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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, dhowells@redhat.com
Subject: [PATCH 26/27] debugfs: Restrict debugfs when the kernel is locked down
Date: Wed,  6 Mar 2019 15:59:12 -0800
Message-ID: <20190306235913.6631-27-matthewgarrett@google.com> (raw)
In-Reply-To: <20190306235913.6631-1-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>
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(-)

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);
-- 
2.21.0.352.gf09ad66450-goog


  parent reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-06 23:58 [PULL REQUEST] Kernel lockdown patches for 5.2 Matthew Garrett
2019-03-06 23:58 ` [PATCH 01/27] Add the ability to lock down access to the running kernel image Matthew Garrett
2019-03-06 23:58 ` [PATCH 02/27] Add a SysRq option to lift kernel lockdown Matthew Garrett
2019-03-07  0:09   ` Randy Dunlap
2019-03-07  0:12     ` Matthew Garrett
2019-03-06 23:58 ` [PATCH 03/27] Enforce module signatures if the kernel is locked down Matthew Garrett
2019-03-08 23:00   ` James Morris
2019-03-08 23:30     ` Matthew Garrett
2019-03-09  4:45       ` James Morris
2019-03-06 23:58 ` [PATCH 04/27] Restrict /dev/{mem,kmem,port} when " Matthew Garrett
2019-03-06 23:58 ` [PATCH 05/27] kexec_load: Disable at runtime if " Matthew Garrett
2019-03-06 23:58 ` [PATCH 06/27] Copy secure_boot flag in boot params across kexec reboot Matthew Garrett
2019-03-06 23:58 ` [PATCH 07/27] kexec_file: split KEXEC_VERIFY_SIG into KEXEC_SIG and KEXEC_SIG_FORCE Matthew Garrett
2019-03-06 23:58 ` [PATCH 08/27] kexec_file: Restrict at runtime if the kernel is locked down Matthew Garrett
2019-03-06 23:58 ` [PATCH 09/27] hibernate: Disable when " Matthew Garrett
2019-03-07 14:55   ` Alan Cox
2019-03-07 17:32     ` Matthew Garrett
2019-03-18 18:55       ` Alan Cox
2019-03-06 23:58 ` [PATCH 10/27] uswsusp: " Matthew Garrett
2019-03-06 23:58 ` [PATCH 11/27] PCI: Lock down BAR access " Matthew Garrett
2019-03-06 23:58 ` [PATCH 12/27] x86: Lock down IO port " Matthew Garrett
2019-03-06 23:58 ` [PATCH 13/27] x86/msr: Restrict MSR " Matthew Garrett
2019-03-06 23:59 ` [PATCH 14/27] ACPI: Limit access to custom_method " Matthew Garrett
2019-03-06 23:59 ` [PATCH 15/27] acpi: Ignore acpi_rsdp kernel param when the kernel has been " Matthew Garrett
2019-03-06 23:59 ` [PATCH 16/27] acpi: Disable ACPI table override if the kernel is " Matthew Garrett
2019-03-06 23:59 ` [PATCH 17/27] acpi: Disable APEI error injection " Matthew Garrett
2019-03-06 23:59 ` [PATCH 18/27] Prohibit PCMCIA CIS storage when " Matthew Garrett
2019-03-06 23:59 ` [PATCH 19/27] Lock down TIOCSSERIAL Matthew Garrett
2019-03-06 23:59 ` [PATCH 20/27] Lock down module params that specify hardware parameters (eg. ioport) Matthew Garrett
2019-03-06 23:59 ` [PATCH 21/27] x86/mmiotrace: Lock down the testmmiotrace module Matthew Garrett
2019-03-06 23:59 ` [PATCH 22/27] Lock down /proc/kcore Matthew Garrett
2019-03-06 23:59 ` [PATCH 23/27] Lock down kprobes Matthew Garrett
2019-03-06 23:59 ` [PATCH 24/27] bpf: Restrict kernel image access functions when the kernel is locked down Matthew Garrett
2019-03-06 23:59 ` [PATCH 25/27] Lock down perf Matthew Garrett
2019-03-06 23:59 ` Matthew Garrett [this message]
2019-04-25 10:49   ` [PATCH 26/27] debugfs: Restrict debugfs when the kernel is locked down Vasily Gorbik
2019-04-25 21:44     ` Matthew Garrett
2019-03-06 23:59 ` [PATCH 27/27] lockdown: Print current->comm in restriction messages Matthew Garrett
2019-03-07  3:56 ` [PULL REQUEST] Kernel lockdown patches for 5.2 Mimi Zohar
2019-03-07  4:24   ` Matthew Garrett
2019-03-12  0:42     ` Matthew Garrett
2019-03-12  1:52       ` Mimi Zohar
2019-03-07 15:59 ` [PATCH 02/27] Add a SysRq option to lift kernel lockdown David Howells
  -- strict thread matches above, loose matches on Subject: below --
2019-02-28 21:28 [PULL REQUEST] Lock down patches Matthew Garrett
2019-02-28 23:11 ` [PATCH 01/27] Add the ability to lock down access to the running kernel image Matthew Garrett
2019-02-28 23:12   ` [PATCH 26/27] debugfs: Restrict debugfs when the kernel is locked down Matthew Garrett

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