diff mbox series

[25/27] debugfs: Restrict debugfs when the kernel is locked down

Message ID 20190325220954.29054-26-matthewgarrett@google.com (mailing list archive)
State New, archived
Headers show
Series [PULL,REQUEST] Lockdown patches for 5.2 | expand

Commit Message

Matthew Garrett March 25, 2019, 10:09 p.m. UTC 
From: David Howells <dhowells@redhat.com>

Disallow opening of debugfs files that might be used to muck around when
the kernel is locked down as various drivers give raw access to hardware
through debugfs.  Given the effort of auditing all 2000 or so files and
manually fixing each one as necessary, I've chosen to apply a heuristic
instead.  The following changes are made:

 (1) chmod and chown are disallowed on debugfs objects (though the root dir
     can be modified by mount and remount, but I'm not worried about that).

 (2) When the kernel is locked down, only files with the following criteria
     are permitted to be opened:

	- The file must have mode 00444
	- The file must not have ioctl methods
	- The file must not have mmap

 (3) When the kernel is locked down, files may only be opened for reading.

Normal device interaction should be done through configfs, sysfs or a
miscdev, not debugfs.

Note that this makes it unnecessary to specifically lock down show_dsts(),
show_devs() and show_call() in the asus-wmi driver.

I would actually prefer to lock down all files by default and have the
the files unlocked by the creator.  This is tricky to manage correctly,
though, as there are 19 creation functions and ~1600 call sites (some of
them in loops scanning tables).

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Andy Shevchenko <andy.shevchenko@gmail.com>
cc: acpi4asus-user@lists.sourceforge.net
cc: platform-driver-x86@vger.kernel.org
cc: Matthew Garrett <mjg59@srcf.ucam.org>
cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Matthew Garrett <matthewgarrett@google.com>
---
 fs/debugfs/file.c  | 28 ++++++++++++++++++++++++++++
 fs/debugfs/inode.c | 30 ++++++++++++++++++++++++++++--
 2 files changed, 56 insertions(+), 2 deletions(-)

Comments

Greg Kroah-Hartman March 26, 2019, 12:31 a.m. UTC | #1 
On Mon, Mar 25, 2019 at 03:09:52PM -0700, Matthew Garrett wrote:
> From: David Howells <dhowells@redhat.com>
> 
> Disallow opening of debugfs files that might be used to muck around when
> the kernel is locked down as various drivers give raw access to hardware
> through debugfs.  Given the effort of auditing all 2000 or so files and
> manually fixing each one as necessary, I've chosen to apply a heuristic
> instead.  The following changes are made:
> 
>  (1) chmod and chown are disallowed on debugfs objects (though the root dir
>      can be modified by mount and remount, but I'm not worried about that).
> 
>  (2) When the kernel is locked down, only files with the following criteria
>      are permitted to be opened:
> 
> 	- The file must have mode 00444
> 	- The file must not have ioctl methods
> 	- The file must not have mmap
> 
>  (3) When the kernel is locked down, files may only be opened for reading.
> 
> Normal device interaction should be done through configfs, sysfs or a
> miscdev, not debugfs.

Then why not just not allow debugfs at all if it is such a "big
problem"?

Actually, it is a big problem, and I am pushing for systems to not mount
debugfs at all these days, but that's a different story...

Just "disabling" it entirely for this crazy dream of a "locked down"
kernel is fine for me.

thanks,

greg k-h
Matthew Garrett March 26, 2019, 12:38 a.m. UTC | #2 
On Mon, Mar 25, 2019 at 5:35 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> On Mon, Mar 25, 2019 at 03:09:52PM -0700, Matthew Garrett wrote:
> > Normal device interaction should be done through configfs, sysfs or a
> > miscdev, not debugfs.
>
> Then why not just not allow debugfs at all if it is such a "big
> problem"?

Previous attempts to do so have resulted in strong pushback from
various maintainers. If you're happy just having any complaints
reassigned to you then I'm more than happy to turn it off entirely.
Greg Kroah-Hartman March 26, 2019, 12:43 a.m. UTC | #3 
On Mon, Mar 25, 2019 at 05:38:09PM -0700, Matthew Garrett wrote:
> On Mon, Mar 25, 2019 at 5:35 PM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> > On Mon, Mar 25, 2019 at 03:09:52PM -0700, Matthew Garrett wrote:
> > > Normal device interaction should be done through configfs, sysfs or a
> > > miscdev, not debugfs.
> >
> > Then why not just not allow debugfs at all if it is such a "big
> > problem"?
> 
> Previous attempts to do so have resulted in strong pushback from
> various maintainers. If you're happy just having any complaints
> reassigned to you then I'm more than happy to turn it off entirely.

Sure, send them my way :)
diff mbox series

Patch

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 4fce1da7db23..c33042c1eff3 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -136,6 +136,25 @@  void debugfs_file_put(struct dentry *dentry)
 }
 EXPORT_SYMBOL_GPL(debugfs_file_put);
 
+/*
+ * Only permit access to world-readable files when the kernel is locked down.
+ * We also need to exclude any file that has ways to write or alter it as root
+ * can bypass the permissions check.
+ */
+static bool debugfs_is_locked_down(struct inode *inode,
+				   struct file *filp,
+				   const struct file_operations *real_fops)
+{
+	if ((inode->i_mode & 07777) == 0444 &&
+	    !(filp->f_mode & FMODE_WRITE) &&
+	    !real_fops->unlocked_ioctl &&
+	    !real_fops->compat_ioctl &&
+	    !real_fops->mmap)
+		return false;
+
+	return kernel_is_locked_down("debugfs");
+}
+
 static int open_proxy_open(struct inode *inode, struct file *filp)
 {
 	struct dentry *dentry = F_DENTRY(filp);
@@ -147,6 +166,11 @@  static int open_proxy_open(struct inode *inode, struct file *filp)
 		return r == -EIO ? -ENOENT : r;
 
 	real_fops = debugfs_real_fops(filp);
+
+	r = -EPERM;
+	if (debugfs_is_locked_down(inode, filp, real_fops))
+		goto out;
+
 	real_fops = fops_get(real_fops);
 	if (!real_fops) {
 		/* Huh? Module did not clean up after itself at exit? */
@@ -272,6 +296,10 @@  static int full_proxy_open(struct inode *inode, struct file *filp)
 		return r == -EIO ? -ENOENT : r;
 
 	real_fops = debugfs_real_fops(filp);
+	r = -EPERM;
+	if (debugfs_is_locked_down(inode, filp, real_fops))
+		goto out;
+
 	real_fops = fops_get(real_fops);
 	if (!real_fops) {
 		/* Huh? Module did not cleanup after itself at exit? */
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 13b01351dd1c..4daec17b8215 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -32,6 +32,31 @@  static struct vfsmount *debugfs_mount;
 static int debugfs_mount_count;
 static bool debugfs_registered;
 
+/*
+ * Don't allow access attributes to be changed whilst the kernel is locked down
+ * so that we can use the file mode as part of a heuristic to determine whether
+ * to lock down individual files.
+ */
+static int debugfs_setattr(struct dentry *dentry, struct iattr *ia)
+{
+	if ((ia->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)) &&
+	    kernel_is_locked_down("debugfs"))
+		return -EPERM;
+	return simple_setattr(dentry, ia);
+}
+
+static const struct inode_operations debugfs_file_inode_operations = {
+	.setattr	= debugfs_setattr,
+};
+static const struct inode_operations debugfs_dir_inode_operations = {
+	.lookup		= simple_lookup,
+	.setattr	= debugfs_setattr,
+};
+static const struct inode_operations debugfs_symlink_inode_operations = {
+	.get_link	= simple_get_link,
+	.setattr	= debugfs_setattr,
+};
+
 static struct inode *debugfs_get_inode(struct super_block *sb)
 {
 	struct inode *inode = new_inode(sb);
@@ -356,6 +381,7 @@  static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
 	inode->i_mode = mode;
 	inode->i_private = data;
 
+	inode->i_op = &debugfs_file_inode_operations;
 	inode->i_fop = proxy_fops;
 	dentry->d_fsdata = (void *)((unsigned long)real_fops |
 				DEBUGFS_FSDATA_IS_REAL_FOPS_BIT);
@@ -513,7 +539,7 @@  struct dentry *debugfs_create_dir(const char *name, struct dentry *parent)
 		return failed_creating(dentry);
 
 	inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
-	inode->i_op = &simple_dir_inode_operations;
+	inode->i_op = &debugfs_dir_inode_operations;
 	inode->i_fop = &simple_dir_operations;
 
 	/* directory inodes start off with i_nlink == 2 (for "." entry) */
@@ -608,7 +634,7 @@  struct dentry *debugfs_create_symlink(const char *name, struct dentry *parent,
 		return failed_creating(dentry);
 	}
 	inode->i_mode = S_IFLNK | S_IRWXUGO;
-	inode->i_op = &simple_symlink_inode_operations;
+	inode->i_op = &debugfs_symlink_inode_operations;
 	inode->i_link = link;
 	d_instantiate(dentry, inode);
 	return end_creating(dentry);