linux-kernel - Re: binderfs interferes with syzkaller?

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Date:   Sat, 25 Jan 2020 19:13:03 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Christian Brauner <christian@...uner.io>,
        Hridya Valsaraju <hridya@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: binderfs interferes with syzkaller?

On Sat, Jan 25, 2020 at 6:49 PM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> Hi binder maintainers,
>
> It seems that something has happened and now syzbot has 0 coverage in
> drivers/android/binder.c:
> https://storage.googleapis.com/syzkaller/cover/ci-upstream-kasan-gce-root.html
> It covered at least something there before as it found some bugs in binder code.
> I _suspect_ it may be related to introduction binderfs, but it's
> purely based on the fact that binderfs changed lots of things there.
> And I see it claims to be backward compatible.
>
> syzkaller strategy to reach binder devices is to use
> CONFIG_ANDROID_BINDER_DEVICES to create a bunch of binderN devices (to
> give each test process a private one):
> https://github.com/google/syzkaller/blob/master/dashboard/config/upstream-kasan.config#L5671
>
> Then it knows how to open these /dev/binderN devices:
> https://github.com/google/syzkaller/blob/master/sys/linux/dev_binder.txt#L22
> and do stuff with them.
>
> Did these devices disappear or something?

Oh, I see, it's backwards compatible if it's not enabled, right?

if (!IS_ENABLED(CONFIG_ANDROID_BINDERFS) &&
    strcmp(binder_devices_param, "") != 0) {
/*
* Copy the module_parameter string, because we don't want to
* tokenize it in-place.
*/
device_names = kstrdup(binder_devices_param, GFP_KERNEL);
if (!device_names) {
ret = -ENOMEM;
goto err_alloc_device_names_failed;
}

device_tmp = device_names;
while ((device_name = strsep(&device_tmp, ","))) {
ret = init_binder_device(device_name);
if (ret)
goto err_init_binder_device_failed;
}
}

And we enabled it because, well, enabling things generally gives more
coverage. I guess I will disable CONFIG_ANDROID_BINDERFS for now to
restore coverage in the binder itself.