[PATCH] net: change capability used by socket options IP{,V6}_TRANSPARENT

["Maciej Żenczykowski" <zenczykowski@gmail.com>]

From: Maciej Żenczykowski <maze@google.com>

Up till now the IP{,V6}_TRANSPARENT socket options (which actually set
the same bit in the socket struct) have required CAP_NET_ADMIN
privileges to set or clear the option.

- we make clearing the bit not require any privileges.
- we deprecate using CAP_NET_ADMIN for this purpose.
- we introduce a new capability CAP_NET_TRANSPARENT,
  which is tailored to allow setting just this bit.
- we allow either one of CAP_NET_TRANSPARENT or CAP_NET_RAW
  to set this bit, because raw sockets already effectively
  allow you to emulate socket transparency, and make the
  transition easier for apps not desiring to use a brand
  new capability (because of header file or glibc support)
- we print a warning (but allow it) if you try to set
  the socket option with CAP_NET_ADMIN privs, but without
  either one of CAP_NET_TRANSPARENT or CAP_NET_RAW.

The reason for introducing a new capability is that while
transparent sockets are potentially dangerous (and can let you
spoof your source IP on traffic), they don't normally give you
the full 'freedom' of eavesdropping and/or spoofing that raw sockets
give you.

Signed-off-by: Maciej Żenczykowski <maze@google.com>
CC: Balazs Scheidler <bazsi@balabit.hu>
---
 include/linux/capability.h |   13 +++++++++----
 net/ipv4/ip_sockglue.c     |   26 ++++++++++++++++++++++----
 net/ipv6/ipv6_sockglue.c   |   29 ++++++++++++++++++++++++-----
 3 files changed, 55 insertions(+), 13 deletions(-)

diff --git a/include/linux/capability.h b/include/linux/capability.h
index c421123..a115ed4 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -198,7 +198,7 @@ struct cpu_vfs_cap_data {
 /* Allow modification of routing tables */
 /* Allow setting arbitrary process / process group ownership on
    sockets */
-/* Allow binding to any address for transparent proxying */
+/* Allow binding to any address for transparent proxying (deprecated) */
 /* Allow setting TOS (type of service) */
 /* Allow setting promiscuous mode */
 /* Allow clearing driver statistics */
@@ -210,6 +210,7 @@ struct cpu_vfs_cap_data {
 
 /* Allow use of RAW sockets */
 /* Allow use of PACKET sockets */
+/* Allow binding to any address for transparent proxying */
 
 #define CAP_NET_RAW          13
 
@@ -332,7 +333,7 @@ struct cpu_vfs_cap_data {
 
 #define CAP_AUDIT_CONTROL    30
 
-#define CAP_SETFCAP	     31
+#define CAP_SETFCAP          31
 
 /* Override MAC access.
    The base kernel enforces no MAC policy.
@@ -357,10 +358,14 @@ struct cpu_vfs_cap_data {
 
 /* Allow triggering something that will wake the system */
 
-#define CAP_WAKE_ALARM            35
+#define CAP_WAKE_ALARM       35
+
+/* Allow binding to any address for transparent proxying */
+
+#define CAP_NET_TRANSPARENT  36
 
 
-#define CAP_LAST_CAP         CAP_WAKE_ALARM
+#define CAP_LAST_CAP         CAP_NET_TRANSPARENT
 
 #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
 
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 8905e92..44efa39 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -961,12 +961,30 @@ mc_msf_out:
 		break;
 
 	case IP_TRANSPARENT:
-		if (!capable(CAP_NET_ADMIN)) {
-			err = -EPERM;
-			break;
-		}
 		if (optlen < 1)
 			goto e_inval;
+		/* Always allow clearing the transparent proxy socket option.
+		 * The pre-3.2 permission for setting this was CAP_NET_ADMIN,
+		 * and this is still supported - but deprecated.  As of Linux
+		 * 3.2 the proper permission is one of CAP_NET_TRANSPARENT
+		 * (preferred, a new capability) or CAP_NET_RAW.  The latter
+		 * is supported to make the transition easier (and because
+		 * raw sockets already effectively allow one to emulate
+		 * socket transparency).
+		 */
+		if (!!val && !capable(CAP_NET_TRANSPARENT)
+		          && !capable(CAP_NET_RAW)) {
+			if (!capable(CAP_NET_ADMIN)) {
+				err = -EPERM;
+				break;
+			}
+			printk_once(KERN_WARNING "%s (%d): "
+				 "deprecated: attempt to set socket option "
+				 "IP_TRANSPARENT with CAP_NET_ADMIN but "
+				 "without either one of CAP_NET_TRANSPARENT "
+				 "or CAP_NET_RAW.\n",
+				 current->comm, task_pid_nr(current));
+		}
 		inet->transparent = !!val;
 		break;
 
diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index 147ede38..c840098 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -343,13 +343,32 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 		break;
 
 	case IPV6_TRANSPARENT:
-		if (!capable(CAP_NET_ADMIN)) {
-			retv = -EPERM;
-			break;
-		}
 		if (optlen < sizeof(int))
 			goto e_inval;
-		/* we don't have a separate transparent bit for IPV6 we use the one in the IPv4 socket */
+		/* Always allow clearing the transparent proxy socket option.
+		 * The pre-3.2 permission for setting this was CAP_NET_ADMIN,
+		 * and this is still supported - but deprecated.  As of Linux
+		 * 3.2 the proper permission is one of CAP_NET_TRANSPARENT
+		 * (preferred, a new capability) or CAP_NET_RAW.  The latter
+		 * is supported to make the transition easier (and because
+		 * raw sockets already effectively allow one to emulate
+		 * socket transparency).
+		 */
+		if (valbool && !capable(CAP_NET_TRANSPARENT)
+		            && !capable(CAP_NET_RAW)) {
+			if (!capable(CAP_NET_ADMIN)) {
+				retv = -EPERM;
+				break;
+			}
+			printk_once(KERN_WARNING "%s (%d): "
+				 "deprecated: attempt to set socket option "
+				 "IPV6_TRANSPARENT with CAP_NET_ADMIN but "
+				 "without either one of CAP_NET_TRANSPARENT "
+				 "or CAP_NET_RAW.\n",
+				 current->comm, task_pid_nr(current));
+		}
+		/* we don't have a separate transparent bit for IPV6 we use the
+		 * one in the IPv4 socket */
 		inet_sk(sk)->transparent = valbool;
 		retv = 0;
 		break;
-- 
1.7.3.1