[PATCH RFC UEK5 5/7] debugfs: Restrict debugfs when the kernel is locked down

* [PATCH RFC UEK5 5/7] debugfs: Restrict debugfs when the kernel is locked down
       [not found] <20201020210004.18977-1-konrad.wilk@oracle.com>
@ 2020-10-20 21:00 ` Konrad Rzeszutek Wilk
  2020-10-20 21:08   ` Konrad Rzeszutek Wilk
  0 siblings, 1 reply; 3+ messages in thread
From: Konrad Rzeszutek Wilk @ 2020-10-20 21:00 UTC (permalink / raw)
  To: eric.snowberg, john.haxby, todd.vierling
  Cc: Konrad Rzeszutek Wilk, David Howells, Andy Shevchenko,
	acpi4asus-user, platform-driver-x86, Matthew Garrett,
	Thomas Gleixner, Greg KH, Rafael J . Wysocki, Matthew Garrett,
	James Morris

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 KH <greg@kroah.com>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Matthew Garrett <matthewgarrett@google.com>
Signed-off-by: James Morris <jmorris@namei.org>

[Backport:
 Since UEK5 by default is confidentiality we have to outright
 disallow debugfs if the default mode is selected. Hence the
 call to __kernel_is_confidentiality_mode to help us.

 If we are in integrity lockdown mode, we can enable debugfs
 IF they match with the above 1-3 criteria]

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 fs/debugfs/file.c    | 36 +++++++++++++++++++++++++++++++++---
 fs/debugfs/inode.c   | 34 ++++++++++++++++++++++++++++++++--
 security/lock_down.c |  1 +
 3 files changed, 66 insertions(+), 5 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 32b5168a7e910..86c7235dfd57b 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -97,15 +97,35 @@ EXPORT_SYMBOL_GPL(debugfs_use_file_finish);
 
 #define F_DENTRY(filp) ((filp)->f_path.dentry)
 
+
+/*
+ * 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 (__kernel_is_confidentiality_mode())
+		return true;
+
+	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)
 {
 	const struct dentry *dentry = F_DENTRY(filp);
 	const struct file_operations *real_fops = NULL;
 	int srcu_idx, r;
 
-	if (kernel_is_locked_down("debugfs"))
-		return -EPERM;
-
 	r = debugfs_use_file_start(dentry, &srcu_idx);
 	if (r) {
 		r = -ENOENT;
@@ -113,6 +133,11 @@ static int open_proxy_open(struct inode *inode, struct file *filp)
 	}
 
 	real_fops = debugfs_real_fops(filp);
+
+	r = debugfs_locked_down(inode, filp, real_fops);
+	if (r)
+		goto out;
+
 	real_fops = fops_get(real_fops);
 	if (!real_fops) {
 		/* Huh? Module did not clean up after itself at exit? */
@@ -245,6 +270,11 @@ static int full_proxy_open(struct inode *inode, struct file *filp)
 	}
 
 	real_fops = debugfs_real_fops(filp);
+
+	r = debugfs_is_locked_down(inode, filp, real_fops);
+	if (r)
+		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 f4df6feec2713..5a42b2387dd07 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -39,6 +39,35 @@ 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)
+{
+	int ret;
+
+	if (kernel_is_locked_down("debugfs"))
+		ret = -EPERM;
+
+	if (ret && (ia->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)))
+		return ret;
+	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);
@@ -362,6 +391,7 @@ 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 *)real_fops;
 
@@ -518,7 +548,7 @@ 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) */
@@ -613,7 +643,7 @@ 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);
diff --git a/security/lock_down.c b/security/lock_down.c
index 1301b25137127..c709c70701235 100644
--- a/security/lock_down.c
+++ b/security/lock_down.c
@@ -61,6 +61,7 @@ bool __kernel_is_locked_down(const char *what, bool first)
 		/* If we are in integrity mode we allow certain callsites */
 		if (!lockdown_confidentiality) {
 			if ((strcmp(what, "BPF") == 0) ||
+			    (strcmp(what, "debugfs") == 0) ||
 			    (strcmp(what, "DTRACE") == 0)) {
 				return 0;
 			}
-- 
2.13.6


^ permalink raw reply related	[flat|nested] 3+ messages in thread