Smack: limited capability for changing process label

This feature introduces new kernel interface:

- <smack_fs>/relabel-self - for setting transition labels list

This list is used to control smack label transition mechanism.
List is set by, and per process. Process can transit to new label only if
label is on the list. Only process with CAP_MAC_ADMIN capability can add
labels to this list. With this list, process can change it's label without
CAP_MAC_ADMIN but only once. After label changing, list is unset.

Changes in v2:
* use list_for_each_entry instead of _rcu during label write
* added missing description in security/Smack.txt

Changes in v3:
* squashed into one commit

Changes in v4:
* switch from global list to per-task list
* since the per-task list is accessed only by the task itself
  there is no need to use synchronization mechanisms on it

Changes in v5:
* change smackfs interface of relabel-self to the one used for onlycap
  multiple labels are accepted, separated by space, which
  replace the previous list upon write

Signed-off-by: Zbigniew Jasinski <z.jasinski@samsung.com>
Signed-off-by: Rafal Krypa <r.krypa@samsung.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
[Jin-gyu Kim: Backported from mainline]
Signed-off-by: Jin-gyu Kim <jin-gyu.kim@samsung.com>

Change-Id: I4d50ca43463fe38184aa56ae3f275508eb60365b

5 files changed, 244 insertions, 43 deletions

diff --git a/Documentation/security/Smack.txt b/Documentation/security/Smack.txt
index 0e8c1ab4a464..73a1f69f4a2b 100644
--- a/Documentation/security/Smack.txt
+++ b/Documentation/security/Smack.txt
@@ -217,6 +217,16 @@ ptrace
 revoke-subject
 	Writing a Smack label here sets the access to '-' for all access
 	rules with that subject label.
+relabel-self
+	This interface contains a list of labels to which the process can
+	transition to, by writing to /proc/self/attr/current.
+	Normally a process can change its own label to any legal value, but only
+	if it has CAP_MAC_ADMIN. This interface allows a process without
+	CAP_MAC_ADMIN to relabel itself to one of labels from predefined list.
+	A process without CAP_MAC_ADMIN can change its label only once. When it
+	does, this list will be cleared.
+	The values are set by writing the desired labels, separated
+	by spaces, to the file or cleared by writing "-" to the file.
 
 You can add access rules in /etc/smack/accesses. They take the form:
 
diff --git a/security/smack/smack.h b/security/smack/smack.h
index 56f8e4e76cc9..ed66a9c3f20c 100644
--- a/security/smack/smack.h
+++ b/security/smack/smack.h
@@ -100,6 +100,7 @@ struct task_smack {
 	struct smack_known	*smk_forked;	/* label when forked */
 	struct list_head	smk_rules;	/* per task access rules */
 	struct mutex		smk_rules_lock;	/* lock for the rules */
+	struct list_head	smk_relabel;	/* transit allowed labels */
 };
 
 #define	SMK_INODE_INSTANT	0x01	/* inode is instantiated */
@@ -138,7 +139,7 @@ struct smk_port_label {
 	struct smack_known	*smk_out;	/* outgoing label */
 };
 
-struct smack_onlycap {
+struct smack_known_list_elem {
 	struct list_head	list;
 	struct smack_known	*smk_label;
 };
@@ -259,8 +260,9 @@ char *smk_import(const char *, int);
 struct smack_known *smk_import_entry(const char *, int);
 void smk_insert_entry(struct smack_known *skp);
 struct smack_known *smk_find_entry(const char *);
-u32 smack_to_secid(const char *);
 int smack_privileged(int cap);
+u32 smack_to_secid(const char *);
+void smk_destroy_label_list(struct list_head *list);
 
 /*
  * Shared data.
diff --git a/security/smack/smack_access.c b/security/smack/smack_access.c
index 970400934d42..75292315b0a9 100644
--- a/security/smack/smack_access.c
+++ b/security/smack/smack_access.c
@@ -667,7 +667,7 @@ DEFINE_MUTEX(smack_onlycap_lock);
 int smack_privileged(int cap)
 {
 	struct smack_known *skp = smk_of_current();
-	struct smack_onlycap *sop;
+	struct smack_known_list_elem *sklep;
 
 	/*
 	 * All kernel tasks are privileged
@@ -684,8 +684,8 @@ int smack_privileged(int cap)
 		return 1;
 	}
 
-	list_for_each_entry_rcu(sop, &smack_onlycap_list, list) {
-		if (sop->smk_label == skp) {
+	list_for_each_entry_rcu(sklep, &smack_onlycap_list, list) {
+		if (sklep->smk_label == skp) {
 			rcu_read_unlock();
 			return 1;
 		}
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 3a927fd5ec60..ce60cdaa908b 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -306,6 +306,7 @@ static struct task_smack *new_task_smack(struct smack_known *task,
 	tsp->smk_task = task;
 	tsp->smk_forked = forked;
 	INIT_LIST_HEAD(&tsp->smk_rules);
+	INIT_LIST_HEAD(&tsp->smk_relabel);
 	mutex_init(&tsp->smk_rules_lock);
 
 	return tsp;
@@ -340,6 +341,35 @@ static int smk_copy_rules(struct list_head *nhead, struct list_head *ohead,
 }
 
 /**
+ * smk_copy_relabel - copy smk_relabel labels list
+ * @nhead: new rules header pointer
+ * @ohead: old rules header pointer
+ * @gfp: type of the memory for the allocation
+ *
+ * Returns 0 on success, -ENOMEM on error
+ */
+static int smk_copy_relabel(struct list_head *nhead, struct list_head *ohead,
+				gfp_t gfp)
+{
+	struct smack_known_list_elem *nklep;
+	struct smack_known_list_elem *oklep;
+
+	INIT_LIST_HEAD(nhead);
+
+	list_for_each_entry(oklep, ohead, list) {
+		nklep = kzalloc(sizeof(struct smack_known_list_elem), gfp);
+		if (nklep == NULL) {
+			smk_destroy_label_list(nhead);
+			return -ENOMEM;
+		}
+		nklep->smk_label = oklep->smk_label;
+		list_add(&nklep->list, nhead);
+	}
+
+	return 0;
+}
+
+/**
  * smk_ptrace_mode - helper function for converting PTRACE_MODE_* into MAY_*
  * @mode - input mode in form of PTRACE_MODE_*
  *
@@ -1778,6 +1808,8 @@ static void smack_cred_free(struct cred *cred)
 		return;
 	cred->security = NULL;
 
+	smk_destroy_label_list(&tsp->smk_relabel);
+
 	list_for_each_safe(l, n, &tsp->smk_rules) {
 		rp = list_entry(l, struct smack_rule, list);
 		list_del(&rp->list);
@@ -1809,6 +1841,11 @@ static int smack_cred_prepare(struct cred *new, const struct cred *old,
 	if (rc != 0)
 		return rc;
 
+	rc = smk_copy_relabel(&new_tsp->smk_relabel, &old_tsp->smk_relabel,
+				 gfp);
+	if (rc != 0)
+		return rc;
+
 	new->security = new_tsp;
 	return 0;
 }
@@ -3304,9 +3341,11 @@ static int smack_getprocattr(struct task_struct *p, char *name, char **value)
 static int smack_setprocattr(struct task_struct *p, char *name,
 			     void *value, size_t size)
 {
-	struct task_smack *tsp;
+	struct task_smack *tsp = current_security();
 	struct cred *new;
 	struct smack_known *skp;
+	struct smack_known_list_elem *sklep;
+	int rc;
 
 	/*
 	 * Changing another process' Smack value is too dangerous
@@ -3315,7 +3354,7 @@ static int smack_setprocattr(struct task_struct *p, char *name,
 	if (p != current)
 		return -EPERM;
 
-	if (!smack_privileged(CAP_MAC_ADMIN))
+	if (!smack_privileged(CAP_MAC_ADMIN) && list_empty(&tsp->smk_relabel))
 		return -EPERM;
 
 	if (value == NULL || size == 0 || size >= SMK_LONGLABEL)
@@ -3334,12 +3373,27 @@ static int smack_setprocattr(struct task_struct *p, char *name,
 	if (skp == &smack_known_web)
 		return -EPERM;
 
+	if (!smack_privileged(CAP_MAC_ADMIN)) {
+		rc = -EPERM;
+		list_for_each_entry(sklep, &tsp->smk_relabel, list)
+			if (sklep->smk_label == skp) {
+				rc = 0;
+				break;
+			}
+		if (rc)
+			return rc;
+	}
+
 	new = prepare_creds();
 	if (new == NULL)
 		return -ENOMEM;
 
 	tsp = new->security;
 	tsp->smk_task = skp;
+	/*
+	 * process can change its label only once
+	 */
+	smk_destroy_label_list(&tsp->smk_relabel);
 
 	commit_creds(new);
 	return size;
diff --git a/security/smack/smackfs.c b/security/smack/smackfs.c
index 3e6c2e5c52fe..c40e2d46dba0 100644
--- a/security/smack/smackfs.c
+++ b/security/smack/smackfs.c
@@ -57,6 +57,8 @@ enum smk_inos {
 #ifdef CONFIG_SECURITY_SMACK_BRINGUP
 	SMK_UNCONFINED  = 22,   /* define an unconfined label */
 #endif
+    SMK_RELABEL_SELF = 24, /* relabel possible without CAP_MAC_ADMIN */
+
 };
 
 /*
@@ -1633,10 +1635,10 @@ static void *onlycap_seq_next(struct seq_file *s, void *v, loff_t *pos)
 static int onlycap_seq_show(struct seq_file *s, void *v)
 {
 	struct list_head *list = v;
-	struct smack_onlycap *sop =
-		list_entry_rcu(list, struct smack_onlycap, list);
+	struct smack_known_list_elem *sklep =
+		list_entry_rcu(list, struct smack_known_list_elem, list);
 
-	seq_puts(s, sop->smk_label->smk_known);
+	seq_puts(s, sklep->smk_label->smk_known);
 	seq_putc(s, ' ');
 
 	return 0;
@@ -1693,6 +1695,54 @@ static void smk_list_swap_rcu(struct list_head *public,
 }
 
 /**
+ * smk_parse_label_list - parse list of Smack labels, separated by spaces
+ *
+ * @data: the string to parse
+ * @private: destination list
+ *
+ * Returns zero on success or error code, as appropriate
+ */
+static int smk_parse_label_list(char *data, struct list_head *list)
+{
+	char *tok;
+	struct smack_known *skp;
+	struct smack_known_list_elem *sklep;
+
+	while ((tok = strsep(&data, " ")) != NULL) {
+		if (!*tok)
+			continue;
+
+		skp = smk_import_entry(tok, 0);
+		if (IS_ERR(skp))
+			return PTR_ERR(skp);
+
+		sklep = kzalloc(sizeof(*sklep), GFP_KERNEL);
+			if (sklep == NULL)
+				return -ENOMEM;
+
+		sklep->smk_label = skp;
+		list_add(&sklep->list, list);
+	}
+
+	return 0;
+}
+
+/**
+ * smk_destroy_label_list - destroy a list of smack_known_list_elem
+ * @head: header pointer of the list to destroy
+ */
+void smk_destroy_label_list(struct list_head *list)
+{
+	struct smack_known_list_elem *sklep;
+	struct smack_known_list_elem *sklep2;
+
+	list_for_each_entry_safe(sklep, sklep2, list, list)
+		kfree(sklep);
+
+	INIT_LIST_HEAD(list);
+}
+
+/**
  * smk_write_onlycap - write() for smackfs/onlycap
  * @file: file pointer, not actually used
  * @buf: where to get the data from
@@ -1705,13 +1755,8 @@ static ssize_t smk_write_onlycap(struct file *file, const char __user *buf,
 				 size_t count, loff_t *ppos)
 {
 	char *data;
-	char *data_parse;
-	char *tok;
-	struct smack_known *skp;
-	struct smack_onlycap *sop;
-	struct smack_onlycap *sop2;
 	LIST_HEAD(list_tmp);
-	int rc = count;
+	int rc;
 
 	if (!smack_privileged(CAP_MAC_ADMIN))
 		return -EPERM;
@@ -1724,27 +1769,8 @@ static ssize_t smk_write_onlycap(struct file *file, const char __user *buf,
 		kfree(data);
 		return -EFAULT;
 	}
-
-	data_parse = data;
-	while ((tok = strsep(&data_parse, " ")) != NULL) {
-		if (!*tok)
-			continue;
-
-		skp = smk_import_entry(tok, 0);
-		if (skp == NULL) {
-			rc = -EINVAL;
-			break;
-		}
-
-		sop = kzalloc(sizeof(*sop), GFP_KERNEL);
-		if (sop == NULL) {
-			rc = -ENOMEM;
-			break;
-		}
-
-		sop->smk_label = skp;
-		list_add_rcu(&sop->list, &list_tmp);
-	}
+	
+    rc = smk_parse_label_list(data, &list_tmp);
 	kfree(data);
 
 	/*
@@ -1757,17 +1783,14 @@ static ssize_t smk_write_onlycap(struct file *file, const char __user *buf,
 	 * But do so only on invalid label, not on system errors.
 	 * The invalid label must be first to count as clearing attempt.
 	 */
-	if (rc == -EINVAL && list_empty(&list_tmp))
-		rc = count;
-
-	if (rc >= 0) {
+	if (!rc || (rc == -EINVAL && list_empty(&list_tmp))) {
 		mutex_lock(&smack_onlycap_lock);
 		smk_list_swap_rcu(&smack_onlycap_list, &list_tmp);
 		mutex_unlock(&smack_onlycap_lock);
+		rc = count;
 	}
 
-	list_for_each_entry_safe(sop, sop2, &list_tmp, list)
-		kfree(sop);
+	smk_destroy_label_list(&list_tmp);
 
 	return rc;
 }
@@ -2326,6 +2349,115 @@ static ssize_t smk_write_change_rule(struct file *file, const char __user *buf,
 				    SMK_CHANGE_FMT);
 }
 
+/*
+ * Seq_file read operations for /smack/relabel-self
+ */
+
+static void *relabel_self_seq_start(struct seq_file *s, loff_t *pos)
+{
+	struct task_smack *tsp = current_security();
+
+	return smk_seq_start(s, pos, &tsp->smk_relabel);
+}
+
+static void *relabel_self_seq_next(struct seq_file *s, void *v, loff_t *pos)
+{
+	struct task_smack *tsp = current_security();
+
+	return smk_seq_next(s, v, pos, &tsp->smk_relabel);
+}
+
+static int relabel_self_seq_show(struct seq_file *s, void *v)
+{
+	struct list_head *list = v;
+	struct smack_known_list_elem *sklep =
+		list_entry(list, struct smack_known_list_elem, list);
+
+	seq_puts(s, sklep->smk_label->smk_known);
+	seq_putc(s, ' ');
+
+	return 0;
+}
+
+static const struct seq_operations relabel_self_seq_ops = {
+	.start = relabel_self_seq_start,
+	.next  = relabel_self_seq_next,
+	.show  = relabel_self_seq_show,
+	.stop  = smk_seq_stop,
+};
+
+
+/**
+ * smk_open_relabel_self - open() for /smack/relabel-self
+ * @inode: inode structure representing file
+ * @file: "relabel-self" file pointer
+ *
+ * Connect our relabel_self_seq_* operations with /smack/relabel-self
+ * file_operations
+ */
+static int smk_open_relabel_self(struct inode *inode, struct file *file)
+{
+	return seq_open(file, &relabel_self_seq_ops);
+}
+
+/**
+ * smk_write_relabel_self - write() for /smack/relabel-self
+ * @file: file pointer, not actually used
+ * @buf: where to get the data from
+ * @count: bytes sent
+ * @ppos: where to start - must be 0
+ *
+ */
+static ssize_t smk_write_relabel_self(struct file *file, const char __user *buf,
+				size_t count, loff_t *ppos)
+{
+	struct task_smack *tsp = current_security();
+	char *data;
+	int rc;
+	LIST_HEAD(list_tmp);
+
+	/*
+	 * Must have privilege.
+	 */
+	if (!smack_privileged(CAP_MAC_ADMIN))
+		return -EPERM;
+
+	/*
+	 * Enough data must be present.
+	 */
+	if (*ppos != 0)
+		return -EINVAL;
+
+	data = kzalloc(count + 1, GFP_KERNEL);
+	if (data == NULL)
+		return -ENOMEM;
+
+	if (copy_from_user(data, buf, count) != 0) {
+		kfree(data);
+		return -EFAULT;
+	}
+
+	rc = smk_parse_label_list(data, &list_tmp);
+	kfree(data);
+
+	if (!rc || (rc == -EINVAL && list_empty(&list_tmp))) {
+		smk_destroy_label_list(&tsp->smk_relabel);
+		list_splice(&list_tmp, &tsp->smk_relabel);
+		return count;
+	}
+
+	smk_destroy_label_list(&list_tmp);
+	return rc;
+}
+
+static const struct file_operations smk_relabel_self_ops = {
+	.open		= smk_open_relabel_self,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.write		= smk_write_relabel_self,
+	.release	= seq_release,
+};
+
 static const struct file_operations smk_change_rule_ops = {
 	.write		= smk_write_change_rule,
 	.read		= simple_transaction_read,
@@ -2531,6 +2663,9 @@ static int smk_fill_super(struct super_block *sb, void *data, int silent)
 		[SMK_UNCONFINED] = {
 			"unconfined", &smk_unconfined_ops, S_IRUGO|S_IWUSR},
 #endif
+		[SMK_RELABEL_SELF] = {
+			"relabel-self", &smk_relabel_self_ops,
+				S_IRUGO|S_IWUGO},
 		/* last one */
 			{""}
 	};