debugfs: Add access restriction option

Since debugfs include sensitive information it need to be treated
carefully. But it also has many very useful debug functions for userspace.
With this option we can have same configuration for system with
need of debugfs and a way to turn it off. This gives a extra protection
for exposure on systems where user-space services with system
access are attacked.

It is controlled by a configurable default value that can be override
with a kernel command line parameter. (debugfs=)

It can be on or off, but also internally on but not seen from user-space.
This no-mount mode do not register a debugfs as filesystem, but client can
register their parts in the internal structures. This data can be readed
with a debugger or saved with a crashkernel. When it is off clients
get EPERM error when accessing the functions for registering their
components.

Signed-off-by: Peter Enderborg <peter.enderborg@sony.com>
Link: https://lore.kernel.org/r/20200716071511.26864-3-peter.enderborg@sony.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 53 insertions, 0 deletions

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index b7f2e971ecbc..2fcf66473436 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -35,6 +35,7 @@
 static struct vfsmount *debugfs_mount;
 static int debugfs_mount_count;
 static bool debugfs_registered;
+static unsigned int debugfs_allow = DEFAULT_DEBUGFS_ALLOW_BITS;
 
 /*
  * Don't allow access attributes to be changed whilst the kernel is locked down
@@ -266,6 +267,9 @@ static struct dentry *debug_mount(struct file_system_type *fs_type,
 			int flags, const char *dev_name,
 			void *data)
 {
+	if (!(debugfs_allow & DEBUGFS_ALLOW_API))
+		return ERR_PTR(-EPERM);
+
 	return mount_single(fs_type, flags, data, debug_fill_super);
 }
 
@@ -311,6 +315,9 @@ static struct dentry *start_creating(const char *name, struct dentry *parent)
 	struct dentry *dentry;
 	int error;
 
+	if (!(debugfs_allow & DEBUGFS_ALLOW_API))
+		return ERR_PTR(-EPERM);
+
 	pr_debug("creating file '%s'\n", name);
 
 	if (IS_ERR(parent))
@@ -385,6 +392,11 @@ static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
 	if (IS_ERR(dentry))
 		return dentry;
 
+	if (!(debugfs_allow & DEBUGFS_ALLOW_API)) {
+		failed_creating(dentry);
+		return ERR_PTR(-EPERM);
+	}
+
 	inode = debugfs_get_inode(dentry->d_sb);
 	if (unlikely(!inode)) {
 		pr_err("out of free dentries, can not create file '%s'\n",
@@ -541,6 +553,11 @@ struct dentry *debugfs_create_dir(const char *name, struct dentry *parent)
 	if (IS_ERR(dentry))
 		return dentry;
 
+	if (!(debugfs_allow & DEBUGFS_ALLOW_API)) {
+		failed_creating(dentry);
+		return ERR_PTR(-EPERM);
+	}
+
 	inode = debugfs_get_inode(dentry->d_sb);
 	if (unlikely(!inode)) {
 		pr_err("out of free dentries, can not create directory '%s'\n",
@@ -583,6 +600,11 @@ struct dentry *debugfs_create_automount(const char *name,
 	if (IS_ERR(dentry))
 		return dentry;
 
+	if (!(debugfs_allow & DEBUGFS_ALLOW_API)) {
+		failed_creating(dentry);
+		return ERR_PTR(-EPERM);
+	}
+
 	inode = debugfs_get_inode(dentry->d_sb);
 	if (unlikely(!inode)) {
 		pr_err("out of free dentries, can not create automount '%s'\n",
@@ -786,10 +808,27 @@ bool debugfs_initialized(void)
 }
 EXPORT_SYMBOL_GPL(debugfs_initialized);
 
+static int __init debugfs_kernel(char *str)
+{
+	if (str) {
+		if (!strcmp(str, "on"))
+			debugfs_allow = DEBUGFS_ALLOW_API | DEBUGFS_ALLOW_MOUNT;
+		else if (!strcmp(str, "no-mount"))
+			debugfs_allow = DEBUGFS_ALLOW_API;
+		else if (!strcmp(str, "off"))
+			debugfs_allow = 0;
+	}
+
+	return 0;
+}
+early_param("debugfs", debugfs_kernel);
 static int __init debugfs_init(void)
 {
 	int retval;
 
+	if (!(debugfs_allow & DEBUGFS_ALLOW_MOUNT))
+		return -EPERM;
+
 	retval = sysfs_create_mount_point(kernel_kobj, "debug");
 	if (retval)
 		return retval;
diff --git a/fs/debugfs/internal.h b/fs/debugfs/internal.h
index 034e6973cead..92af8ae31313 100644
--- a/fs/debugfs/internal.h
+++ b/fs/debugfs/internal.h
@@ -29,4 +29,18 @@ struct debugfs_fsdata {
  */
 #define DEBUGFS_FSDATA_IS_REAL_FOPS_BIT BIT(0)
 
+/* Access BITS */
+#define DEBUGFS_ALLOW_API	BIT(0)
+#define DEBUGFS_ALLOW_MOUNT	BIT(1)
+
+#ifdef CONFIG_DEBUG_FS_ALLOW_ALL
+#define DEFAULT_DEBUGFS_ALLOW_BITS (DEBUGFS_ALLOW_MOUNT | DEBUGFS_ALLOW_API)
+#endif
+#ifdef CONFIG_DEBUG_FS_DISALLOW_MOUNT
+#define DEFAULT_DEBUGFS_ALLOW_BITS (DEBUGFS_ALLOW_API)
+#endif
+#ifdef CONFIG_DEBUG_FS_ALLOW_NONE
+#define DEFAULT_DEBUGFS_ALLOW_BITS (0)
+#endif
+
 #endif /* _DEBUGFS_INTERNAL_H_ */