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

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Message-Id: <20200128191656.111902-1-cgzones@googlemail.com>
Date:   Tue, 28 Jan 2020 20:16:48 +0100
From:   Christian Göttsche <cgzones@...glemail.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     cgzones@...glemail.com, Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Joshua Brindle <joshua.brindle@...nchydata.com>,
        David Howells <dhowells@...hat.com>,
        Jeff Vander Stoep <jeffv@...gle.com>,
        Richard Guy Briggs <rgb@...hat.com>,
        YueHaibing <yuehaibing@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kent Overstreet <kent.overstreet@...il.com>,
        selinux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2] selinux: allow kernfs symlinks to inherit parent directory context

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@...glemail.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