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    Subject[PATCH 5.4 221/260] net/af_unix: fix a data-race in unix_dgram_poll
    Date
    From: Eric Dumazet <edumazet@google.com>

    commit 04f08eb44b5011493d77b602fdec29ff0f5c6cd5 upstream.

    syzbot reported another data-race in af_unix [1]

    Lets change __skb_insert() to use WRITE_ONCE() when changing
    skb head qlen.

    Also, change unix_dgram_poll() to use lockless version
    of unix_recvq_full()

    It is verry possible we can switch all/most unix_recvq_full()
    to the lockless version, this will be done in a future kernel version.

    [1] HEAD commit: 8596e589b787732c8346f0482919e83cc9362db1

    BUG: KCSAN: data-race in skb_queue_tail / unix_dgram_poll

    write to 0xffff88814eeb24e0 of 4 bytes by task 25815 on cpu 0:
    __skb_insert include/linux/skbuff.h:1938 [inline]
    __skb_queue_before include/linux/skbuff.h:2043 [inline]
    __skb_queue_tail include/linux/skbuff.h:2076 [inline]
    skb_queue_tail+0x80/0xa0 net/core/skbuff.c:3264
    unix_dgram_sendmsg+0xff2/0x1600 net/unix/af_unix.c:1850
    sock_sendmsg_nosec net/socket.c:703 [inline]
    sock_sendmsg net/socket.c:723 [inline]
    ____sys_sendmsg+0x360/0x4d0 net/socket.c:2392
    ___sys_sendmsg net/socket.c:2446 [inline]
    __sys_sendmmsg+0x315/0x4b0 net/socket.c:2532
    __do_sys_sendmmsg net/socket.c:2561 [inline]
    __se_sys_sendmmsg net/socket.c:2558 [inline]
    __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2558
    do_syscall_x64 arch/x86/entry/common.c:50 [inline]
    do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
    entry_SYSCALL_64_after_hwframe+0x44/0xae

    read to 0xffff88814eeb24e0 of 4 bytes by task 25834 on cpu 1:
    skb_queue_len include/linux/skbuff.h:1869 [inline]
    unix_recvq_full net/unix/af_unix.c:194 [inline]
    unix_dgram_poll+0x2bc/0x3e0 net/unix/af_unix.c:2777
    sock_poll+0x23e/0x260 net/socket.c:1288
    vfs_poll include/linux/poll.h:90 [inline]
    ep_item_poll fs/eventpoll.c:846 [inline]
    ep_send_events fs/eventpoll.c:1683 [inline]
    ep_poll fs/eventpoll.c:1798 [inline]
    do_epoll_wait+0x6ad/0xf00 fs/eventpoll.c:2226
    __do_sys_epoll_wait fs/eventpoll.c:2238 [inline]
    __se_sys_epoll_wait fs/eventpoll.c:2233 [inline]
    __x64_sys_epoll_wait+0xf6/0x120 fs/eventpoll.c:2233
    do_syscall_x64 arch/x86/entry/common.c:50 [inline]
    do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
    entry_SYSCALL_64_after_hwframe+0x44/0xae

    value changed: 0x0000001b -> 0x00000001

    Reported by Kernel Concurrency Sanitizer on:
    CPU: 1 PID: 25834 Comm: syz-executor.1 Tainted: G W 5.14.0-syzkaller #0
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

    Fixes: 86b18aaa2b5b ("skbuff: fix a data race in skb_queue_len()")
    Cc: Qian Cai <cai@lca.pw>
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    include/linux/skbuff.h | 2 +-
    net/unix/af_unix.c | 2 +-
    2 files changed, 2 insertions(+), 2 deletions(-)

    --- a/include/linux/skbuff.h
    +++ b/include/linux/skbuff.h
    @@ -1887,7 +1887,7 @@ static inline void __skb_insert(struct s
    WRITE_ONCE(newsk->prev, prev);
    WRITE_ONCE(next->prev, newsk);
    WRITE_ONCE(prev->next, newsk);
    - list->qlen++;
    + WRITE_ONCE(list->qlen, list->qlen + 1);
    }

    static inline void __skb_queue_splice(const struct sk_buff_head *list,
    --- a/net/unix/af_unix.c
    +++ b/net/unix/af_unix.c
    @@ -2734,7 +2734,7 @@ static __poll_t unix_dgram_poll(struct f

    other = unix_peer(sk);
    if (other && unix_peer(other) != sk &&
    - unix_recvq_full(other) &&
    + unix_recvq_full_lockless(other) &&
    unix_dgram_peer_wake_me(sk, other))
    writable = 0;


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