IMA: policy can now be updated multiple times

The new rules get appended to the original policy, forming a queue.
The new rules are first added to a temporary list, which on error
get released without disturbing the normal IMA operations.  On
success both lists (the current policy and the new rules) are spliced.

IMA policy reads are many orders of magnitude more numerous compared to
writes, the match code is RCU protected.  The updater side also does
list splice in RCU manner.

Signed-off-by: Petko Manolov <petkan@mip-labs.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

security/integrity/ima/ima_fs.c

@@ -25,6 +25,8 @@
 
 #include "ima.h"
 
+static DEFINE_MUTEX(ima_write_mutex);
+
 static int valid_policy = 1;
 #define TMPBUFLEN 12
 static ssize_t ima_show_htable_value(char __user *buf, size_t count,
@@ -261,6 +263,11 @@ static ssize_t ima_write_policy(struct file *file, const char __user *buf,
 {
 	char *data = NULL;
 	ssize_t result;
+	int res;
+
+	res = mutex_lock_interruptible(&ima_write_mutex);
+	if (res)
+		return res;
 
 	if (datalen >= PAGE_SIZE)
 		datalen = PAGE_SIZE - 1;
@@ -286,6 +293,8 @@ out:
 	if (result < 0)
 		valid_policy = 0;
 	kfree(data);
+	mutex_unlock(&ima_write_mutex);
+
 	return result;
 }
 
@@ -337,8 +346,12 @@ static int ima_release_policy(struct inode *inode, struct file *file)
 		return 0;
 	}
 	ima_update_policy();
+#ifndef	CONFIG_IMA_WRITE_POLICY
 	securityfs_remove(ima_policy);
 	ima_policy = NULL;
+#else
+	clear_bit(IMA_FS_BUSY, &ima_fs_flags);
+#endif
 	return 0;
 }