[PATCH v2] selinux: allow kernfs symlinks to inherit parent directory context

["Christian Göttsche" <cgzones@googlemail.com>]

Currently symlinks on kernel filesystems, like sysfs, are labeled on
creation with the parent filesystem root sid.

Allow symlinks to inherit the parent directory context, so fine-grained
kernfs labeling can be applied to symlinks too and checking contexts
doesn't complain about them.

For backward-compatibility this behavior is contained in a new policy
capability: genfs_seclabel_symlinks

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
---
v2:
incorporate feedback from Stephen Smalley
  - changed polcap name
  - extended affected filesystems from SE_SBGENFS_XATTR to SE_SBGENFS

 security/selinux/hooks.c            | 4 +++-
 security/selinux/include/security.h | 8 ++++++++
 security/selinux/ss/services.c      | 3 ++-
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index d9e8b2131a65..60a0b3553c70 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -1475,7 +1475,9 @@ static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dent
 		/* Default to the fs superblock SID. */
 		sid = sbsec->sid;
 
-		if ((sbsec->flags & SE_SBGENFS) && !S_ISLNK(inode->i_mode)) {
+		if ((sbsec->flags & SE_SBGENFS) &&
+		     (!S_ISLNK(inode->i_mode) ||
+		      selinux_policycap_genfs_seclabel_symlinks())) {
 			/* We must have a dentry to determine the label on
 			 * procfs inodes */
 			if (opt_dentry) {
diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
index a39f9565d80b..863ccf2bb629 100644
--- a/security/selinux/include/security.h
+++ b/security/selinux/include/security.h
@@ -79,6 +79,7 @@ enum {
 	POLICYDB_CAPABILITY_ALWAYSNETWORK,
 	POLICYDB_CAPABILITY_CGROUPSECLABEL,
 	POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
+	POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS,
 	__POLICYDB_CAPABILITY_MAX
 };
 #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
@@ -209,6 +210,13 @@ static inline bool selinux_policycap_nnp_nosuid_transition(void)
 	return state->policycap[POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION];
 }
 
+static inline bool selinux_policycap_genfs_seclabel_symlinks(void)
+{
+	struct selinux_state *state = &selinux_state;
+
+	return state->policycap[POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS];
+}
+
 int security_mls_enabled(struct selinux_state *state);
 int security_load_policy(struct selinux_state *state,
 			 void *data, size_t len);
diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index 216ce602a2b5..d9306f489060 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -73,7 +73,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
 	"extended_socket_class",
 	"always_check_network",
 	"cgroup_seclabel",
-	"nnp_nosuid_transition"
+	"nnp_nosuid_transition",
+	"genfs_seclabel_symlinks"
 };
 
 static struct selinux_ss selinux_ss;
-- 
2.25.0