Smack: fix seq operations in smackfs

Use proper RCU functions and read locking in smackfs seq_operations.

Smack gets away with not using proper RCU functions in smackfs, because
it never removes entries from these lists. But now one list will be
needed (with interface in smackfs) that will have both elements added and
removed to it.
This change will also help any future changes implementing removal of
unneeded entries from other Smack lists.

The patch also fixes handling of pos argument in smk_seq_start and
smk_seq_next. This fixes a bug in case when smackfs is read with a small
buffer:

Kernel panic - not syncing: Kernel mode fault at addr 0xfa0000011b
CPU: 0 PID: 1292 Comm: dd Not tainted 4.1.0-rc1-00012-g98179b8 #13
Stack:
 00000003 0000000d 7ff39e48 7f69fd00
 7ff39ce0 601ae4b0 7ff39d50 600e587b
 00000010 6039f690 7f69fd40 00612003
Call Trace:
 [<601ae4b0>] load2_seq_show+0x19/0x1d
 [<600e587b>] seq_read+0x168/0x331
 [<600c5943>] __vfs_read+0x21/0x101
 [<601a595e>] ? security_file_permission+0xf8/0x105
 [<600c5ec6>] ? rw_verify_area+0x86/0xe2
 [<600c5fc3>] vfs_read+0xa1/0x14c
 [<600c68e2>] SyS_read+0x57/0xa0
 [<6001da60>] handle_syscall+0x60/0x80
 [<6003087d>] userspace+0x442/0x548
 [<6001aa77>] ? interrupt_end+0x0/0x80
 [<6001daae>] ? copy_chunk_to_user+0x0/0x2b
 [<6002cb6b>] ? save_registers+0x1f/0x39
 [<60032ef7>] ? arch_prctl+0xf5/0x170
 [<6001a92d>] fork_handler+0x85/0x87

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>

security/smack/smackfs.c

@@ -566,23 +566,17 @@ static void *smk_seq_start(struct seq_file *s, loff_t *pos,
 				struct list_head *head)
 {
 	struct list_head *list;
+	int i = *pos;
+
+	rcu_read_lock();
+	for (list = rcu_dereference(list_next_rcu(head));
+		list != head;
+		list = rcu_dereference(list_next_rcu(list))) {
+		if (i-- == 0)
+			return list;
+	}
 
-	/*
-	 * This is 0 the first time through.
-	 */
-	if (s->index == 0)
-		s->private = head;
-
-	if (s->private == NULL)
-		return NULL;
-
-	list = s->private;
-	if (list_empty(list))
-		return NULL;
-
-	if (s->index == 0)
-		return list->next;
-	return list;
+	return NULL;
 }
 
 static void *smk_seq_next(struct seq_file *s, void *v, loff_t *pos,
@@ -590,17 +584,15 @@ static void *smk_seq_next(struct seq_file *s, void *v, loff_t *pos,
 {
 	struct list_head *list = v;
 
-	if (list_is_last(list, head)) {
-		s->private = NULL;
-		return NULL;
-	}
-	s->private = list->next;
-	return list->next;
+	++*pos;
+	list = rcu_dereference(list_next_rcu(list));
+
+	return (list == head) ? NULL : list;
 }
 
 static void smk_seq_stop(struct seq_file *s, void *v)
 {
-	/* No-op */
+	rcu_read_unlock();
 }
 
 static void smk_rule_show(struct seq_file *s, struct smack_rule *srp, int max)
@@ -660,7 +652,7 @@ static int load_seq_show(struct seq_file *s, void *v)
 {
 	struct list_head *list = v;
 	struct smack_master_list *smlp =
-		 list_entry(list, struct smack_master_list, list);
+		list_entry_rcu(list, struct smack_master_list, list);
 
 	smk_rule_show(s, smlp->smk_rule, SMK_LABELLEN);
 
@@ -808,7 +800,7 @@ static int cipso_seq_show(struct seq_file *s, void *v)
 {
 	struct list_head  *list = v;
 	struct smack_known *skp =
-		 list_entry(list, struct smack_known, list);
+		list_entry_rcu(list, struct smack_known, list);
 	struct netlbl_lsm_catmap *cmp = skp->smk_netlabel.attr.mls.cat;
 	char sep = '/';
 	int i;
@@ -999,7 +991,7 @@ static int cipso2_seq_show(struct seq_file *s, void *v)
 {
 	struct list_head  *list = v;
 	struct smack_known *skp =
-		 list_entry(list, struct smack_known, list);
+		list_entry_rcu(list, struct smack_known, list);
 	struct netlbl_lsm_catmap *cmp = skp->smk_netlabel.attr.mls.cat;
 	char sep = '/';
 	int i;
@@ -1083,7 +1075,7 @@ static int netlbladdr_seq_show(struct seq_file *s, void *v)
 {
 	struct list_head *list = v;
 	struct smk_netlbladdr *skp =
-			 list_entry(list, struct smk_netlbladdr, list);
+			list_entry_rcu(list, struct smk_netlbladdr, list);
 	unsigned char *hp = (char *) &skp->smk_host.sin_addr.s_addr;
 	int maskn;
 	u32 temp_mask = be32_to_cpu(skp->smk_mask.s_addr);
@@ -1917,7 +1909,7 @@ static int load_self_seq_show(struct seq_file *s, void *v)
 {
 	struct list_head *list = v;
 	struct smack_rule *srp =
-		 list_entry(list, struct smack_rule, list);
+		list_entry_rcu(list, struct smack_rule, list);
 
 	smk_rule_show(s, srp, SMK_LABELLEN);
 
@@ -2046,7 +2038,7 @@ static int load2_seq_show(struct seq_file *s, void *v)
 {
 	struct list_head *list = v;
 	struct smack_master_list *smlp =
-		 list_entry(list, struct smack_master_list, list);
+		list_entry_rcu(list, struct smack_master_list, list);
 
 	smk_rule_show(s, smlp->smk_rule, SMK_LONGLABEL);
 
@@ -2123,7 +2115,7 @@ static int load_self2_seq_show(struct seq_file *s, void *v)
 {
 	struct list_head *list = v;
 	struct smack_rule *srp =
-		 list_entry(list, struct smack_rule, list);
+		list_entry_rcu(list, struct smack_rule, list);
 
 	smk_rule_show(s, srp, SMK_LONGLABEL);