Re: [PATCH 3/3] vt: keyboard, extend func_buf_lock to readers

[Jiri Slaby]

On 16. 10. 20, 15:20, Greg KH wrote:
> On Fri, Oct 16, 2020 at 02:24:12PM +0200, Jiri Slaby wrote:
> > Both read-side users of func_table/func_buf need locking. Without that,
> > one can easily confuse the code by repeatedly setting altering strings
> > like:
> > while (1)
> > 	for (a = 0; a < 2; a++) {
> > 		struct kbsentry kbs = {};
> > 		strcpy((char *)kbs.kb_string, a ? ".\n" : "88888\n");
> > 		ioctl(fd, KDSKBSENT, &kbs);
> > 	}
> >
> > When that program runs, one can get unexpected output by holding F1
> > (note the unxpected period on the last line):
> > .
> > 88888
> > .8888
> >
> > So protect all accesses to 'func_table' (and func_buf) by preexisting
> > 'func_buf_lock'.
> >
> > It is easy in 'k_fn' handler as 'puts_queue' is expected not to sleep.
> > On the other hand, KDGKBSENT needs a local (atomic) copy of the string
> > because copy_to_user can sleep.
> >
> > Likely fixes CVE-2020-25656.
> >
> > Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
> > Reported-by: Minh Yuan <yuanmingbuaa@xxxxxxxxx>
> > ---
> >   drivers/tty/vt/keyboard.c | 26 +++++++++++++++++++++-----
> >   1 file changed, 21 insertions(+), 5 deletions(-)
>
> So all 3 of these should go to 5.10-final?

Let me try to eliminate also patch 1/3 which I now think is possible.

--
js
suse labs