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    Subject[PATCH] netlink: Replace rhash_portid with load_acquire protected boolean
    Hello,

    Here's an updated version of Herbert's patch which always uses
    load_acquire through a helper.

    Thanks.
    ----- 8< -----
    The commit 1f770c0a09da855a2b51af6d19de97fb955eca85 ("netlink: Fix
    autobind race condition that leads to zero port ID") created some new
    races that can occur due to inconsistencies between the two port IDs -
    a reader may see zero nlk->portid after seeing non-zero
    nlk->rhash_portid.

    This patch reverts the original patch and instead uses a load_acquire
    protected boolean to indicate that a user netlink socket has been
    bound. The boolean is set with store_release only after the portid is
    assigned and the socket is hashed. The readers test with load_acquire
    so that the socket is guaranteed to be visible with a valid port
    number and hashed on a true return.

    As this sort of lockless tests can be broken in ways which are very
    difficult to track down, the boolean field is prefixed with double
    underscores and a dedicated test helper with load_acquire is always
    used. While a couple test sites might not strictly require
    load_acquire, micro-optimization at this level doens't make sense
    given the danger of subtle breakages.

    tj: Took Herbert's patch and updated so that all readers test via a
    helper which does load_acquire.

    Fixes: 1f770c0a09da ("netlink: Fix autobind race condition that leads to zero port ID")
    Reported-by: Tejun Heo <tj@kernel.org>
    Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
    Original-patch-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: Tejun Heo <tj@kernel.org>
    ---
    net/netlink/af_netlink.c | 24 ++++++++++++++----------
    net/netlink/af_netlink.h | 20 +++++++++++++++++++-
    2 files changed, 33 insertions(+), 11 deletions(-)

    --- a/net/netlink/af_netlink.c
    +++ b/net/netlink/af_netlink.c
    @@ -1015,7 +1015,7 @@ static inline int netlink_compare(struct
    const struct netlink_compare_arg *x = arg->key;
    const struct netlink_sock *nlk = ptr;

    - return nlk->rhash_portid != x->portid ||
    + return nlk->portid != x->portid ||
    !net_eq(sock_net(&nlk->sk), read_pnet(&x->pnet));
    }

    @@ -1041,7 +1041,7 @@ static int __netlink_insert(struct netli
    {
    struct netlink_compare_arg arg;

    - netlink_compare_arg_init(&arg, sock_net(sk), nlk_sk(sk)->rhash_portid);
    + netlink_compare_arg_init(&arg, sock_net(sk), nlk_sk(sk)->portid);
    return rhashtable_lookup_insert_key(&table->hash, &arg,
    &nlk_sk(sk)->node,
    netlink_rhashtable_params);
    @@ -1095,7 +1095,7 @@ static int netlink_insert(struct sock *s
    lock_sock(sk);

    err = -EBUSY;
    - if (nlk_sk(sk)->portid)
    + if (nlk_bound(nlk_sk(sk)))
    goto err;

    err = -ENOMEM;
    @@ -1103,7 +1103,7 @@ static int netlink_insert(struct sock *s
    unlikely(atomic_read(&table->hash.nelems) >= UINT_MAX))
    goto err;

    - nlk_sk(sk)->rhash_portid = portid;
    + nlk_sk(sk)->portid = portid;
    sock_hold(sk);

    err = __netlink_insert(table, sk);
    @@ -1119,7 +1119,8 @@ static int netlink_insert(struct sock *s
    goto err;
    }

    - nlk_sk(sk)->portid = portid;
    + /* See nlk_bound(). */
    + smp_store_release(&nlk_sk(sk)->__bound, portid);

    err:
    release_sock(sk);
    @@ -1521,9 +1522,11 @@ static int netlink_bind(struct socket *s
    return err;
    }

    - if (nlk->portid)
    + /* Ensure nlk->portid is up-to-date. */
    + if (nlk_bound(nlk)) {
    if (nladdr->nl_pid != nlk->portid)
    return -EINVAL;
    + }

    if (nlk->netlink_bind && groups) {
    int group;
    @@ -1539,7 +1542,7 @@ static int netlink_bind(struct socket *s
    }
    }

    - if (!nlk->portid) {
    + if (!nlk_bound(nlk)) {
    err = nladdr->nl_pid ?
    netlink_insert(sk, nladdr->nl_pid) :
    netlink_autobind(sock);
    @@ -1587,7 +1590,7 @@ static int netlink_connect(struct socket
    !netlink_allowed(sock, NL_CFG_F_NONROOT_SEND))
    return -EPERM;

    - if (!nlk->portid)
    + if (!nlk_bound(nlk))
    err = netlink_autobind(sock);

    if (err == 0) {
    @@ -2428,7 +2431,8 @@ static int netlink_sendmsg(struct socket
    dst_group = nlk->dst_group;
    }

    - if (!nlk->portid) {
    + /* Ensure nlk->portid is up-to-date. */
    + if (!nlk_bound(nlk)) {
    err = netlink_autobind(sock);
    if (err)
    goto out;
    @@ -3257,7 +3261,7 @@ static inline u32 netlink_hash(const voi
    const struct netlink_sock *nlk = data;
    struct netlink_compare_arg arg;

    - netlink_compare_arg_init(&arg, sock_net(&nlk->sk), nlk->rhash_portid);
    + netlink_compare_arg_init(&arg, sock_net(&nlk->sk), nlk->portid);
    return jhash2((u32 *)&arg, netlink_compare_arg_len / sizeof(u32), seed);
    }

    --- a/net/netlink/af_netlink.h
    +++ b/net/netlink/af_netlink.h
    @@ -3,6 +3,7 @@

    #include <linux/rhashtable.h>
    #include <linux/atomic.h>
    +#include <asm/barrier.h>
    #include <net/sock.h>

    #define NLGRPSZ(x) (ALIGN(x, sizeof(unsigned long) * 8) / 8)
    @@ -25,7 +26,6 @@ struct netlink_ring {
    struct netlink_sock {
    /* struct sock has to be the first member of netlink_sock */
    struct sock sk;
    - u32 rhash_portid;
    u32 portid;
    u32 dst_portid;
    u32 dst_group;
    @@ -36,6 +36,7 @@ struct netlink_sock {
    unsigned long state;
    size_t max_recvmsg_len;
    wait_queue_head_t wait;
    + bool __bound; /* always use nlk_bound() */
    bool cb_running;
    struct netlink_callback cb;
    struct mutex *cb_mutex;
    @@ -60,6 +61,23 @@ static inline struct netlink_sock *nlk_s
    return container_of(sk, struct netlink_sock, sk);
    }

    +/**
    + * nlk_bound - test whether a netlink_sock is bound to a port number
    + * @nlk: netlink_sock of interest
    + *
    + * Test whether @nlk is bound to a port number. Can be called without any
    + * locks and guarantees no false positive - @nlk has a valid port number
    + * and is hashed on a true return.
    + */
    +static inline bool nlk_bound(struct netlink_sock *nlk)
    +{
    + /*
    + * Paired with smp_store_release() in netlink_insert() to guarantee
    + * the visibility of port number and hashing.
    + */
    + return smp_load_acquire(&nlk->__bound);
    +}
    +
    struct netlink_table {
    struct rhashtable hash;
    struct hlist_head mc_list;

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