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Date:   Tue, 10 Jan 2023 00:58:58 -0800
From:   Saeed Mahameed <saeed@kernel.org>
To:     Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Cc:     Ying Hsu <yinghsu@chromium.org>, linux-bluetooth@vger.kernel.org,
        marcel@holtmann.org, leon@kernel.org,
        chromeos-bluetooth-upstreaming@chromium.org,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org
Subject: Re: [PATCH v2] Bluetooth: Fix possible deadlock in
 rfcomm_sk_state_change
Message-ID: <Y70o0gPKZRCKS93n@x130>
References: <20230104150642.v2.1.I1f29bb547a03e9adfe2e6754212f9d14a2e39c4b@changeid>
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On 06 Jan 11:44, Luiz Augusto von Dentz wrote:
>Hi Saeed,
>
>On Thu, Jan 5, 2023 at 5:18 PM Saeed Mahameed <saeed@kernel.org> wrote:
>>
>> On 04 Jan 14:21, Luiz Augusto von Dentz wrote:
>> >Hi Ying,
>> >
>> >On Wed, Jan 4, 2023 at 7:07 AM Ying Hsu <yinghsu@chromium.org> wrote:
>> >>
>> >> There's a possible deadlock when two processes are connecting
>> >> and closing a RFCOMM socket concurrently. Here's the call trace:
>> >
>> >Are you sure it is 2 different processes? Usually that would mean 2
>> >different sockets (sk) then so they wouldn't share the same lock, so
>> >this sounds more like 2 different threads, perhaps it is worth
>> >creating a testing case in our rfcomm-tester so we are able to detect
>> >this sort of thing in the future.
>> >
>> >> -> #2 (&d->lock){+.+.}-{3:3}:
>> >>        __mutex_lock_common kernel/locking/mutex.c:603 [inline]
>> >>        __mutex_lock0x12f/0x1360 kernel/locking/mutex.c:747
>> >>        __rfcomm_dlc_close+0x15d/0x890 net/bluetooth/rfcomm/core.c:487
>> >>        rfcomm_dlc_close+1e9/0x240 net/bluetooth/rfcomm/core.c:520
>> >>        __rfcomm_sock_close+0x13c/0x250 net/bluetooth/rfcomm/sock.c:220
>> >>        rfcomm_sock_shutdown+0xd8/0x230 net/bluetooth/rfcomm/sock.c:907
>> >>        rfcomm_sock_release+0x68/0x140 net/bluetooth/rfcomm/sock.c:928
>> >>        __sock_release+0xcd/0x280 net/socket.c:650
>> >>        sock_close+0x1c/0x20 net/socket.c:1365
>> >>        __fput+0x27c/0xa90 fs/file_table.c:320
>> >>        task_work_run+0x16f/0x270 kernel/task_work.c:179
>> >>        exit_task_work include/linux/task_work.h:38 [inline]
>> >>        do_exit+0xaa8/0x2950 kernel/exit.c:867
>> >>        do_group_exit+0xd4/0x2a0 kernel/exit.c:1012
>> >>        get_signal+0x21c3/0x2450 kernel/signal.c:2859
>> >>        arch_do_signal_or_restart+0x79/0x5c0 arch/x86/kernel/signal.c:306
>> >>        exit_to_user_mode_loop kernel/entry/common.c:168 [inline]
>> >>        exit_to_user_mode_prepare+0x15f/0x250 kernel/entry/common.c:203
>> >>        __syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
>> >>        syscall_exit_to_user_mode+0x1d/0x50 kernel/entry/common.c:296
>> >>        do_syscall_64+0x46/0xb0 arch/x86/entry/common.c:86
>> >>        entry_SYSCALL_64_after_hwframe+0x63/0xcd
>> >>
>> >> -> #1 (rfcomm_mutex){+.+.}-{3:3}:
>> >>        __mutex_lock_common kernel/locking/mutex.c:603 [inline]
>> >>        __mutex_lock+0x12f/0x1360 kernel/locking/mutex.c:747
>> >>        rfcomm_dlc_open+0x93/0xa80 net/bluetooth/rfcomm/core.c:425
>> >>        rfcomm_sock_connect+0x329/0x450 net/bluetooth/rfcomm/sock.c:413
>> >>        __sys_connect_file+0x153/0x1a0 net/socket.c:1976
>> >>        __sys_connect+0x165/0x1a0 net/socket.c:1993
>> >>        __do_sys_connect net/socket.c:2003 [inline]
>> >>        __se_sys_connect net/socket.c:2000 [inline]
>> >>        __x64_sys_connect+0x73/0xb0 net/socket.c:2000
>> >>        do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>> >>        do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
>> >>        entry_SYSCALL_64_after_hwframe+0x63/0xcd
>> >>
>> >> -> #0 (sk_lock-AF_BLUETOOTH-BTPROTO_RFCOMM){+.+.}-{0:0}:
>> >>        check_prev_add kernel/locking/lockdep.c:3097 [inline]
>> >>        check_prevs_add kernel/locking/lockdep.c:3216 [inline]
>> >>        validate_chain kernel/locking/lockdep.c:3831 [inline]
>> >>        __lock_acquire+0x2a43/0x56d0 kernel/locking/lockdep.c:5055
>> >>        lock_acquire kernel/locking/lockdep.c:5668 [inline]
>> >>        lock_acquire+0x1e3/0x630 kernel/locking/lockdep.c:5633
>> >>        lock_sock_nested+0x3a/0xf0 net/core/sock.c:3470
>> >>        lock_sock include/net/sock.h:1725 [inline]
>> >>        rfcomm_sk_state_change+0x6d/0x3a0 net/bluetooth/rfcomm/sock.c:73
>> >>        __rfcomm_dlc_close+0x1b1/0x890 net/bluetooth/rfcomm/core.c:489
>> >>        rfcomm_dlc_close+0x1e9/0x240 net/bluetooth/rfcomm/core.c:520
>> >>        __rfcomm_sock_close+0x13c/0x250 net/bluetooth/rfcomm/sock.c:220
>> >>        rfcomm_sock_shutdown+0xd8/0x230 net/bluetooth/rfcomm/sock.c:907
>> >>        rfcomm_sock_release+0x68/0x140 net/bluetooth/rfcomm/sock.c:928
>> >>        __sock_release+0xcd/0x280 net/socket.c:650
>> >>        sock_close+0x1c/0x20 net/socket.c:1365
>> >>        __fput+0x27c/0xa90 fs/file_table.c:320
>> >>        task_work_run+0x16f/0x270 kernel/task_work.c:179
>> >>        exit_task_work include/linux/task_work.h:38 [inline]
>> >>        do_exit+0xaa8/0x2950 kernel/exit.c:867
>> >>        do_group_exit+0xd4/0x2a0 kernel/exit.c:1012
>> >>        get_signal+0x21c3/0x2450 kernel/signal.c:2859
>> >>        arch_do_signal_or_restart+0x79/0x5c0 arch/x86/kernel/signal.c:306
>> >>        exit_to_user_mode_loop kernel/entry/common.c:168 [inline]
>> >>        exit_to_user_mode_prepare+0x15f/0x250 kernel/entry/common.c:203
>> >>        __syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
>> >>        syscall_exit_to_user_mode+0x1d/0x50 kernel/entry/common.c:296
>> >>        do_syscall_64+0x46/0xb0 arch/x86/entry/common.c:86
>> >>        entry_SYSCALL_64_after_hwframe+0x63/0xcd
>> >>
>> >> Signed-off-by: Ying Hsu <yinghsu@chromium.org>
>> >> ---
>> >> This commit has been tested with a C reproducer on qemu-x86_64
>> >> and a ChromeOS device.
>> >>
>> >> Changes in v2:
>> >> - Fix potential use-after-free in rfc_comm_sock_connect.
>> >>
>> >>  net/bluetooth/rfcomm/sock.c | 7 ++++++-
>> >>  1 file changed, 6 insertions(+), 1 deletion(-)
>> >>
>> >> diff --git a/net/bluetooth/rfcomm/sock.c b/net/bluetooth/rfcomm/sock.c
>> >> index 21e24da4847f..4397e14ff560 100644
>> >> --- a/net/bluetooth/rfcomm/sock.c
>> >> +++ b/net/bluetooth/rfcomm/sock.c
>> >> @@ -391,6 +391,7 @@ static int rfcomm_sock_connect(struct socket *sock, struct sockaddr *addr, int a
>> >>             addr->sa_family != AF_BLUETOOTH)
>> >>                 return -EINVAL;
>> >>
>> >> +       sock_hold(sk);
>> >>         lock_sock(sk);
>> >>
>> >>         if (sk->sk_state != BT_OPEN && sk->sk_state != BT_BOUND) {
>> >> @@ -410,14 +411,18 @@ static int rfcomm_sock_connect(struct socket *sock, struct sockaddr *addr, int a
>> >>         d->sec_level = rfcomm_pi(sk)->sec_level;
>> >>         d->role_switch = rfcomm_pi(sk)->role_switch;
>> >>
>> >> +       /* Drop sock lock to avoid potential deadlock with the RFCOMM lock */
>> >> +       release_sock(sk);
>> >>         err = rfcomm_dlc_open(d, &rfcomm_pi(sk)->src, &sa->rc_bdaddr,
>> >>                               sa->rc_channel);
>> >> -       if (!err)
>> >> +       lock_sock(sk);
>> >> +       if (!err && !sock_flag(sk, SOCK_ZAPPED))
>> >>                 err = bt_sock_wait_state(sk, BT_CONNECTED,
>> >>                                 sock_sndtimeo(sk, flags & O_NONBLOCK));
>> >>
>> >>  done:
>> >>         release_sock(sk);
>> >> +       sock_put(sk);
>> >>         return err;
>> >>  }
>> >
>> >This sounds like a great solution to hold the reference and then
>>
>> Why do you need sock_hold/put in the same proto_ops.callback sock opts ?
>> it should be guaranteed by the caller the sk will remain valid
>> or if you are paranoid then sock_hold() on your proto_ops.bind() and put()
>> on your proto_ops.release()
>
>It doesn't looks like there is a sock_hold done in the likes of
>__sys_connect/__sys_connect_file so afaik it is possible that the sk
>is freed in the meantime if we attempt to release and lock afterward,
>and about being paranoid I guess we are past that already since with
>the likes of fuzzing testing is already paranoid in itself.
>

My point is, if you claim that the sk can be freed from another process
after you call release_sock(sk);  this means it also can be free by 
another process before you call lock_sock(sk); so what makes the first
lock_sock(sk); safe in first place ? or after you changed the code to do
sock_hold(sk) what makes sock_hold(sk) safe if another process can free it
before you hold it ?