It'd be a really nice idea if all the consoles had a configurable amount
> of scrollback buffer which is always remembered. IMHO with todays machines
> having a scrollback buffer of 1000 lines for 6 or a little bit more
> consoles (at most 63 IIRC) is affordable as well as the processor time
> needed to copy the data from/to vga/normal memory on each console switch
> and at every Nth Shift+PageUp (no matter what N is). But this is a whole
> different story.
>
> What I'm talking about is: normally after people switch away from a
> console they assume that the scrollback buffer is no longer available
> since this is the behavior they experience normally. E.g. Z does a 'cat
> my-long-private-file' and then logs out. Then even if getty clears the
> screen, one can press Shift+PageUp to go back and read parts of this file.
> Z is about to leave the computer but don't want others to be able to
> scroll back with Shift+PageUp. So switches console (Alt+Fx) and the
> scrollback buffer is gone. He is happy. But shouldn't be.
>
> With the trick I described it is possible to bring back some random parts
> of previous texts, often some garbage with stupid flashing characters, but
> maybe parts of Z's my-long-private-file. The behavior seems to be random
> to me, uncontrollable by the user (I see no way to force private data to
> be cleared from the vga buffer) and clearly not intentional.
>
> Please try what I wrote, I'm sure that you misunderstood me (I'm trying to
> write as clear as I can but I'm not native English speaker and not even
> good in English, so it might be that my bugreport is a little bit hard to
> understand). I'm sure not talking about a feature, nor am I a Linux newbie
> who has just seen Shift+PageUp a few days ago for the first time (even
> though I'm very far from being a kernel hacker ;-))
OK, I think I got what you are trying to point out.
To reproduce:
1. login to a (vga) console.
2. less /etc/services; press space t oscroll a few screens
3. logout
4. login again on the same console (possibly as a different user)
5. less /etc/resolv.conf
6. press UpArrow, then Shift+PgUp

What is expected:
screen should not scroll past your file.

What happens:
You can view the previous text (from /etc/services)!!!

So the point is that this buffer is persistend across logout/login, which is a security bug.
And I guess LKML is not the place for it, logout should clear the buffer IMHO.

BTW, using agetty here.

Kalin.

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