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Subjectdisablenetwork (v5): Update documentation for PR_NETWORK_ENABLE_DN.
Signed-off-by: Michael Stone <>
Documentation/disablenetwork.txt | 43 ++++++++++++++++++++++++++++---------
1 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/Documentation/disablenetwork.txt b/Documentation/disablenetwork.txt
index c885502..5d376e6 100644
--- a/Documentation/disablenetwork.txt
+++ b/Documentation/disablenetwork.txt
@@ -30,11 +30,19 @@ Implementation

The initial userland interface for accessing the disablenetwork functionality
is provided through the prctl() framework via a new pair of options named
-PR_{GET,SET}_NETWORK and a new flag named PR_NETWORK_OFF.
+PR_{GET,SET}_NETWORK and a pair of flags named PR_NETWORK_ENABLE_DN and

The PR_{GET,SET}_NETWORK options access and modify a new (conditionally
compiled) task_struct flags field named "network".

+prctl(PR_SET_NETWORK) takes its argument by value rather than by address to
+avoid a time-of-check-to-time-of-use race between security_prctl() and
+However, prctl(PR_GET_NETWORK) returns the value of the "network" field via a
+pointer argument so that its return code can represent errors like ENOSYS.
Finally, the pre-existing

@@ -58,22 +66,35 @@ Writes which attempt to clear bits in current->network return -EPERM.
The default value for current->network is named PR_NETWORK_ON and is defined
to be 0.

-Presently, only one flag is defined: PR_NETWORK_OFF.
+Presently, two flags are defined: PR_NETWORK_ENABLE_DN and PR_NETWORK_OFF.
More flags may be defined in the future if they become needed.

Attempts to set undefined flags result in -EINVAL.

-When PR_NETWORK_OFF is set, the disablenetwork security hooks for socket(),
-bind(), connect(), sendmsg(), and ptrace() will return -EPERM or 0.
+CAP_SETPCAP is required in order to set PR_NETWORK_ENABLE_DN. We believe that
+this restriction will protect privileged legacy system configurations from
+unprivileged misuse of disablenetwork. (Thanks to Pavel Machek and Serge Hallyn
+for analyzing the problem and for suggesting this compromise approach.)
+We say that "disablenetwork is active" when both PR_NETWORK_ENABLE_DN and
+PR_NETWORK_OFF are set. Otherwise, we say that it is "inactive".
+When the disablenetwork is inactive, the disablenetwork_* security hooks for
+socket(), bind(), connect(), sendmsg(), and ptrace() will return 0.
+When disablenetwork is active -EPERM or 0 as follows:
+The hooks will return 0 when disablenetwork is active for the current process
+and when an exception applies: i.e. when the current process is

-Exceptions are made for
+ * manipulating an AF_UNIX socket or,
+ * calling sendmsg() on a previously connected socket (i.e. one with
+ msg.msg_name == NULL && msg.msg_namelen == 0) or
+ * calling ptrace() on a target process in which disablenetwork is also
+ active.

- * processes manipulating an AF_UNIX socket or,
- * processes calling sendmsg() on a previously connected socket
- (i.e. one with msg.msg_name == NULL && msg.msg_namelen == 0) or
- * processes calling ptrace() on a target process which shares every
- networking restriction flag set in current->network.
+When disablenetwork is active in non-exceptional circumstances, the hooks will
+return -EPERM.


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