SELinux: allow fstype unknown to policy to use xattrs if present

Currently if a FS is mounted for which SELinux policy does not define an
fs_use_* that FS will either be genfs labeled or not labeled at all.
This decision is based on the existence of a genfscon rule in policy and
is irrespective of the capabilities of the filesystem itself.  This
patch allows the kernel to check if the filesystem supports security
xattrs and if so will use those if there is no fs_use_* rule in policy.
An fstype with a no fs_use_* rule but with a genfs rule will use xattrs
if available and will follow the genfs rule.

This can be particularly interesting for things like ecryptfs which
actually overlays a real underlying FS.  If we define excryptfs in
policy to use xattrs we will likely get this wrong at times, so with
this path we just don't need to define it!

Overlay ecryptfs on top of NFS with no xattr support:
SELinux: initialized (dev ecryptfs, type ecryptfs), uses genfs_contexts
Overlay ecryptfs on top of ext4 with xattr support:
SELinux: initialized (dev ecryptfs, type ecryptfs), uses xattr

It is also useful as the kernel adds new FS we don't need to add them in
policy if they support xattrs and that is how we want to handle them.
Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>

security/selinux/hooks.c

@@ -556,13 +556,15 @@ static int selinux_set_mnt_opts(struct super_block *sb,
 	struct task_security_struct *tsec = current->security;
 	struct superblock_security_struct *sbsec = sb->s_security;
 	const char *name = sb->s_type->name;
-	struct inode *inode = sbsec->sb->s_root->d_inode;
-	struct inode_security_struct *root_isec = inode->i_security;
+	struct dentry *root = sb->s_root;
+	struct inode *root_inode = root->d_inode;
+	struct inode_security_struct *root_isec = root_inode->i_security;
 	u32 fscontext_sid = 0, context_sid = 0, rootcontext_sid = 0;
 	u32 defcontext_sid = 0;
 	char **mount_options = opts->mnt_opts;
 	int *flags = opts->mnt_opts_flags;
 	int num_opts = opts->num_mnt_opts;
+	bool can_xattr = false;
 
 	mutex_lock(&sbsec->lock);
 
@@ -666,14 +668,24 @@ static int selinux_set_mnt_opts(struct super_block *sb,
 		goto out;
 	}
 
-	if (strcmp(sb->s_type->name, "proc") == 0)
+	if (strcmp(name, "proc") == 0)
 		sbsec->proc = 1;
 
+	/*
+	 * test if the fs supports xattrs, fs_use might make use of this if the
+	 * fs has no definition in policy.
+	 */
+	if (root_inode->i_op->getxattr) {
+		rc = root_inode->i_op->getxattr(root, XATTR_NAME_SELINUX, NULL, 0);
+		if (rc >= 0 || rc == -ENODATA)
+			can_xattr = true;
+	}
+
 	/* Determine the labeling behavior to use for this filesystem type. */
-	rc = security_fs_use(sb->s_type->name, &sbsec->behavior, &sbsec->sid);
+	rc = security_fs_use(name, &sbsec->behavior, &sbsec->sid, can_xattr);
 	if (rc) {
 		printk(KERN_WARNING "%s: security_fs_use(%s) returned %d\n",
-		       __func__, sb->s_type->name, rc);
+		       __func__, name, rc);
 		goto out;
 	}
 

security/selinux/include/security.h

@@ -136,7 +136,7 @@ int security_get_allow_unknown(void);
 #define SECURITY_FS_USE_MNTPOINT	6 /* use mountpoint labeling */
 
 int security_fs_use(const char *fstype, unsigned int *behavior,
-	u32 *sid);
+	u32 *sid, bool can_xattr);
 
 int security_genfs_sid(const char *fstype, char *name, u16 sclass,
 	u32 *sid);

security/selinux/ss/services.c

@@ -1934,7 +1934,8 @@ int security_genfs_sid(const char *fstype,
 int security_fs_use(
 	const char *fstype,
 	unsigned int *behavior,
-	u32 *sid)
+	u32 *sid,
+	bool can_xattr)
 {
 	int rc = 0;
 	struct ocontext *c;
@@ -1948,6 +1949,7 @@ int security_fs_use(
 		c = c->next;
 	}
 
+	/* look for labeling behavior defined in policy */
 	if (c) {
 		*behavior = c->v.behavior;
 		if (!c->sid[0]) {
@@ -1958,7 +1960,17 @@ int security_fs_use(
 				goto out;
 		}
 		*sid = c->sid[0];
-	} else {
+		goto out;
+	}
+
+	/* labeling behavior not in policy, use xattrs if possible */
+	if (can_xattr) {
+		*behavior = SECURITY_FS_USE_XATTR;
+		*sid = SECINITSID_FS;
+		goto out;
+	}
+
+	/* no behavior in policy and can't use xattrs, try GENFS */
 	rc = security_genfs_sid(fstype, "/", SECCLASS_DIR, sid);
 	if (rc) {
 		*behavior = SECURITY_FS_USE_NONE;
@@ -1966,7 +1978,6 @@ int security_fs_use(
 	} else {
 		*behavior = SECURITY_FS_USE_GENFS;
 	}
-	}
 
 out:
 	read_unlock(&policy_rwlock);