eCryptfs: Privileged kthread for lower file opens

eCryptfs would really like to have read-write access to all files in the
lower filesystem.  Right now, the persistent lower file may be opened
read-only if the attempt to open it read-write fails.  One way to keep
from having to do that is to have a privileged kthread that can open the
lower persistent file on behalf of the user opening the eCryptfs file;
this patch implements this functionality.

This patch will properly allow a less-privileged user to open the eCryptfs
file, followed by a more-privileged user opening the eCryptfs file, with
the first user only being able to read and the second user being able to
both read and write.  eCryptfs currently does this wrong; it will wind up
calling vfs_write() on a file that was opened read-only.  This is fixed in
this patch.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Cc: Dave Kleikamp <shaggy@austin.ibm.com>
Cc: Serge Hallyn <serue@us.ibm.com>
Cc: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 203 insertions, 0 deletions

diff --git a/fs/ecryptfs/kthread.c b/fs/ecryptfs/kthread.c
new file mode 100644
index 0000000..c440c6b
--- /dev/null
+++ b/fs/ecryptfs/kthread.c
@@ -0,0 +1,203 @@
+/**
+ * eCryptfs: Linux filesystem encryption layer
+ *
+ * Copyright (C) 2008 International Business Machines Corp.
+ *   Author(s): Michael A. Halcrow <mahalcro@us.ibm.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ */
+
+#include <linux/kthread.h>
+#include <linux/freezer.h>
+#include <linux/wait.h>
+#include <linux/mount.h>
+#include "ecryptfs_kernel.h"
+
+struct kmem_cache *ecryptfs_open_req_cache;
+
+static struct ecryptfs_kthread_ctl {
+#define ECRYPTFS_KTHREAD_ZOMBIE 0x00000001
+	u32 flags;
+	struct mutex mux;
+	struct list_head req_list;
+	wait_queue_head_t wait;
+} ecryptfs_kthread_ctl;
+
+static struct task_struct *ecryptfs_kthread;
+
+/**
+ * ecryptfs_threadfn
+ * @ignored: ignored
+ *
+ * The eCryptfs kernel thread that has the responsibility of getting
+ * the lower persistent file with RW permissions.
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+static int ecryptfs_threadfn(void *ignored)
+{
+	set_freezable();
+	while (1)  {
+		struct ecryptfs_open_req *req;
+
+		wait_event_freezable(
+			ecryptfs_kthread_ctl.wait,
+			(!list_empty(&ecryptfs_kthread_ctl.req_list)
+			 || kthread_should_stop()));
+		mutex_lock(&ecryptfs_kthread_ctl.mux);
+		if (ecryptfs_kthread_ctl.flags & ECRYPTFS_KTHREAD_ZOMBIE) {
+			mutex_unlock(&ecryptfs_kthread_ctl.mux);
+			goto out;
+		}
+		while (!list_empty(&ecryptfs_kthread_ctl.req_list)) {
+			req = list_first_entry(&ecryptfs_kthread_ctl.req_list,
+					       struct ecryptfs_open_req,
+					       kthread_ctl_list);
+			mutex_lock(&req->mux);
+			list_del(&req->kthread_ctl_list);
+			if (!(req->flags & ECRYPTFS_REQ_ZOMBIE)) {
+				dget(req->lower_dentry);
+				mntget(req->lower_mnt);
+				(*req->lower_file) = dentry_open(
+					req->lower_dentry, req->lower_mnt,
+					(O_RDWR | O_LARGEFILE));
+				req->flags |= ECRYPTFS_REQ_PROCESSED;
+			}
+			wake_up(&req->wait);
+			mutex_unlock(&req->mux);
+		}
+		mutex_unlock(&ecryptfs_kthread_ctl.mux);
+	}
+out:
+	return 0;
+}
+
+int ecryptfs_init_kthread(void)
+{
+	int rc = 0;
+
+	mutex_init(&ecryptfs_kthread_ctl.mux);
+	init_waitqueue_head(&ecryptfs_kthread_ctl.wait);
+	INIT_LIST_HEAD(&ecryptfs_kthread_ctl.req_list);
+	ecryptfs_kthread = kthread_run(&ecryptfs_threadfn, NULL,
+				       "ecryptfs-kthread");
+	if (IS_ERR(ecryptfs_kthread)) {
+		rc = PTR_ERR(ecryptfs_kthread);
+		printk(KERN_ERR "%s: Failed to create kernel thread; rc = [%d]"
+		       "\n", __func__, rc);
+	}
+	return rc;
+}
+
+void ecryptfs_destroy_kthread(void)
+{
+	struct ecryptfs_open_req *req;
+
+	mutex_lock(&ecryptfs_kthread_ctl.mux);
+	ecryptfs_kthread_ctl.flags |= ECRYPTFS_KTHREAD_ZOMBIE;
+	list_for_each_entry(req, &ecryptfs_kthread_ctl.req_list,
+			    kthread_ctl_list) {
+		mutex_lock(&req->mux);
+		req->flags |= ECRYPTFS_REQ_ZOMBIE;
+		wake_up(&req->wait);
+		mutex_unlock(&req->mux);
+	}
+	mutex_unlock(&ecryptfs_kthread_ctl.mux);
+	kthread_stop(ecryptfs_kthread);
+	wake_up(&ecryptfs_kthread_ctl.wait);
+}
+
+/**
+ * ecryptfs_privileged_open
+ * @lower_file: Result of dentry_open by root on lower dentry
+ * @lower_dentry: Lower dentry for file to open
+ * @lower_mnt: Lower vfsmount for file to open
+ *
+ * This function gets a r/w file opened againt the lower dentry.
+ *
+ * Returns zero on success; non-zero otherwise
+ */
+int ecryptfs_privileged_open(struct file **lower_file,
+			     struct dentry *lower_dentry,
+			     struct vfsmount *lower_mnt)
+{
+	struct ecryptfs_open_req *req;
+	int rc = 0;
+
+	/* Corresponding dput() and mntput() are done when the
+	 * persistent file is fput() when the eCryptfs inode is
+	 * destroyed. */
+	dget(lower_dentry);
+	mntget(lower_mnt);
+	(*lower_file) = dentry_open(lower_dentry, lower_mnt,
+				    (O_RDWR | O_LARGEFILE));
+	if (!IS_ERR(*lower_file))
+		goto out;
+	req = kmem_cache_alloc(ecryptfs_open_req_cache, GFP_KERNEL);
+	if (!req) {
+		rc = -ENOMEM;
+		goto out;
+	}
+	mutex_init(&req->mux);
+	req->lower_file = lower_file;
+	req->lower_dentry = lower_dentry;
+	req->lower_mnt = lower_mnt;
+	init_waitqueue_head(&req->wait);
+	req->flags = 0;
+	mutex_lock(&ecryptfs_kthread_ctl.mux);
+	if (ecryptfs_kthread_ctl.flags & ECRYPTFS_KTHREAD_ZOMBIE) {
+		rc = -EIO;
+		mutex_unlock(&ecryptfs_kthread_ctl.mux);
+		printk(KERN_ERR "%s: We are in the middle of shutting down; "
+		       "aborting privileged request to open lower file\n",
+			__func__);
+		goto out_free;
+	}
+	list_add_tail(&req->kthread_ctl_list, &ecryptfs_kthread_ctl.req_list);
+	mutex_unlock(&ecryptfs_kthread_ctl.mux);
+	wake_up(&ecryptfs_kthread_ctl.wait);
+	wait_event(req->wait, (req->flags != 0));
+	mutex_lock(&req->mux);
+	BUG_ON(req->flags == 0);
+	if (req->flags & ECRYPTFS_REQ_DROPPED
+	    || req->flags & ECRYPTFS_REQ_ZOMBIE) {
+		rc = -EIO;
+		printk(KERN_WARNING "%s: Privileged open request dropped\n",
+		       __func__);
+		goto out_unlock;
+	}
+	if (IS_ERR(*req->lower_file)) {
+		rc = PTR_ERR(*req->lower_file);
+		dget(lower_dentry);
+		mntget(lower_mnt);
+		(*lower_file) = dentry_open(lower_dentry, lower_mnt,
+					    (O_RDONLY | O_LARGEFILE));
+		if (IS_ERR(*lower_file)) {
+			rc = PTR_ERR(*req->lower_file);
+			(*lower_file) = NULL;
+			printk(KERN_WARNING "%s: Error attempting privileged "
+			       "open of lower file with either RW or RO "
+			       "perms; rc = [%d]. Giving up.\n",
+			       __func__, rc);
+		}
+	}
+out_unlock:
+	mutex_unlock(&req->mux);
+out_free:
+	kmem_cache_free(ecryptfs_open_req_cache, req);
+out:
+	return rc;
+}