Date:   Fri, 11 May 2018 11:33:59 -0700
From:   Andrei Vagin <avagin@...tuozzo.com>
To:     syzbot <syzbot+c1872be62e587eae9669@...kaller.appspotmail.com>
Cc:     avagin@...nvz.org, davem@...emloft.net,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: possible deadlock in sk_diag_fill

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.

> 
> 
> 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.
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