[PATCH v3] selinux: enable per-file labeling for functionfs

[Neill Kapron <nkapron@google.com>]

This patch adds support for genfscon per-file labeling of functionfs
files as well as support for userspace to apply labels after new
functionfs endpoints are created.

This allows for separate labels and therefore access control on a
per-endpoint basis. An example use case would be for the default
endpoint EP0 used as a restricted control endpoint, and additional
usb endpoints to be used by other more permissive domains.

It should be noted that if there are multiple functionfs mounts on a
system, genfs file labels will apply to all mounts, and therefore will not
likely be as useful as the userspace relabeling portion of this patch -
the addition to selinux_is_genfs_special_handling().

This patch introduces the functionfs_seclabel policycap to maintain
existing functionfs genfscon behavior unless explicitly enabled.

Signed-off-by: Neill Kapron <nkapron@google.com>

Changes since v1:
- Add functionfs_seclabel policycap
- Move new functionality to the end of existing lists

Changes since v2:
- Sending as separate patches
---
 security/selinux/hooks.c                   | 8 ++++++--
 security/selinux/include/policycap.h       | 1 +
 security/selinux/include/policycap_names.h | 1 +
 security/selinux/include/security.h        | 6 ++++++
 4 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index e474cd7398ef..333bb6cba25e 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -476,7 +476,9 @@ static int selinux_is_genfs_special_handling(struct super_block *sb)
 		!strcmp(sb->s_type->name, "rootfs") ||
 		(selinux_policycap_cgroupseclabel() &&
 		 (!strcmp(sb->s_type->name, "cgroup") ||
-		  !strcmp(sb->s_type->name, "cgroup2")));
+		  !strcmp(sb->s_type->name, "cgroup2"))) ||
+		(selinux_policycap_functionfs_seclabel() &&
+		 !strcmp(sb->s_type->name, "functionfs"));
 }
 
 static int selinux_is_sblabel_mnt(struct super_block *sb)
@@ -741,7 +743,9 @@ static int selinux_set_mnt_opts(struct super_block *sb,
 	    !strcmp(sb->s_type->name, "binder") ||
 	    !strcmp(sb->s_type->name, "bpf") ||
 	    !strcmp(sb->s_type->name, "pstore") ||
-	    !strcmp(sb->s_type->name, "securityfs"))
+	    !strcmp(sb->s_type->name, "securityfs") ||
+	    (selinux_policycap_functionfs_seclabel() &&
+	     strcmp(sb->s_type->name, "functionfs")))
 		sbsec->flags |= SE_SBGENFS;
 
 	if (!strcmp(sb->s_type->name, "sysfs") ||
diff --git a/security/selinux/include/policycap.h b/security/selinux/include/policycap.h
index 7405154e6c42..135a969f873c 100644
--- a/security/selinux/include/policycap.h
+++ b/security/selinux/include/policycap.h
@@ -17,6 +17,7 @@ enum {
 	POLICYDB_CAP_NETLINK_XPERM,
 	POLICYDB_CAP_NETIF_WILDCARD,
 	POLICYDB_CAP_GENFS_SECLABEL_WILDCARD,
+	POLICYDB_CAP_FUNCTIONFS_SECLABEL,
 	__POLICYDB_CAP_MAX
 };
 #define POLICYDB_CAP_MAX (__POLICYDB_CAP_MAX - 1)
diff --git a/security/selinux/include/policycap_names.h b/security/selinux/include/policycap_names.h
index d8962fcf2ff9..ff8882887651 100644
--- a/security/selinux/include/policycap_names.h
+++ b/security/selinux/include/policycap_names.h
@@ -20,6 +20,7 @@ const char *const selinux_policycap_names[__POLICYDB_CAP_MAX] = {
 	"netlink_xperm",
 	"netif_wildcard",
 	"genfs_seclabel_wildcard",
+	"functionfs_seclabel",
 };
 /* clang-format on */
 
diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
index 7f19972f7922..0f954a40d3fc 100644
--- a/security/selinux/include/security.h
+++ b/security/selinux/include/security.h
@@ -203,6 +203,12 @@ static inline bool selinux_policycap_netlink_xperm(void)
 		selinux_state.policycap[POLICYDB_CAP_NETLINK_XPERM]);
 }
 
+static inline bool selinux_policycap_functionfs_seclabel(void)
+{
+	return READ_ONCE(
+		selinux_state.policycap[POLICYDB_CAP_FUNCTIONFS_SECLABEL]);
+}
+
 struct selinux_policy_convert_data;
 
 struct selinux_load_state {
-- 
2.51.0.318.gd7df087d1a-goog