connect.2: Can return EACCES because of SELinux

Recently I had to troubleshoot a problem where a connect() call
was returning EACCES:

17648 socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 37
17648 connect(37, {sa_family=AF_INET, sin_port=htons(8081),
sin_addr=inet_addr("10.12.1.201")}, 16) = -1 EACCES (Permission
denied)

I've traced this to SELinux policy denying the connection. This is
on a Fedora 23 VM:

$ cat /etc/redhat-release
Fedora release 23 (Twenty Three)
$ uname -a
Linux mako-fedora-01 4.8.13-100.fc23.x86_64 #1 SMP Fri Dec 9 14:51:40
UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

The manpage says this can happen when connecting to a broadcast
address, or when a local firewall rule blocks the connection.
However, the address above is unicast, and using 'wget' from
another account to access the URL works fine.

The context is that we're building an OS image, and this involves
downloading RPMs through a proxy. The proxy (polipo) is labelled
by SELinux, and I guess there is some sort of policy that says
"proxy can only connect to HTTP ports". When trying to connect to
a server listening on a port that is not labeled as an HTTP server
port, I guess SELinux steps in. With 'setenforce 0', the build
works fine. In the kernel sources I see connect() calls
security_socket_connect() (see
https://elixir.bootlin.com/linux/latest/source/net/socket.c#L1855),
which calls whatever security hooks are registered. I see the
SELinux hook getting registered at
https://elixir.bootlin.com/linux/latest/source/security/selinux/hooks.c#L7047,
and setting a perf probe on the call proves that the
selinux_socket_connect function gets called (while
tcp_v4_connect() is not).

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

diff --git a/man2/connect.2 b/man2/connect.2
index 41296cf4ec6fc51fe51f2d7c2c002638aa3c6743..d051d1c282443aaca37ffe00f3fd91032c830b49 100644 (file)
--- a/man2/connect.2
+++ b/man2/connect.2
@@ -155,6 +155,13 @@ in the path prefix.
 The user tried to connect to a broadcast address without having the socket
 broadcast flag enabled or the connection request failed because of a local
 firewall rule.
+
+.B EACCES
+can also be returned if a SELinux policy denied a connection (for
+example, if there is a policy saying that an HTTP proxy can only
+connect to ports associated with HTTP servers, and the proxy tries to
+connect to a different port).
+
 .TP
 .B EADDRINUSE
 Local address is already in use.
@@ -297,4 +304,5 @@ is shown in
 .BR getsockname (2),
 .BR listen (2),
 .BR socket (2),
-.BR path_resolution (7)
+.BR path_resolution (7),
+.BR selinux (8)