[PATCH V3 Notebook] How to add a new policy capability

[PATCH V3 Notebook] How to add a new policy capability

[Richard Haines]

Describes the steps required to add a new policy capability to:
kernel, libsepol, and policy.

Also add the ioctl_skip_cloexec capability description.

Signed-off-by: Richard Haines <richard_c_haines@btinternet.com>
---
V2 Changes:
Clarify naming conventions.
Change enums from POLICYDB_CAPABILITY to POLICYDB_CAP
Add ioctl_skip_cloexec
V3 Changes:
Fix typos and 'Policy Updates' text.

 src/lsm_selinux.md              |   4 +
 src/policy_config_statements.md | 139 ++++++++++++++++++++++++++++++++
 2 files changed, 143 insertions(+)

diff --git a/src/lsm_selinux.md b/src/lsm_selinux.md
index cb8189b..7284912 100644
--- a/src/lsm_selinux.md
+++ b/src/lsm_selinux.md
@@ -714,6 +714,10 @@ or *libsepol* library.
 - Enables fine-grained labeling of symlinks in pseudo filesystems based
   on *genfscon* rules.
 
+*policy_capabilities/ioctl_skip_cloexec*
+
+- If true always allow FIOCLEX and FIONCLEXE ioctl permissions (from kernel 5.18).
+
 *policy_capabilities/network_peer_controls*
 
 - If true the following *network_peer_controls* are enabled:
diff --git a/src/policy_config_statements.md b/src/policy_config_statements.md
index d4eee48..9699766 100644
--- a/src/policy_config_statements.md
+++ b/src/policy_config_statements.md
@@ -1,5 +1,12 @@
 # Policy Configuration Statements
 
+- [*policycap*](#policycap)
+  - [Adding A New Policy Capability](#adding-a-new-policy-capability)
+    - [Kernel Updates](#kernel-updates)
+    - [*libsepol* Library Updates](#libsepol-library-updates)
+    - [Reference Policy Updates](#reference-policy-updates)
+    - [CIL Policy Updates](#cil-policy-updates)
+
 ## *policycap*
 
 Policy version 22 introduced the *policycap* statement to allow new
@@ -47,6 +54,138 @@ Conditional Policy Statements
 policycap network_peer_controls;
 ```
 
+## Adding A New Policy Capability
+
+The kernel, userspace libsepol library and policy must be updated to enable
+the new capability as described below. For readability, the new capability
+should follow a consistent naming convention, where:
+
+- policy capability identifier is a lower-case string.
+- enum definition is ```POLICYDB_CAP_``` with the identifier appended in
+  upper-case.
+- kernel function call is ```selinux_policycap_``` with the identifier
+  appended in lower-case.
+
+### Kernel Updates
+
+In kernel source update the following three files with the new capability:
+
+***security/selinux/include/policycap_names.h***
+
+Add new entry at end of this list:
+
+```
+/* Policy capability names */
+const char *selinux_policycap_names[__POLICYDB_CAP_MAX] = {
+	...
+	"genfs_seclabel_symlinks",
+	"ioctl_skip_cloexec",
+	"new_name"
+};
+```
+
+***security/selinux/include/policycap.h***
+
+Add new entry at end of this list:
+
+```
+/* Policy capabilities */
+enum {
+	...
+	POLICYDB_CAP_GENFS_SECLABEL_SYMLINKS,
+	POLICYDB_CAP_IOCTL_SKIP_CLOEXEC,
+	POLICYDB_CAP_NEW_NAME,
+	__POLICYDB_CAP_MAX
+};
+```
+
+***security/selinux/include/security.h***
+
+Add a new call to retrieve the loaded policy capability state:
+
+```
+static inline bool selinux_policycap_new_name(void)
+{
+	struct selinux_state *state = &selinux_state;
+
+	return READ_ONCE(state->policycap[POLICYDB_CAP_NEW_NAME]);
+}
+```
+
+Finally in the updated code that utilises the new policy capability do
+something like:
+
+```
+if (selinux_policycap_new_name())
+	do this;
+else
+	do that;
+```
+
+### *libsepol* Library Updates
+
+In selinux userspace source update the following two files with the new
+capability:
+
+***selinux/libsepol/src/polcaps.c***
+
+Add new entry at end of this list:
+
+```
+static const char * const polcap_names[] = {
+	...
+	"genfs_seclabel_symlinks",	/* POLICYDB_CAP_GENFS_SECLABEL_SYMLINKS */
+	"ioctl_skip_cloexec",		/* POLICYDB_CAP_IOCTL_SKIP_CLOEXEC */
+	"new_name",			/* POLICYDB_CAP_NEW_NAME */
+	NULL
+};
+```
+
+***selinux/libsepol/include/sepol/policydb/polcaps.h***
+
+Add new entry at end of this list:
+
+```
+/* Policy capabilities */
+enum {
+	...
+	POLICYDB_CAP_GENFS_SECLABEL_SYMLINKS,
+	POLICYDB_CAP_IOCTL_SKIP_CLOEXEC,
+	POLICYDB_CAP_NEW_NAME,
+	__POLICYDB_CAP_MAX
+};
+```
+
+### Reference Policy Updates
+
+To enable the new capability in Reference Policy, add a new entry to this file:
+
+***policy/policy_capabilities***
+
+New *policycap* statement added to end of file:
+
+```
+# A description of the capability
+policycap new_name;
+```
+
+To disable the capability, comment out the entry:
+
+```
+# A description of the capability
+#policycap new_name;
+```
+
+### CIL Policy Updates
+
+To enable the capability in policy, add the following entry to a CIL
+source file:
+
+```
+; A description of the capability
+(policycap new_name)
+```
+
 <!-- %CUTHERE% -->
 
 ---
-- 
2.35.1

Re: [PATCH V3 Notebook] How to add a new policy capability

[Paul Moore]

On Tue, Apr 5, 2022 at 4:19 AM Richard Haines
<richard_c_haines@btinternet.com> wrote:
>
> Describes the steps required to add a new policy capability to:
> kernel, libsepol, and policy.
>
> Also add the ioctl_skip_cloexec capability description.
>
> Signed-off-by: Richard Haines <richard_c_haines@btinternet.com>
> ---
> V2 Changes:
> Clarify naming conventions.
> Change enums from POLICYDB_CAPABILITY to POLICYDB_CAP
> Add ioctl_skip_cloexec
> V3 Changes:
> Fix typos and 'Policy Updates' text.
>
>  src/lsm_selinux.md              |   4 +
>  src/policy_config_statements.md | 139 ++++++++++++++++++++++++++++++++
>  2 files changed, 143 insertions(+)

Merged, thanks everyone!

-- 
paul-moore.com