apparmor: add policy revision file interface

Add a policy revision file to find the current revision of a ns's policy.
There is a revision file per ns, as well as a virtualized global revision
file in the base apparmor fs directory. The global revision file when
opened will provide the revision of the opening task namespace.

The revision file can be waited on via select/poll to detect apparmor
policy changes from the last read revision of the opened file. This
means that the revision file must be read after the select/poll other
wise update data will remain ready for reading.

Signed-off-by: John Johansen <[e-mail]>

security/apparmor/apparmorfs.c

···
 #include <linux/capability.h>
 #include <linux/rcupdate.h>
 #include <linux/fs.h>
+#include <linux/poll.h>
 #include <uapi/linux/major.h>
 #include <uapi/linux/magic.h>
 
···
 #include "include/audit.h"
 #include "include/context.h"
 #include "include/crypto.h"
+#include "include/policy_ns.h"
 #include "include/policy.h"
 #include "include/policy_ns.h"
 #include "include/resource.h"
···
 	.llseek = default_llseek,
 };
 
+struct aa_revision {
+	struct aa_ns *ns;
+	long last_read;
+};
+
+/* revision file hook fn for policy loads */
+static int ns_revision_release(struct inode *inode, struct file *file)
+{
+	struct aa_revision *rev = file->private_data;
+
+	if (rev) {
+		aa_put_ns(rev->ns);
+		kfree(rev);
+	}
+
+	return 0;
+}
+
+static ssize_t ns_revision_read(struct file *file, char __user *buf,
+				size_t size, loff_t *ppos)
+{
+	struct aa_revision *rev = file->private_data;
+	char buffer[32];
+	long last_read;
+	int avail;
+
+	mutex_lock(&rev->ns->lock);
+	last_read = rev->last_read;
+	if (last_read == rev->ns->revision) {
+		mutex_unlock(&rev->ns->lock);
+		if (file->f_flags & O_NONBLOCK)
+			return -EAGAIN;
+		if (wait_event_interruptible(rev->ns->wait,
+					     last_read !=
+					     READ_ONCE(rev->ns->revision)))
+			return -ERESTARTSYS;
+		mutex_lock(&rev->ns->lock);
+	}
+
+	avail = sprintf(buffer, "%ld\n", rev->ns->revision);
+	if (*ppos + size > avail) {
+		rev->last_read = rev->ns->revision;
+		*ppos = 0;
+	}
+	mutex_unlock(&rev->ns->lock);
+
+	return simple_read_from_buffer(buf, size, ppos, buffer, avail);
+}
+
+static int ns_revision_open(struct inode *inode, struct file *file)
+{
+	struct aa_revision *rev = kzalloc(sizeof(*rev), GFP_KERNEL);
+
+	if (!rev)
+		return -ENOMEM;
+
+	rev->ns = aa_get_ns(inode->i_private);
+	if (!rev->ns)
+		rev->ns = aa_get_current_ns();
+	file->private_data = rev;
+
+	return 0;
+}
+
+static unsigned int ns_revision_poll(struct file *file, poll_table *pt)
+{
+	struct aa_revision *rev = file->private_data;
+	unsigned int mask = 0;
+
+	if (rev) {
+		mutex_lock(&rev->ns->lock);
+		poll_wait(file, &rev->ns->wait, pt);
+		if (rev->last_read < rev->ns->revision)
+			mask |= POLLIN | POLLRDNORM;
+		mutex_unlock(&rev->ns->lock);
+	}
+
+	return mask;
+}
+
 void __aa_bump_ns_revision(struct aa_ns *ns)
 {
 	ns->revision++;
+	wake_up_interruptible(&ns->wait);
 }
+
+static const struct file_operations aa_fs_ns_revision_fops = {
+	.owner		= THIS_MODULE,
+	.open		= ns_revision_open,
+	.poll		= ns_revision_poll,
+	.read		= ns_revision_read,
+	.llseek		= generic_file_llseek,
+	.release	= ns_revision_release,
+};
 
 /**
  * query_data - queries a policy and writes its data to buf
···
 		sub = d_inode(ns_subremove(ns))->i_private;
 		aa_put_ns(sub);
 	}
+	if (ns_subrevision(ns)) {
+		sub = d_inode(ns_subrevision(ns))->i_private;
+		aa_put_ns(sub);
+	}
 
 	for (i = AAFS_NS_SIZEOF - 1; i >= 0; --i) {
 		aafs_remove(ns->dents[i]);
···
 	if (IS_ERR(dent))
 		return PTR_ERR(dent);
 	ns_subdata_dir(ns) = dent;
+
+	dent = aafs_create_file("revision", 0444, dir, ns,
+				&aa_fs_ns_revision_fops);
+	if (IS_ERR(dent))
+		return PTR_ERR(dent);
+	aa_get_ns(ns);
+	ns_subrevision(ns) = dent;
 
 	dent = aafs_create_file(".load", 0640, dir, ns,
 				      &aa_fs_profile_load);
···
 	}
 	ns_subremove(root_ns) = dent;
 
+	dent = securityfs_create_file("revision", 0444, aa_sfs_entry.dentry,
+				      NULL, &aa_fs_ns_revision_fops);
+	if (IS_ERR(dent)) {
+		error = PTR_ERR(dent);
+		goto error;
+	}
+	ns_subrevision(root_ns) = dent;
+
+	/* policy tree referenced by magic policy symlink */
 	mutex_lock(&root_ns->lock);
 	error = __aafs_ns_mkdir(root_ns, aafs_mnt->mnt_root, ".policy",
 				aafs_mnt->mnt_root);
 	mutex_unlock(&root_ns->lock);
-
 	if (error)
 		goto error;
 

security/apparmor/include/apparmorfs.h

···
 	AAFS_NS_LOAD,
 	AAFS_NS_REPLACE,
 	AAFS_NS_REMOVE,
+	AAFS_NS_REVISION,
 	AAFS_NS_COUNT,
 	AAFS_NS_MAX_COUNT,
 	AAFS_NS_SIZE,
···
 #define ns_subload(X) ((X)->dents[AAFS_NS_LOAD])
 #define ns_subreplace(X) ((X)->dents[AAFS_NS_REPLACE])
 #define ns_subremove(X) ((X)->dents[AAFS_NS_REMOVE])
+#define ns_subrevision(X) ((X)->dents[AAFS_NS_REVISION])
 
 #define prof_dir(X) ((X)->dents[AAFS_PROF_DIR])
 #define prof_child_dir(X) ((X)->dents[AAFS_PROF_PROFS])

security/apparmor/include/policy_ns.h

···
 	long uniq_id;
 	int level;
 	long revision;
+	wait_queue_head_t wait;
 
 	struct list_head rawdata_list;
 

security/apparmor/policy_ns.c

···
 	INIT_LIST_HEAD(&ns->sub_ns);
 	INIT_LIST_HEAD(&ns->rawdata_list);
 	mutex_init(&ns->lock);
+	init_waitqueue_head(&ns->wait);
 
 	/* released by aa_free_ns() */
 	ns->unconfined = aa_alloc_profile("unconfined", GFP_KERNEL);