netlink: Always copy on mmap TX.

commit 4682a0358639b29cf69437ed909c6221f8c89847 upstream.

Checking the file f_count and the nlk->mapped count is not completely
sufficient to prevent the mmap'd area contents from changing from
under us during netlink mmap sendmsg() operations.

Be careful to sample the header's length field only once, because this
could change from under us as well.

Fixes: 5fd96123ee19 ("netlink: implement memory mapped sendmsg()")
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 0ed3c2b65fb7b71b5c2d112cfdf362e9cb3747a0..625593d866ac2a31ad9b28c5e282364e8fce88f6 100644 (file)
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -510,14 +510,14 @@ out:
        return err;
 }
 
-static void netlink_frame_flush_dcache(const struct nl_mmap_hdr *hdr)
+static void netlink_frame_flush_dcache(const struct nl_mmap_hdr *hdr, unsigned int nm_len)
 {
 #if ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE == 1
        struct page *p_start, *p_end;
 
        /* First page is flushed through netlink_{get,set}_status */
        p_start = pgvec_to_page(hdr + PAGE_SIZE);
-       p_end   = pgvec_to_page((void *)hdr + NL_MMAP_HDRLEN + hdr->nm_len - 1);
+       p_end   = pgvec_to_page((void *)hdr + NL_MMAP_HDRLEN + nm_len - 1);
        while (p_start <= p_end) {
                flush_dcache_page(p_start);
                p_start++;
@@ -699,24 +699,16 @@ static int netlink_mmap_sendmsg(struct sock *sk, struct msghdr *msg,
        struct nl_mmap_hdr *hdr;
        struct sk_buff *skb;
        unsigned int maxlen;
-       bool excl = true;
        int err = 0, len = 0;
 
-       /* Netlink messages are validated by the receiver before processing.
-        * In order to avoid userspace changing the contents of the message
-        * after validation, the socket and the ring may only be used by a
-        * single process, otherwise we fall back to copying.
-        */
-       if (atomic_long_read(&sk->sk_socket->file->f_count) > 1 ||
-           atomic_read(&nlk->mapped) > 1)
-               excl = false;
-
        mutex_lock(&nlk->pg_vec_lock);
 
        ring   = &nlk->tx_ring;
        maxlen = ring->frame_size - NL_MMAP_HDRLEN;
 
        do {
+               unsigned int nm_len;
+
                hdr = netlink_current_frame(ring, NL_MMAP_STATUS_VALID);
                if (hdr == NULL) {
                        if (!(msg->msg_flags & MSG_DONTWAIT) &&
@@ -724,35 +716,23 @@ static int netlink_mmap_sendmsg(struct sock *sk, struct msghdr *msg,
                                schedule();
                        continue;
                }
-               if (hdr->nm_len > maxlen) {
+
+               nm_len = ACCESS_ONCE(hdr->nm_len);
+               if (nm_len > maxlen) {
                        err = -EINVAL;
                        goto out;
                }
 
-               netlink_frame_flush_dcache(hdr);
+               netlink_frame_flush_dcache(hdr, nm_len);
 
-               if (likely(dst_portid == 0 && dst_group == 0 && excl)) {
-                       skb = alloc_skb_head(GFP_KERNEL);
-                       if (skb == NULL) {
-                               err = -ENOBUFS;
-                               goto out;
-                       }
-                       sock_hold(sk);
-                       netlink_ring_setup_skb(skb, sk, ring, hdr);
-                       NETLINK_CB(skb).flags |= NETLINK_SKB_TX;
-                       __skb_put(skb, hdr->nm_len);
-                       netlink_set_status(hdr, NL_MMAP_STATUS_RESERVED);
-                       atomic_inc(&ring->pending);
-               } else {
-                       skb = alloc_skb(hdr->nm_len, GFP_KERNEL);
-                       if (skb == NULL) {
-                               err = -ENOBUFS;
-                               goto out;
-                       }
-                       __skb_put(skb, hdr->nm_len);
-                       memcpy(skb->data, (void *)hdr + NL_MMAP_HDRLEN, hdr->nm_len);
-                       netlink_set_status(hdr, NL_MMAP_STATUS_UNUSED);
+               skb = alloc_skb(nm_len, GFP_KERNEL);
+               if (skb == NULL) {
+                       err = -ENOBUFS;
+                       goto out;
                }
+               __skb_put(skb, nm_len);
+               memcpy(skb->data, (void *)hdr + NL_MMAP_HDRLEN, nm_len);
+               netlink_set_status(hdr, NL_MMAP_STATUS_UNUSED);
 
                netlink_increment_head(ring);
 
@@ -798,7 +778,7 @@ static void netlink_queue_mmaped_skb(struct sock *sk, struct sk_buff *skb)
        hdr->nm_pid     = NETLINK_CB(skb).creds.pid;
        hdr->nm_uid     = from_kuid(sk_user_ns(sk), NETLINK_CB(skb).creds.uid);
        hdr->nm_gid     = from_kgid(sk_user_ns(sk), NETLINK_CB(skb).creds.gid);
-       netlink_frame_flush_dcache(hdr);
+       netlink_frame_flush_dcache(hdr, hdr->nm_len);
        netlink_set_status(hdr, NL_MMAP_STATUS_VALID);
 
        NETLINK_CB(skb).flags |= NETLINK_SKB_DELIVERED;