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From: Matthew Garrett <matthewgarrett@google.com>
To: jmorris@namei.org
Cc: linux-security-module@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	David Howells <dhowells@redhat.com>,
	Andy Shevchenko <andy.shevchenko@gmail.com>,
	acpi4asus-user@lists.sourceforge.net,
	platform-driver-x86@vger.kernel.org,
	Matthew Garrett <mjg59@srcf.ucam.org>,
	Thomas Gleixner <tglx@linutronix.de>, Greg KH <greg@kroah.com>,
	"Rafael J . Wysocki" <rafael@kernel.org>,
	Matthew Garrett <matthewgarrett@google.com>
Subject: [PATCH V38 26/29] debugfs: Restrict debugfs when the kernel is locked down
Date: Wed,  7 Aug 2019 17:07:18 -0700
Message-ID: <20190808000721.124691-27-matthewgarrett@google.com> (raw)
In-Reply-To: <20190808000721.124691-1-matthewgarrett@google.com>

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 KH <greg@kroah.com>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Matthew Garrett <matthewgarrett@google.com>
---
 fs/debugfs/file.c            | 30 ++++++++++++++++++++++++++++++
 fs/debugfs/inode.c           | 32 ++++++++++++++++++++++++++++++--
 include/linux/security.h     |  1 +
 security/lockdown/lockdown.c |  1 +
 4 files changed, 62 insertions(+), 2 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 93e4ca6b2ad7..87846aad594b 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -19,6 +19,7 @@
 #include <linux/atomic.h>
 #include <linux/device.h>
 #include <linux/poll.h>
+#include <linux/security.h>
 
 #include "internal.h"
 
@@ -136,6 +137,25 @@ 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 security_locked_down(LOCKDOWN_DEBUGFS);
+}
+
 static int open_proxy_open(struct inode *inode, struct file *filp)
 {
 	struct dentry *dentry = F_DENTRY(filp);
@@ -147,6 +167,11 @@ static int open_proxy_open(struct inode *inode, struct file *filp)
 		return r == -EIO ? -ENOENT : r;
 
 	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 clean up after itself at exit? */
@@ -272,6 +297,11 @@ static int full_proxy_open(struct inode *inode, struct file *filp)
 		return r == -EIO ? -ENOENT : r;
 
 	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 ec5c197985ec..51ced4ae9280 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -27,6 +27,7 @@
 #include <linux/seq_file.h>
 #include <linux/magic.h>
 #include <linux/slab.h>
+#include <linux/security.h>
 
 #include "internal.h"
 
@@ -36,6 +37,32 @@ 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 = security_locked_down(LOCKDOWN_DEBUGFS);
+
+	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);
@@ -353,6 +380,7 @@ 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);
@@ -516,7 +544,7 @@ struct dentry *debugfs_create_dir(const char *name, struct dentry *parent)
 	}
 
 	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) */
@@ -616,7 +644,7 @@ 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/include/linux/security.h b/include/linux/security.h
index 8ef366de70b0..d92323b44a3f 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -115,6 +115,7 @@ enum lockdown_reason {
 	LOCKDOWN_TIOCSSERIAL,
 	LOCKDOWN_MODULE_PARAMETERS,
 	LOCKDOWN_MMIOTRACE,
+	LOCKDOWN_DEBUGFS,
 	LOCKDOWN_INTEGRITY_MAX,
 	LOCKDOWN_KCORE,
 	LOCKDOWN_KPROBES,
diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index fb437a7ef5f2..88064ce1c844 100644
--- a/security/lockdown/lockdown.c
+++ b/security/lockdown/lockdown.c
@@ -30,6 +30,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_TIOCSSERIAL] = "reconfiguration of serial port IO",
 	[LOCKDOWN_MODULE_PARAMETERS] = "unsafe module parameters",
 	[LOCKDOWN_MMIOTRACE] = "unsafe mmio",
+	[LOCKDOWN_DEBUGFS] = "debugfs access",
 	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
 	[LOCKDOWN_KCORE] = "/proc/kcore access",
 	[LOCKDOWN_KPROBES] = "use of kprobes",
-- 
2.22.0.770.g0f2c4a37fd-goog


  parent reply index

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-08  0:06 [PATCH V38 00/29] security: Add support for locking down the kernel Matthew Garrett
2019-08-08  0:06 ` [PATCH V38 01/29] security: Support early LSMs Matthew Garrett
2019-08-08  0:06 ` [PATCH V38 02/29] security: Add a "locked down" LSM hook Matthew Garrett
2019-08-08  0:06 ` [PATCH V38 03/29] security: Add a static lockdown policy LSM Matthew Garrett
2019-08-08  0:06 ` [PATCH V38 04/29] Enforce module signatures if the kernel is locked down Matthew Garrett
2019-08-08  0:06 ` [PATCH V38 05/29] Restrict /dev/{mem,kmem,port} when " Matthew Garrett
2019-08-08  0:06 ` [PATCH V38 06/29] kexec_load: Disable at runtime if " Matthew Garrett
2019-08-08  0:06 ` [PATCH V38 07/29] Copy secure_boot flag in boot params across kexec reboot Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 08/29] kexec_file: split KEXEC_VERIFY_SIG into KEXEC_SIG and KEXEC_SIG_FORCE Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 09/29] kexec_file: Restrict at runtime if the kernel is locked down Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 10/29] hibernate: Disable when " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 11/29] PCI: Lock down BAR access " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 12/29] x86: Lock down IO port " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 13/29] x86/msr: Restrict MSR " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 14/29] ACPI: Limit access to custom_method " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 15/29] acpi: Ignore acpi_rsdp kernel param when the kernel has been " Matthew Garrett
2019-08-14  2:51   ` Dave Young
2019-08-14  7:26   ` Borislav Petkov
2019-08-14 17:14     ` Matthew Garrett
2019-08-14 17:47       ` Borislav Petkov
2019-08-14 18:02         ` Matthew Garrett
2019-08-14  7:28   ` Borislav Petkov
2019-08-08  0:07 ` [PATCH V38 16/29] acpi: Disable ACPI table override if the kernel is " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 17/29] Prohibit PCMCIA CIS storage when " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 18/29] Lock down TIOCSSERIAL Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 19/29] Lock down module params that specify hardware parameters (eg. ioport) Matthew Garrett
2019-08-08 11:12   ` Jessica Yu
2019-08-08 16:33     ` James Morris
2019-08-09 20:58     ` [PATCH V39] " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 20/29] x86/mmiotrace: Lock down the testmmiotrace module Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 21/29] Lock down /proc/kcore Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 22/29] Lock down tracing and perf kprobes when in confidentiality mode Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 23/29] bpf: Restrict bpf when kernel lockdown is " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 24/29] Lock down perf when " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 25/29] kexec: Allow kexec_file() with appropriate IMA policy when locked down Matthew Garrett
2019-08-08  0:07 ` Matthew Garrett [this message]
2019-08-08  0:07 ` [PATCH V38 27/29] tracefs: Restrict tracefs when the kernel is " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 28/29] efi: Restrict efivar_ssdt_load " Matthew Garrett
2019-08-08  0:07 ` [PATCH V38 29/29] lockdown: Print current->comm in restriction messages Matthew Garrett
2019-08-10  6:08 ` [PATCH V38 00/29] security: Add support for locking down the kernel James Morris
2019-08-12 17:06   ` Matthew Garrett
2019-08-12 17:39     ` James Morris
2019-08-12 22:29       ` James Morris

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