[PATCH V32 27/27] tracefs: Restrict tracefs when the kernel is locked down

[Matthew Garrett]

Tracefs may release more information about the kernel than desirable, so
restrict it when the kernel is locked down in confidentiality mode by
preventing open().

Signed-off-by: Matthew Garrett <mjg59 at google.com>
Cc: Steven Rostedt <rostedt at goodmis.org>
---
 fs/tracefs/inode.c | 40 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index 7098c49f3693..576327ffd9d1 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -31,6 +31,21 @@ static struct vfsmount *tracefs_mount;
 static int tracefs_mount_count;
 static bool tracefs_registered;
 
+static int default_open_file(struct inode *inode, struct file *filp)
+{
+	struct dentry *dentry = filp->f_path.dentry;
+	struct file_operations *real_fops;
+
+	if (!dentry)
+		return -EINVAL;
+
+	if (kernel_is_locked_down("tracefs", LOCKDOWN_CONFIDENTIALITY))
+		return -EPERM;
+
+	real_fops = dentry->d_fsdata;
+	return real_fops->open(inode, filp);
+}
+
 static ssize_t default_read_file(struct file *file, char __user *buf,
 				 size_t count, loff_t *ppos)
 {
@@ -50,6 +65,13 @@ static const struct file_operations tracefs_file_operations = {
 	.llseek =	noop_llseek,
 };
 
+static const struct file_operations tracefs_proxy_file_operations = {
+	.read =		default_read_file,
+	.write =	default_write_file,
+	.open =		default_open_file,
+	.llseek =	noop_llseek,
+};
+
 static struct tracefs_dir_ops {
 	int (*mkdir)(const char *name);
 	int (*rmdir)(const char *name);
@@ -225,6 +247,12 @@ static int tracefs_apply_options(struct super_block *sb)
 	return 0;
 }
 
+static void tracefs_destroy_inode(struct inode *inode)
+{
+	if S_ISREG(inode->i_mode)
+		kfree(inode->i_fop);
+}
+
 static int tracefs_remount(struct super_block *sb, int *flags, char *data)
 {
 	int err;
@@ -260,6 +288,7 @@ static int tracefs_show_options(struct seq_file *m, struct dentry *root)
 
 static const struct super_operations tracefs_super_operations = {
 	.statfs		= simple_statfs,
+	.destroy_inode  = tracefs_destroy_inode,
 	.remount_fs	= tracefs_remount,
 	.show_options	= tracefs_show_options,
 };
@@ -393,6 +422,7 @@ struct dentry *tracefs_create_file(const char *name, umode_t mode,
 {
 	struct dentry *dentry;
 	struct inode *inode;
+	struct file_operations *proxy_fops;
 
 	if (!(mode & S_IFMT))
 		mode |= S_IFREG;
@@ -406,8 +436,16 @@ struct dentry *tracefs_create_file(const char *name, umode_t mode,
 	if (unlikely(!inode))
 		return failed_creating(dentry);
 
+	proxy_fops = kzalloc(sizeof(struct file_operations), GFP_KERNEL);
+	if (!proxy_fops)
+		return failed_creating(dentry);
+
+	dentry->d_fsdata = fops ? (void *)fops :
+		(void *)&tracefs_file_operations;
+	memcpy(proxy_fops, dentry->d_fsdata, sizeof(struct file_operations));
+	proxy_fops->open = default_open_file;
 	inode->i_mode = mode;
-	inode->i_fop = fops ? fops : &tracefs_file_operations;
+	inode->i_fop = proxy_fops;
 	inode->i_private = data;
 	d_instantiate(dentry, inode);
 	fsnotify_create(dentry->d_parent->d_inode, dentry);
-- 
2.21.0.392.gf8f6787159e-goog