netlink: Fix handling of error from netlink_dump().

netlink_dump() returns a negative errno value on error.  Until now,
netlink_recvmsg() directly recorded that negative value in sk->sk_err, but
that's wrong since sk_err takes positive errno values.  (This manifests as
userspace receiving a positive return value from the recv() system call,
falsely indicating success.) This bug was introduced in the commit that
started checking the netlink_dump() return value, commit b44d211 (netlink:
handle errors from netlink_dump()).

Multithreaded Netlink dumps are one way to trigger this behavior in
practice, as described in the commit message for the userspace workaround
posted here:
    http://openvswitch.org/pipermail/dev/2014-June/042339.html

This commit also fixes the same bug in netlink_poll(), introduced in commit
cd1df525d (netlink: add flow control for memory mapped I/O).

Signed-off-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 2 deletions

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 15c731f03fa6..e6fac7e3db52 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -636,7 +636,7 @@ static unsigned int netlink_poll(struct file *file, struct socket *sock,
                 while (nlk->cb_running && netlink_dump_space(nlk)) {
                         err = netlink_dump(sk);
                         if (err < 0) {
-                                sk->sk_err = err;
+                                sk->sk_err = -err;
                                 sk->sk_error_report(sk);
                                 break;
                         }
@@ -2483,7 +2483,7 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
             atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf / 2) {
                 ret = netlink_dump(sk);
                 if (ret) {
-                        sk->sk_err = ret;
+                        sk->sk_err = -ret;
                         sk->sk_error_report(sk);
                 }
         }