[PATCH v1] security: Add a new hook: inode_touch_atime

From: "Mickaël Salaün" <mic@digikod.net>
To: linux-kernel@vger.kernel.org
Cc: "Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Andreas Gruenbacher" <agruenba@redhat.com>,
	"Andy Lutomirski" <luto@amacapital.net>,
	"Casey Schaufler" <casey@schaufler-ca.com>,
	"Dmitry Kasatkin" <dmitry.kasatkin@gmail.com>,
	"Eric Paris" <eparis@parisplace.org>,
	"James Morris" <james.l.morris@oracle.com>,
	"John Johansen" <john.johansen@canonical.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Kentaro Takeda" <takedakn@nttdata.co.jp>,
	"Mimi Zohar" <zohar@linux.vnet.ibm.com>,
	"Paul Moore" <paul@paul-moore.com>,
	"Serge E . Hallyn" <serge@hallyn.com>,
	"Stephen Smalley" <sds@tycho.nsa.gov>,
	"Tetsuo Handa" <penguin-kernel@I-love.SAKURA.ne.jp>,
	"Vivek Goyal" <vgoyal@redhat.com>,
	linux-fsdevel@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	"Mickaël Salaün" <mic@digikod.net>
Subject: [PATCH v1] security: Add a new hook: inode_touch_atime
Date: Thu, 22 Dec 2016 00:15:06 +0100	[thread overview]
Message-ID: <20161221231506.19800-1-mic@digikod.net> (raw)

Add a new LSM hook named inode_touch_atime which is needed to deny
indirect update of extended file attributes (i.e. access time) which are
not catched by the inode_setattr hook. By creating a new hook instead of
calling inode_setattr, we avoid to simulate a useless struct iattr.

This hook allows to create read-only environments as with read-only
mount points. It can also take care of anonymous inodes.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
Cc: Eric Paris <eparis@parisplace.org>
Cc: James Morris <james.l.morris@oracle.com>
Cc: John Johansen <john.johansen@canonical.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kentaro Takeda <takedakn@nttdata.co.jp>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Serge E. Hallyn <serge@hallyn.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 fs/inode.c                | 5 ++++-
 include/linux/lsm_hooks.h | 8 ++++++++
 include/linux/security.h  | 8 ++++++++
 security/security.c       | 9 +++++++++
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index 88110fd0b282..8e7519196942 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1706,6 +1706,10 @@ void touch_atime(const struct path *path)
 	if (!__atime_needs_update(path, inode, false))
 		return;
 
+	now = current_time(inode);
+	if (security_inode_touch_atime(path, &now))
+		return;
+
 	if (!sb_start_write_trylock(inode->i_sb))
 		return;
 
@@ -1720,7 +1724,6 @@ void touch_atime(const struct path *path)
 	 * We may also fail on filesystems that have the ability to make parts
 	 * of the fs read only, e.g. subvolumes in Btrfs.
 	 */
-	now = current_time(inode);
 	update_time(inode, &now, S_ATIME);
 	__mnt_drop_write(mnt);
 skip_update:
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 558adfa5c8a8..e77051715e6b 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -428,6 +428,11 @@
  *	security module does not know about attribute or a negative error code
  *	to abort the copy up. Note that the caller is responsible for reading
  *	and writing the xattrs as this hook is merely a filter.
+ * @inode_touch_atime:
+ *	Check permission before updating access time.
+ *	@path contains the path structure for the file.
+ *	@ts contains the current time.
+ *	Return 0 if permission is granted.
  *
  * Security hooks for file operations
  *
@@ -1458,6 +1463,8 @@ union security_list_options {
 	void (*inode_getsecid)(struct inode *inode, u32 *secid);
 	int (*inode_copy_up)(struct dentry *src, struct cred **new);
 	int (*inode_copy_up_xattr)(const char *name);
+	int (*inode_touch_atime)(const struct path *path,
+					const struct timespec *ts);
 
 	int (*file_permission)(struct file *file, int mask);
 	int (*file_alloc_security)(struct file *file);
@@ -1731,6 +1738,7 @@ struct security_hook_heads {
 	struct list_head inode_getsecid;
 	struct list_head inode_copy_up;
 	struct list_head inode_copy_up_xattr;
+	struct list_head inode_touch_atime;
 	struct list_head file_permission;
 	struct list_head file_alloc_security;
 	struct list_head file_free_security;
diff --git a/include/linux/security.h b/include/linux/security.h
index c2125e9093e8..619f44c290a5 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -30,6 +30,7 @@
 #include <linux/string.h>
 #include <linux/mm.h>
 #include <linux/fs.h>
+#include <linux/time.h>
 
 struct linux_binprm;
 struct cred;
@@ -288,6 +289,7 @@ int security_inode_listsecurity(struct inode *inode, char *buffer, size_t buffer
 void security_inode_getsecid(struct inode *inode, u32 *secid);
 int security_inode_copy_up(struct dentry *src, struct cred **new);
 int security_inode_copy_up_xattr(const char *name);
+int security_inode_touch_atime(const struct path *path, const struct timespec *ts);
 int security_file_permission(struct file *file, int mask);
 int security_file_alloc(struct file *file);
 void security_file_free(struct file *file);
@@ -781,6 +783,12 @@ static inline int security_inode_copy_up_xattr(const char *name)
 	return -EOPNOTSUPP;
 }
 
+static inline int security_inode_touch_atime(const struct path *path, const
+					     struct timespec *ts)
+{
+	return 0;
+}
+
 static inline int security_file_permission(struct file *file, int mask)
 {
 	return 0;
diff --git a/security/security.c b/security/security.c
index f825304f04a7..cd093c4b4115 100644
--- a/security/security.c
+++ b/security/security.c
@@ -769,6 +769,13 @@ int security_inode_copy_up_xattr(const char *name)
 }
 EXPORT_SYMBOL(security_inode_copy_up_xattr);
 
+int security_inode_touch_atime(const struct path *path,
+				const struct timespec *ts)
+{
+	return call_int_hook(inode_touch_atime, 0, path, ts);
+}
+EXPORT_SYMBOL(security_inode_touch_atime);
+
 int security_file_permission(struct file *file, int mask)
 {
 	int ret;
@@ -1711,6 +1718,8 @@ struct security_hook_heads security_hook_heads = {
 		LIST_HEAD_INIT(security_hook_heads.inode_copy_up),
 	.inode_copy_up_xattr =
 		LIST_HEAD_INIT(security_hook_heads.inode_copy_up_xattr),
+	.inode_touch_atime =
+		LIST_HEAD_INIT(security_hook_heads.inode_touch_atime),
 	.file_permission =
 		LIST_HEAD_INIT(security_hook_heads.file_permission),
 	.file_alloc_security =
-- 
2.11.0
