[PATCH V34 27/29] 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           | 43 +++++++++++++++++++++++++++++++++++-
 include/linux/security.h     |  1 +
 security/lockdown/lockdown.c |  1 +
 3 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index 7098c49f3693..487d41f234f8 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -24,6 +24,7 @@
 #include <linux/parser.h>
 #include <linux/magic.h>
 #include <linux/slab.h>
+#include <linux/security.h>
 
 #define TRACEFS_DEFAULT_MODE	0700
 
@@ -31,6 +32,23 @@ 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;
+	int ret;
+
+	if (!dentry)
+		return -EINVAL;
+
+	ret = security_locked_down(LOCKDOWN_TRACEFS);
+	if (ret)
+		return ret;
+
+	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 +68,13 @@ 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 +250,12 @@ 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 +291,7 @@ 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 +425,7 @@ 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 +439,16 @@ 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);
diff --git a/include/linux/security.h b/include/linux/security.h
index 097e4b0ce73f..438dc0892b96 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -100,6 +100,7 @@ enum lockdown_reason {
 	LOCKDOWN_KPROBES,
 	LOCKDOWN_BPF_READ,
 	LOCKDOWN_PERF,
+	LOCKDOWN_TRACEFS,
 	LOCKDOWN_CONFIDENTIALITY_MAX,
 };
 
diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index bbcb82985765..98f9ee0026d5 100644
--- a/security/lockdown/lockdown.c
+++ b/security/lockdown/lockdown.c
@@ -36,6 +36,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_KPROBES] = "use of kprobes",
 	[LOCKDOWN_BPF_READ] = "use of bpf to read kernel RAM",
 	[LOCKDOWN_PERF] = "unsafe use of perf",
+	[LOCKDOWN_TRACEFS] = "use of tracefs",
 	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
 };
 
-- 
2.22.0.410.gd8fdbe21b5-goog