linux-kernel - Re: [PATCH v2] Bluetooth: Collect kcov coverage from hci_rx

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Date:   Mon, 4 Jul 2022 14:52:53 +0200
From:   Tamás Koczka <poprdi@...gle.com>
To:     Marcel Holtmann <marcel@...tmann.org>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Aleksandr Nogikh <nogikh@...gle.com>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        linux-bluetooth <linux-bluetooth@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Nguyen <theflow@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v2] Bluetooth: Collect kcov coverage from hci_rx_work

Hello,

If you need any clarification about the patch or if you have questions
or if the patch needs to be modified, please feel free to tell me.

Basically the patch should not have any effect on a kernel which is
not compiled with CONFIG_KCOV and we'd like to use the patch to make
the coverage of the hci_rx_work background thread visible to
Syzkaller, because the BT packet parsing / handling logic happens
there and this way Syzkaller will be able to more effectively mutate
the packets used for fuzzing, hopefully reaching new code paths, maybe
discovering and reporting new vulnerabilities before they reach the
mainline.

Thank you,
Tamas


On Thu, Jun 23, 2022 at 11:18 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> On Wed, 22 Jun 2022 at 12:20, Aleksandr Nogikh <nogikh@...gle.com> wrote:
> >
> > (Resending the reply I sent to the v1 of the patch. I sent it by
> > mistake with HTML content, so it did not reach lore.)
> >
> > I checked out v5.18.1, applied this patch and fuzzed it with syzkaller
> > for a day. The fuzzer was indeed able to find and report more coverage
> > of the BT subsystem than without the patch.
> >
> > Tested-by: Aleksandr Nogikh <nogikh@...gle.com>
> >
> >
> > On Tue, Jun 14, 2022 at 3:34 PM Tamás Koczka <poprdi@...gle.com> wrote:
> > >
> > > Hello Marcel,
> > >
> > > I hope this was the change you originally requested, and I did not
> > > misunderstand anything, but if you need any additional modification to
> > > the code or the commit, please feel free to let me know!
> > >
> > > Thank you,
> > > Tamas
> > >
> > > On Tue, Jun 7, 2022 at 1:44 PM Tamás Koczka <poprdi@...gle.com> wrote:
> > > >
> > > > Hello Marcel,
> > > >
> > > > I added some comments into the code about what the kcov_remote calls do and
> > > > why they were implemented and I also added some reasoning to the commit
> > > > message.
> > > >
> > > > I did not mention in the commit but these functions only run if the kernel
> > > > is compiled with CONFIG_KCOV.
> > > >
> > > > Thank you again for reviewing the patch!
> > > >
> > > > --
> > > > Tamas
> > > >
> > > > On Tue, Jun 7, 2022 at 12:40 PM Tamas Koczka <poprdi@...gle.com> wrote:
> > > > >
> > > > > Annotate hci_rx_work() with kcov_remote_start() and kcov_remote_stop()
> > > > > calls, so remote KCOV coverage is collected while processing the rx_q
> > > > > queue which is the main incoming Bluetooth packet queue.
> > > > >
> > > > > Coverage is associated with the thread which created the packet skb.
> > > > >
> > > > > The collected extra coverage helps kernel fuzzing efforts in finding
> > > > > vulnerabilities.
> > > > >
> > > > > Signed-off-by: Tamas Koczka <poprdi@...gle.com>
> > > > > ---
> > > > > Changelog since v1:
> > > > >  - add comment about why kcov_remote functions are called
> > > > >
> > > > > v1: https://lore.kernel.org/all/20220517094532.2729049-1-poprdi@google.com/
> > > > >
> > > > >  net/bluetooth/hci_core.c | 10 +++++++++-
> > > > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/net/bluetooth/hci_core.c b/net/bluetooth/hci_core.c
> > > > > index 45c2dd2e1590..0af43844c55a 100644
> > > > > --- a/net/bluetooth/hci_core.c
> > > > > +++ b/net/bluetooth/hci_core.c
> > > > > @@ -29,6 +29,7 @@
> > > > >  #include <linux/rfkill.h>
> > > > >  #include <linux/debugfs.h>
> > > > >  #include <linux/crypto.h>
> > > > > +#include <linux/kcov.h>
> > > > >  #include <linux/property.h>
> > > > >  #include <linux/suspend.h>
> > > > >  #include <linux/wait.h>
> > > > > @@ -3780,7 +3781,14 @@ static void hci_rx_work(struct work_struct *work)
> > > > >
> > > > >         BT_DBG("%s", hdev->name);
> > > > >
> > > > > -       while ((skb = skb_dequeue(&hdev->rx_q))) {
> > > > > +       /* The kcov_remote functions used for collecting packet parsing
> > > > > +        * coverage information from this background thread and associate
> > > > > +        * the coverage with the syscall's thread which originally injected
> > > > > +        * the packet. This helps fuzzing the kernel.
> > > > > +        */
> > > > > +       for (; (skb = skb_dequeue(&hdev->rx_q)); kcov_remote_stop()) {
> > > > > +               kcov_remote_start_common(skb_get_kcov_handle(skb));
> > > > > +
> > > > >                 /* Send copy to monitor */
> > > > >                 hci_send_to_monitor(hdev, skb);
>
> Looks good to me.
> Anything else needed to merge this patch?
>
> Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>