disablenetwork (v5): Require CAP_SETPCAP to enable disablenetwork.

[Michael Stone]

We implement this check by allocating a new flag bit in task_struct->network
and by propagating its semantics throughout the disablenetwork facility.

Signed-off-by: Michael Stone <michael@xxxxxxxxxx>
---
  include/linux/prctl.h     |    8 +++++---
  kernel/sys.c              |   14 +++++++++++++-
  security/disablenetwork.c |   15 ++++++++-------
  3 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/include/linux/prctl.h b/include/linux/prctl.h
index 4eb4110..4fdb417 100644
--- a/include/linux/prctl.h
+++ b/include/linux/prctl.h
@@ -105,8 +105,10 @@
  /* Get/set process disable-network flags */
  #define PR_SET_NETWORK	35
  #define PR_GET_NETWORK	36
-# define PR_NETWORK_ON        0
-# define PR_NETWORK_OFF       1
-# define PR_NETWORK_ALL_FLAGS 1
+# define PR_NETWORK_ON		0
+# define PR_NETWORK_ENABLE_DN	1
+# define PR_NETWORK_OFF	2
+# define PR_NETWORK_ALL_FLAGS	3
+# define PR_NETWORK_DN_FLAGS	3
  
  #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 1fadf10..4c7ef83 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1608,7 +1608,19 @@ long prctl_set_network(unsigned long network_flags)
  	if (current->network & ~network_flags)
  		return -EPERM;
  
-	current->network = network_flags;
+	/* Only the superuser may permit a process to enable disablenetwork. */
+	if (!(current->network & PR_NETWORK_ENABLE_DN) &&
+		network_flags & PR_NETWORK_ENABLE_DN &&
+		!capable(CAP_SETPCAP))
+		return -EPERM;
+
+	/* Only dn-enabled processes may activate disablenetwork. */
+	if (!(current->network & PR_NETWORK_OFF) &&
+		network_flags & PR_NETWORK_OFF &&
+		!(current->network & PR_NETWORK_ENABLE_DN))
+		return -EPERM;
+
+	current->network |= network_flags;
  	return 0;
  }
  
diff --git a/security/disablenetwork.c b/security/disablenetwork.c
index 27b88d7..02c0150 100644
--- a/security/disablenetwork.c
+++ b/security/disablenetwork.c
@@ -19,10 +19,11 @@
  #include <net/sock.h>
  #include <linux/socket.h>
  #include <linux/disablenetwork.h>
+#include <linux/prctl.h>
  
-static inline int maybe_allow(void)
+static inline int maybe_allow(unsigned long network_flags)
  {
-	if (current->network)
+	if ((network_flags & PR_NETWORK_DN_FLAGS) == PR_NETWORK_DN_FLAGS)
  		return -EPERM;
  	return 0;
  }
@@ -32,7 +33,7 @@ int disablenetwork_security_socket_create(int family, int type,
  {
  	if (family == AF_UNIX)
  		return 0;
-	return maybe_allow();
+	return maybe_allow(current->network);
  }
  
  int disablenetwork_security_socket_bind(struct socket * sock,
@@ -41,7 +42,7 @@ int disablenetwork_security_socket_bind(struct socket * sock,
  {
  	if (address->sa_family == AF_UNIX)
  		return 0;
-	return maybe_allow();
+	return maybe_allow(current->network);
  }
  
  int disablenetwork_security_socket_connect(struct socket * sock,
@@ -50,7 +51,7 @@ int disablenetwork_security_socket_connect(struct socket * sock,
  {
  	if (address->sa_family == AF_UNIX)
  		return 0;
-	return maybe_allow();
+	return maybe_allow(current->network);
  }
  
  int disablenetwork_security_socket_sendmsg(struct socket * sock,
@@ -59,14 +60,14 @@ int disablenetwork_security_socket_sendmsg(struct socket * sock,
  	/* permit sockets which are PF_UNIX or connected; check others. */
  	if (sock->sk->sk_family == PF_UNIX || msg->msg_name == NULL)
  		return 0;
-	return maybe_allow();
+	return maybe_allow(current->network);
  }
  
  int disablenetwork_security_ptrace_access_check(struct task_struct *child,
                                                  unsigned int mode)
  {
  	/* does current have networking restrictions not shared by child? */
-	if (current->network & ~child->network)
+	if (maybe_allow(current->network) && !maybe_allow(child->network))
  		return -EPERM;
  	return 0;
  }
-- 
1.6.6.rc2
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