Re: [RESEND PATCH] platform/chrome: cros_ec_debugfs: detach log reader wq from devm

From: Tzung-Bi Shih
Date: Mon Oct 25 2021 - 05:30:41 EST


Hi Guenter,

On Tue, Oct 19, 2021 at 1:24 PM Tzung-Bi Shih <tzungbi@xxxxxxxxxx> wrote:
>
> On Mon, Oct 18, 2021 at 4:59 PM Guenter Roeck <groeck@xxxxxxxxxx> wrote:
> >
> > On Mon, Oct 18, 2021 at 2:03 AM Tzung-Bi Shih <tzungbi@xxxxxxxxxx> wrote:
> > >
> > > Debugfs console_log uses devm memory (see struct cros_ec_debugfs in
> > > cros_ec_console_log fops). However, lifecycles of device and debugfs
> > > are independent. An use-after-free issue is observed if userland
> > > program operates the debugfs after the memory has been freed.
> > >
> >
> > It would help to see the backtrace. Without it, it is difficult to
> > determine where the UAF is observed. Also, most if not all of the
> > touched functions access struct cros_ec_debugfs all over the place,
> > not only for the wait queue, so I am not sure if moving the wait queue
> > out of the data structure is the correct fix. It might instead be
> > necessary to disconnect memory allocations from the ec device.
>
> A trimmed backtrace is in the commit message, but the more verbose one:
> [ 426.174308] Call trace:
> [ 426.174314] dump_backtrace+0x0/0x3ec
> [ 426.174318] show_stack+0x20/0x2c
> [ 426.174324] dump_stack+0x11c/0x1ac
> [ 426.174329] print_address_description+0x7c/0x510
> [ 426.174333] kasan_report+0x134/0x174
> [ 426.174337] __asan_report_load4_noabort+0x44/0x50
> [ 426.174341] do_raw_spin_lock+0x214/0x308
> [ 426.174345] _raw_spin_lock_irqsave+0x68/0xf0
> [ 426.174350] remove_wait_queue+0x3c/0x10c
> [ 426.174355] ep_unregister_pollwait+0x120/0x170
> [ 426.174358] ep_remove+0x60/0x2a0
> [ 426.174362] do_epoll_ctl+0x590/0x7f4
>
> I guess only the wait queue in the struct cros_ec_debugfs has
> deep-coupled to console_log debugfs. There are 2 more file operation
> scenarios appended after the "--".

Do you think the backtrace is sufficient to determine the UAF happens
on the wait queue?

How about we keep the fix as is since we have a constantly reproducing
step for the UAF. And look forward to the approach "disconnect memory
allocations from the ec device" if we could discover more UAFs?