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Date:   Sun, 15 Mar 2020 16:59:11 +0100
From:   Vincent Bernat <vincent@...nat.ch>
To:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        David Ahern <dsahern@...il.com>
Cc:     Vincent Bernat <vincent@...nat.ch>
Subject: [RFC PATCH net-next v1] net: core: enable SO_BINDTODEVICE for non-root users

Currently, SO_BINDTODEVICE requires CAP_NET_RAW. This change allows a
non-root user to bind a socket to an interface if it is not already
bound. This is useful to allow an application to bind itself to a
specific VRF for outgoing or incoming connections. Currently, an
application wanting to manage connections through several VRF need to
be privileged. Moreover, I don't see a reason why an application
couldn't restrict its own scope. Such a privilege is already possible
with UDP through IP_UNICAST_IF.

When an application is restricted to a VRF (with `ip vrf exec`), the
socket is bound to an interface at creation and therefore, a
non-privileged call to SO_BINDTODEVICE to escape the VRF fails.

When an application bound a socket to SO_BINDTODEVICE and transmit it
to a non-privileged process through a Unix socket, a tentative to
change the bound device also fails.

Before:

    >>> import socket
    >>> s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    >>> s.setsockopt(socket.SOL_SOCKET, socket.SO_BINDTODEVICE, b"dummy0")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    PermissionError: [Errno 1] Operation not permitted

After:

    >>> import socket
    >>> s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    >>> s.setsockopt(socket.SOL_SOCKET, socket.SO_BINDTODEVICE, b"dummy0")
    >>> s.setsockopt(socket.SOL_SOCKET, socket.SO_BINDTODEVICE, b"dummy0")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    PermissionError: [Errno 1] Operation not permitted

Signed-off-by: Vincent Bernat <vincent@...nat.ch>
---
 net/core/sock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/sock.c b/net/core/sock.c
index 0fc8937a7ff4..e89c6148177b 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -574,7 +574,7 @@ static int sock_setbindtodevice_locked(struct sock *sk, int ifindex)
 
 	/* Sorry... */
 	ret = -EPERM;
-	if (!ns_capable(net->user_ns, CAP_NET_RAW))
+	if (sk->sk_bound_dev_if && !ns_capable(net->user_ns, CAP_NET_RAW))
 		goto out;
 
 	ret = -EINVAL;
-- 
2.25.1

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