Date:   Sat, 12 May 2018 09:46:25 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Andrei Vagin <avagin@...tuozzo.com>
Cc:     syzbot <syzbot+c1872be62e587eae9669@...kaller.appspotmail.com>,
        avagin <avagin@...nvz.org>, David Miller <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: possible deadlock in sk_diag_fill

On Fri, May 11, 2018 at 8:33 PM, Andrei Vagin <avagin@...tuozzo.com> wrote:
> On Sat, May 05, 2018 at 10:59:02AM -0700, syzbot wrote:
>> Hello,
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:    c1c07416cdd4 Merge tag 'kbuild-fixes-v4.17' of git://git.k..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=12164c97800000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=5a1dc06635c10d27
>> dashboard link: https://syzkaller.appspot.com/bug?extid=c1872be62e587eae9669
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> userspace arch: i386
>>
>> Unfortunately, I don't have any reproducer for this crash yet.
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+c1872be62e587eae9669@...kaller.appspotmail.com
>>
>>
>> ======================================================
>> WARNING: possible circular locking dependency detected
>> 4.17.0-rc3+ #59 Not tainted
>> ------------------------------------------------------
>> syz-executor1/25282 is trying to acquire lock:
>> 000000004fddf743 (&(&u->lock)->rlock/1){+.+.}, at: sk_diag_dump_icons
>> net/unix/diag.c:82 [inline]
>> 000000004fddf743 (&(&u->lock)->rlock/1){+.+.}, at:
>> sk_diag_fill.isra.5+0xa43/0x10d0 net/unix/diag.c:144
>>
>> but task is already holding lock:
>> 00000000b6895645 (rlock-AF_UNIX){+.+.}, at: spin_lock
>> include/linux/spinlock.h:310 [inline]
>> 00000000b6895645 (rlock-AF_UNIX){+.+.}, at: sk_diag_dump_icons
>> net/unix/diag.c:64 [inline]
>> 00000000b6895645 (rlock-AF_UNIX){+.+.}, at: sk_diag_fill.isra.5+0x94e/0x10d0
>> net/unix/diag.c:144
>>
>> which lock already depends on the new lock.
>
> In the code, we have a comment which explains why it is safe to take this lock
>
> /*
>  * The state lock is outer for the same sk's
>  * queue lock. With the other's queue locked it's
>  * OK to lock the state.
>  */
> unix_state_lock_nested(req);
>
> It is a question how to explain this to lockdep.

Do I understand it correctly that (&u->lock)->rlock associated with
AF_UNIX is locked under rlock-AF_UNIX, and then rlock-AF_UNIX is
locked under (&u->lock)->rlock associated with AF_NETLINK? If so, I
think we need to split (&u->lock)->rlock by family too, so that we
have u->lock-AF_UNIX and u->lock-AF_NETLINK.



>> the existing dependency chain (in reverse order) is:
>>
>> -> #1 (rlock-AF_UNIX){+.+.}:
>>        __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
>>        _raw_spin_lock_irqsave+0x96/0xc0 kernel/locking/spinlock.c:152
>>        skb_queue_tail+0x26/0x150 net/core/skbuff.c:2900
>>        unix_dgram_sendmsg+0xf77/0x1730 net/unix/af_unix.c:1797
>>        sock_sendmsg_nosec net/socket.c:629 [inline]
>>        sock_sendmsg+0xd5/0x120 net/socket.c:639
>>        ___sys_sendmsg+0x525/0x940 net/socket.c:2117
>>        __sys_sendmmsg+0x3bb/0x6f0 net/socket.c:2205
>>        __compat_sys_sendmmsg net/compat.c:770 [inline]
>>        __do_compat_sys_sendmmsg net/compat.c:777 [inline]
>>        __se_compat_sys_sendmmsg net/compat.c:774 [inline]
>>        __ia32_compat_sys_sendmmsg+0x9f/0x100 net/compat.c:774
>>        do_syscall_32_irqs_on arch/x86/entry/common.c:323 [inline]
>>        do_fast_syscall_32+0x345/0xf9b arch/x86/entry/common.c:394
>>        entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
>>
>> -> #0 (&(&u->lock)->rlock/1){+.+.}:
>>        lock_acquire+0x1dc/0x520 kernel/locking/lockdep.c:3920
>>        _raw_spin_lock_nested+0x28/0x40 kernel/locking/spinlock.c:354
>>        sk_diag_dump_icons net/unix/diag.c:82 [inline]
>>        sk_diag_fill.isra.5+0xa43/0x10d0 net/unix/diag.c:144
>>        sk_diag_dump net/unix/diag.c:178 [inline]
>>        unix_diag_dump+0x35f/0x550 net/unix/diag.c:206
>>        netlink_dump+0x507/0xd20 net/netlink/af_netlink.c:2226
>>        __netlink_dump_start+0x51a/0x780 net/netlink/af_netlink.c:2323
>>        netlink_dump_start include/linux/netlink.h:214 [inline]
>>        unix_diag_handler_dump+0x3f4/0x7b0 net/unix/diag.c:307
>>        __sock_diag_cmd net/core/sock_diag.c:230 [inline]
>>        sock_diag_rcv_msg+0x2e0/0x3d0 net/core/sock_diag.c:261
>>        netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2448
>>        sock_diag_rcv+0x2a/0x40 net/core/sock_diag.c:272
>>        netlink_unicast_kernel net/netlink/af_netlink.c:1310 [inline]
>>        netlink_unicast+0x58b/0x740 net/netlink/af_netlink.c:1336
>>        netlink_sendmsg+0x9f0/0xfa0 net/netlink/af_netlink.c:1901
>>        sock_sendmsg_nosec net/socket.c:629 [inline]
>>        sock_sendmsg+0xd5/0x120 net/socket.c:639
>>        sock_write_iter+0x35a/0x5a0 net/socket.c:908
>>        call_write_iter include/linux/fs.h:1784 [inline]
>>        new_sync_write fs/read_write.c:474 [inline]
>>        __vfs_write+0x64d/0x960 fs/read_write.c:487
>>        vfs_write+0x1f8/0x560 fs/read_write.c:549
>>        ksys_write+0xf9/0x250 fs/read_write.c:598
>>        __do_sys_write fs/read_write.c:610 [inline]
>>        __se_sys_write fs/read_write.c:607 [inline]
>>        __ia32_sys_write+0x71/0xb0 fs/read_write.c:607
>>        do_syscall_32_irqs_on arch/x86/entry/common.c:323 [inline]
>>        do_fast_syscall_32+0x345/0xf9b arch/x86/entry/common.c:394
>>        entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
>>
>> other info that might help us debug this:
>>
>>  Possible unsafe locking scenario:
>>
>>        CPU0                    CPU1
>>        ----                    ----
>>   lock(rlock-AF_UNIX);
>>                                lock(&(&u->lock)->rlock/1);
>>                                lock(rlock-AF_UNIX);
>>   lock(&(&u->lock)->rlock/1);
>>
>>  *** DEADLOCK ***
>>
>> 5 locks held by syz-executor1/25282:
>>  #0: 000000003919e1bd (sock_diag_mutex){+.+.}, at: sock_diag_rcv+0x1b/0x40
>> net/core/sock_diag.c:271
>>  #1: 000000004f328d3e (sock_diag_table_mutex){+.+.}, at: __sock_diag_cmd
>> net/core/sock_diag.c:225 [inline]
>>  #1: 000000004f328d3e (sock_diag_table_mutex){+.+.}, at:
>> sock_diag_rcv_msg+0x169/0x3d0 net/core/sock_diag.c:261
>>  #2: 000000004cc04dbb (nlk_cb_mutex-SOCK_DIAG){+.+.}, at:
>> netlink_dump+0x98/0xd20 net/netlink/af_netlink.c:2182
>>  #3: 00000000accdef41 (unix_table_lock){+.+.}, at: spin_lock
>> include/linux/spinlock.h:310 [inline]
>>  #3: 00000000accdef41 (unix_table_lock){+.+.}, at:
>> unix_diag_dump+0x10a/0x550 net/unix/diag.c:192
>>  #4: 00000000b6895645 (rlock-AF_UNIX){+.+.}, at: spin_lock
>> include/linux/spinlock.h:310 [inline]
>>  #4: 00000000b6895645 (rlock-AF_UNIX){+.+.}, at: sk_diag_dump_icons
>> net/unix/diag.c:64 [inline]
>>  #4: 00000000b6895645 (rlock-AF_UNIX){+.+.}, at:
>> sk_diag_fill.isra.5+0x94e/0x10d0 net/unix/diag.c:144
>>
>> stack backtrace:
>> CPU: 1 PID: 25282 Comm: syz-executor1 Not tainted 4.17.0-rc3+ #59
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>> Google 01/01/2011
>> Call Trace:
>>  __dump_stack lib/dump_stack.c:77 [inline]
>>  dump_stack+0x1b9/0x294 lib/dump_stack.c:113
>>  print_circular_bug.isra.36.cold.54+0x1bd/0x27d
>> kernel/locking/lockdep.c:1223
>>  check_prev_add kernel/locking/lockdep.c:1863 [inline]
>>  check_prevs_add kernel/locking/lockdep.c:1976 [inline]
>>  validate_chain kernel/locking/lockdep.c:2417 [inline]
>>  __lock_acquire+0x343e/0x5140 kernel/locking/lockdep.c:3431
>>  lock_acquire+0x1dc/0x520 kernel/locking/lockdep.c:3920
>>  _raw_spin_lock_nested+0x28/0x40 kernel/locking/spinlock.c:354
>>  sk_diag_dump_icons net/unix/diag.c:82 [inline]
>>  sk_diag_fill.isra.5+0xa43/0x10d0 net/unix/diag.c:144
>>  sk_diag_dump net/unix/diag.c:178 [inline]
>>  unix_diag_dump+0x35f/0x550 net/unix/diag.c:206
>>  netlink_dump+0x507/0xd20 net/netlink/af_netlink.c:2226
>>  __netlink_dump_start+0x51a/0x780 net/netlink/af_netlink.c:2323
>>  netlink_dump_start include/linux/netlink.h:214 [inline]
>>  unix_diag_handler_dump+0x3f4/0x7b0 net/unix/diag.c:307
>>  __sock_diag_cmd net/core/sock_diag.c:230 [inline]
>>  sock_diag_rcv_msg+0x2e0/0x3d0 net/core/sock_diag.c:261
>>  netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2448
>>  sock_diag_rcv+0x2a/0x40 net/core/sock_diag.c:272
>>  netlink_unicast_kernel net/netlink/af_netlink.c:1310 [inline]
>>  netlink_unicast+0x58b/0x740 net/netlink/af_netlink.c:1336
>>  netlink_sendmsg+0x9f0/0xfa0 net/netlink/af_netlink.c:1901
>>  sock_sendmsg_nosec net/socket.c:629 [inline]
>>  sock_sendmsg+0xd5/0x120 net/socket.c:639
>>  sock_write_iter+0x35a/0x5a0 net/socket.c:908
>>  call_write_iter include/linux/fs.h:1784 [inline]
>>  new_sync_write fs/read_write.c:474 [inline]
>>  __vfs_write+0x64d/0x960 fs/read_write.c:487
>>  vfs_write+0x1f8/0x560 fs/read_write.c:549
>>  ksys_write+0xf9/0x250 fs/read_write.c:598
>>  __do_sys_write fs/read_write.c:610 [inline]
>>  __se_sys_write fs/read_write.c:607 [inline]
>>  __ia32_sys_write+0x71/0xb0 fs/read_write.c:607
>>  do_syscall_32_irqs_on arch/x86/entry/common.c:323 [inline]
>>  do_fast_syscall_32+0x345/0xf9b arch/x86/entry/common.c:394
>>  entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
>> RIP: 0023:0xf7f8ccb9
>> RSP: 002b:00000000f5f880ac EFLAGS: 00000282 ORIG_RAX: 0000000000000004
>> RAX: ffffffffffffffda RBX: 0000000000000017 RCX: 000000002058bfe4
>> RDX: 0000000000000029 RSI: 0000000000000000 RDI: 0000000000000000
>> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000296 R12: 0000000000000000
>> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>>
>>
>> ---
>> This bug is generated by a bot. It may contain errors.
>> See https://goo.gl/tpsmEJ for more information about syzbot.
>> syzbot engineers can be reached at syzkaller@...glegroups.com.
>>
>> syzbot will keep track of this bug report.
>> If you forgot to add the Reported-by tag, once the fix for this bug is
>> merged
>> into any tree, please reply to this email with:
>> #syz fix: exact-commit-title
>> To mark this as a duplicate of another syzbot report, please reply with:
>> #syz dup: exact-subject-of-another-report
>> If it's a one-off invalid bug report, please reply with:
>> #syz invalid
>> Note: if the crash happens again, it will cause creation of a new bug
>> report.
>> Note: all commands must start from beginning of the line in the email body.
>
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