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Date:   Thu, 5 Jan 2023 16:24:10 -0800
From:   Saeed Mahameed <saeed@kernel.org>
To:     Leon Romanovsky <leon@kernel.org>
Cc:     Ying Hsu <yinghsu@chromium.org>, linux-bluetooth@vger.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>,
        Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        Marcel Holtmann <marcel@holtmann.org>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org
Subject: Re: [PATCH] Bluetooth: Fix possible deadlock in
 rfcomm_sk_state_change
Message-ID: <Y7dqKnQe8UUeQ/CD@x130>
References: <20230103111221.1.I1f29bb547a03e9adfe2e6754212f9d14a2e39c4b@changeid>
 <Y7UiDn3Gi5YdNIoC@unreal>
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On 04 Jan 08:51, Leon Romanovsky wrote:
>On Tue, Jan 03, 2023 at 11:12:46AM +0000, Ying Hsu wrote:
>> There's a possible deadlock when two processes are connecting
>> and closing concurrently:
>>   + CPU0: __rfcomm_dlc_close locks rfcomm and then calls
>>   rfcomm_sk_state_change which locks the sock.
>>   + CPU1: rfcomm_sock_connect locks the socket and then calls
>>   rfcomm_dlc_open which locks rfcomm.
>>
>> Here's the call trace:
>>
>> -> #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.
>>
>>  net/bluetooth/rfcomm/sock.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/net/bluetooth/rfcomm/sock.c b/net/bluetooth/rfcomm/sock.c
>> index 21e24da4847f..29f9a88a3dc8 100644
>> --- a/net/bluetooth/rfcomm/sock.c
>> +++ b/net/bluetooth/rfcomm/sock.c
>> @@ -410,8 +410,10 @@ 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;
>>
>> +	release_sock(sk);
>>  	err = rfcomm_dlc_open(d, &rfcomm_pi(sk)->src, &sa->rc_bdaddr,
>                                           ^^^^
>Are you sure that "sk" still exists here after you called to release_sock(sk)?
>What prevents from use-after-free here?
>

sk must be valid to be locked in first place.
release_sock() has mutex unlock semantics so it doesn't free anything..

from a quick review it doesn't seem to be necessary to keep the lock held
around rfcomm_dlc_open, and looking into bt_sock_wait_state, it also
releases the lock temporarily internally ..  so it won't matter if the lock
got released temporarily earlier, IMHO. 
Though you might want to make sure rfcomm_pi(sk)->src won't change,
it seems like other functions use &rfcomm_sk_list.lock to protect it.

Reviewed-by: Saeed Mahameed <saeed@kernel.org>

>>  			      sa->rc_channel);
>> +	lock_sock(sk);
>>  	if (!err)
>>  		err = bt_sock_wait_state(sk, BT_CONNECTED,
>>  				sock_sndtimeo(sk, flags & O_NONBLOCK));
>> --
>> 2.39.0.314.g84b9a713c41-goog
>>