[PATCH] tty: hvc: Fix data abort due to race in hvc_open

[Jiri Slaby]

On 15. 05. 20, 1:22, rananta at codeaurora.org wrote:
> On 2020-05-13 00:04, Greg KH wrote:
>> On Tue, May 12, 2020 at 02:39:50PM -0700, rananta at codeaurora.org wrote:
>>> On 2020-05-12 01:25, Greg KH wrote:
>>> > On Tue, May 12, 2020 at 09:22:15AM +0200, Jiri Slaby wrote:
>>> > > commit bdb498c20040616e94b05c31a0ceb3e134b7e829
>>> > > Author: Jiri Slaby <jslaby at suse.cz>
>>> > > Date:   Tue Aug 7 21:48:04 2012 +0200
>>> > >
>>> > >     TTY: hvc_console, add tty install
>>> > >
>>> > > added hvc_install but did not move 'tty->driver_data = NULL;' from
>>> > > hvc_open's fail path to hvc_cleanup.
>>> > >
>>> > > IOW hvc_open now NULLs tty->driver_data even for another task which
>>> > > opened the tty earlier. The same holds for
>>> > > "tty_port_tty_set(&hp->port,
>>> > > NULL);" there. And actually "tty_port_put(&hp->port);" is also
>>> > > incorrect
>>> > > for the 2nd task opening the tty.
>>> > >

...

> These are the traces you get when the issue happens:
> [  154.212291] hvc_install called for pid: 666
> [  154.216705] hvc_open called for pid: 666
> [  154.233657] hvc_open: request_irq failed with rc -22.
> [  154.238934] hvc_open called for pid: 678
> [  154.243012] Unable to handle kernel NULL pointer dereference at
> virtual address 00000000000000c4
> # hvc_install isn't called for pid: 678 as the file wasn't closed yet.

Nice. Does the attached help?

I wonder how comes the tty_port_put in hvc_open does not cause a UAF? I
would say hvc_open fails, tty_port_put is called. It decrements the
reference taken in hvc_install. So far so good.

Now, this should happen IMO:
tty_open
  -> hvc_open (fails)
    -> tty_port_put
  -> tty_release
    -> tty_release_struct
      -> tty_kref_put
        -> queue_release_one_tty
SCHEDULED WORKQUEUE
release_one_tty
  -> hvc_cleanup
    -> tty_port_put (should die terrible death now)

What am I missing?

thanks,
-- 
js
suse labs
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