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    Subject[PATCH v3] skbuff: fix a data race in skb_queue_len()
    Date
    sk_buff.qlen can be accessed concurrently as noticed by KCSAN,

    BUG: KCSAN: data-race in __skb_try_recv_from_queue / unix_dgram_sendmsg

    read to 0xffff8a1b1d8a81c0 of 4 bytes by task 5371 on cpu 96:
    unix_dgram_sendmsg+0x9a9/0xb70 include/linux/skbuff.h:1821
    net/unix/af_unix.c:1761
    ____sys_sendmsg+0x33e/0x370
    ___sys_sendmsg+0xa6/0xf0
    __sys_sendmsg+0x69/0xf0
    __x64_sys_sendmsg+0x51/0x70
    do_syscall_64+0x91/0xb47
    entry_SYSCALL_64_after_hwframe+0x49/0xbe

    write to 0xffff8a1b1d8a81c0 of 4 bytes by task 1 on cpu 99:
    __skb_try_recv_from_queue+0x327/0x410 include/linux/skbuff.h:2029
    __skb_try_recv_datagram+0xbe/0x220
    unix_dgram_recvmsg+0xee/0x850
    ____sys_recvmsg+0x1fb/0x210
    ___sys_recvmsg+0xa2/0xf0
    __sys_recvmsg+0x66/0xf0
    __x64_sys_recvmsg+0x51/0x70
    do_syscall_64+0x91/0xb47
    entry_SYSCALL_64_after_hwframe+0x49/0xbe

    Since only the read is operating as lockless, it could introduce a logic
    bug in unix_recvq_full() due to the load tearing. Fix it by adding
    a lockless variant of skb_queue_len() and unix_recvq_full() where
    READ_ONCE() is on the read while WRITE_ONCE() is on the write similar to
    the commit d7d16a89350a ("net: add skb_queue_empty_lockless()").

    Signed-off-by: Qian Cai <cai@lca.pw>
    ---

    v3: fix minor issues thanks to Eric.
    v2: add lockless variant helpers and WRITE_ONCE().

    include/linux/skbuff.h | 14 +++++++++++++-
    net/unix/af_unix.c | 11 +++++++++--
    2 files changed, 22 insertions(+), 3 deletions(-)

    diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
    index 3d13a4b717e9..ca8806b69388 100644
    --- a/include/linux/skbuff.h
    +++ b/include/linux/skbuff.h
    @@ -1822,6 +1822,18 @@ static inline __u32 skb_queue_len(const struct sk_buff_head *list_)
    }

    /**
    + * skb_queue_len_lockless - get queue length
    + * @list_: list to measure
    + *
    + * Return the length of an &sk_buff queue.
    + * This variant can be used in lockless contexts.
    + */
    +static inline __u32 skb_queue_len_lockless(const struct sk_buff_head *list_)
    +{
    + return READ_ONCE(list_->qlen);
    +}
    +
    +/**
    * __skb_queue_head_init - initialize non-spinlock portions of sk_buff_head
    * @list: queue to initialize
    *
    @@ -2026,7 +2038,7 @@ static inline void __skb_unlink(struct sk_buff *skb, struct sk_buff_head *list)
    {
    struct sk_buff *next, *prev;

    - list->qlen--;
    + WRITE_ONCE(list->qlen, list->qlen - 1);
    next = skb->next;
    prev = skb->prev;
    skb->next = skb->prev = NULL;
    diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
    index 321af97c7bbe..62c12cb5763e 100644
    --- a/net/unix/af_unix.c
    +++ b/net/unix/af_unix.c
    @@ -189,11 +189,17 @@ static inline int unix_may_send(struct sock *sk, struct sock *osk)
    return unix_peer(osk) == NULL || unix_our_peer(sk, osk);
    }

    -static inline int unix_recvq_full(struct sock const *sk)
    +static inline int unix_recvq_full(const struct sock *sk)
    {
    return skb_queue_len(&sk->sk_receive_queue) > sk->sk_max_ack_backlog;
    }

    +static inline int unix_recvq_full_lockless(const struct sock *sk)
    +{
    + return skb_queue_len_lockless(&sk->sk_receive_queue) >
    + READ_ONCE(sk->sk_max_ack_backlog);
    +}
    +
    struct sock *unix_peer_get(struct sock *s)
    {
    struct sock *peer;
    @@ -1758,7 +1764,8 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
    * - unix_peer(sk) == sk by time of get but disconnected before lock
    */
    if (other != sk &&
    - unlikely(unix_peer(other) != sk && unix_recvq_full(other))) {
    + unlikely(unix_peer(other) != sk &&
    + unix_recvq_full_lockless(other))) {
    if (timeo) {
    timeo = unix_wait_for_peer(other, timeo);

    --
    1.8.3.1
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