EVM: Allow runtime modification of the set of verified xattrs

Sites may wish to provide additional metadata alongside files in order
to make more fine-grained security decisions[1]. The security of this is
enhanced if this metadata is protected, something that EVM makes
possible. However, the kernel cannot know about the set of extended
attributes that local admins may wish to protect, and hardcoding this
policy in the kernel makes it difficult to change over time and less
convenient for distributions to enable.

This patch adds a new /sys/kernel/security/integrity/evm/evm_xattrs node,
which can be read to obtain the current set of EVM-protected extended
attributes or written to in order to add new entries. Extending this list
will not change the validity of any existing signatures provided that the
file in question does not have any of the additional extended attributes -
missing xattrs are skipped when calculating the EVM hash.

[1] For instance, a package manager could install information about the
package uploader in an additional extended attribute. Local LSM policy
could then be associated with that extended attribute in order to
restrict the privileges available to packages from less trusted
uploaders.

Signed-off-by: Matthew Garrett <[e-mail]>
Reviewed-by: James Morris <[e-mail]>
Signed-off-by: Mimi Zohar <[e-mail]>

Documentation/ABI/testing/evm

···
 		dracut (via 97masterkey and 98integrity) and systemd (via
 		core/ima-setup) have support for loading keys at boot
 		time.
+
+What:		security/integrity/evm/evm_xattrs
+Date:		April 2018
+Contact:	Matthew Garrett <[e-mail]>
+Description:
+		Shows the set of extended attributes used to calculate or
+		validate the EVM signature, and allows additional attributes
+		to be added at runtime. Any signatures generated after
+		additional attributes are added (and on files posessing those
+		additional attributes) will only be valid if the same
+		additional attributes are configured on system boot. Writing
+		a single period (.) will lock the xattr list from any further
+		modification.

include/uapi/linux/audit.h

···
 #define AUDIT_INTEGRITY_HASH	    1803 /* Integrity HASH type */
 #define AUDIT_INTEGRITY_PCR	    1804 /* PCR invalidation msgs */
 #define AUDIT_INTEGRITY_RULE	    1805 /* policy rule */
+#define AUDIT_INTEGRITY_EVM_XATTR   1806 /* New EVM-covered xattr */
 
 #define AUDIT_KERNEL		2000	/* Asynchronous audit record. NOT A REQUEST. */
 

security/integrity/evm/Kconfig

···
 	  additional info to the calculation, requires existing EVM
 	  labeled file systems to be relabeled.
 
+config EVM_ADD_XATTRS
+	bool "Add additional EVM extended attributes at runtime"
+	depends on EVM
+	default n
+	help
+	  Allow userland to provide additional xattrs for HMAC calculation.
+
+	  When this option is enabled, root can add additional xattrs to the
+	  list used by EVM by writing them into
+	  /sys/kernel/security/integrity/evm/evm_xattrs.
+
 config EVM_LOAD_X509
 	bool "Load an X509 certificate onto the '.evm' trusted keyring"
 	depends on EVM && INTEGRITY_TRUSTED_KEYRING

security/integrity/evm/evm_crypto.c

···
 		return PTR_ERR(desc);
 
 	error = -ENODATA;
-	list_for_each_entry(xattr, &evm_config_xattrnames, list) {
+	list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) {
 		bool is_ima = false;
 
 		if (strcmp(xattr->name, XATTR_NAME_IMA) == 0)

security/integrity/evm/evm_main.c

···
 };
 int evm_hmac_attrs;
 
-static struct xattr_list evm_config_default_xattrnames[] __ro_after_init = {
+static struct xattr_list evm_config_default_xattrnames[] = {
 #ifdef CONFIG_SECURITY_SELINUX
 	{.name = XATTR_NAME_SELINUX},
 #endif
···
 	if (!(inode->i_opflags & IOP_XATTR))
 		return -EOPNOTSUPP;
 
-	list_for_each_entry(xattr, &evm_config_xattrnames, list) {
+	list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) {
 		error = __vfs_getxattr(dentry, inode, xattr->name, NULL, 0);
 		if (error < 0) {
 			if (error == -ENODATA)
···
 	struct xattr_list *xattr;
 
 	namelen = strlen(req_xattr_name);
-	list_for_each_entry(xattr, &evm_config_xattrnames, list) {
+	list_for_each_entry_rcu(xattr, &evm_config_xattrnames, list) {
 		if ((strlen(xattr->name) == namelen)
 		    && (strncmp(req_xattr_name, xattr->name, namelen) == 0)) {
 			found = 1;

security/integrity/evm/evm_secfs.c

···
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
+#include <linux/audit.h>
 #include <linux/uaccess.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include "evm.h"
 
 static struct dentry *evm_dir;
 static struct dentry *evm_init_tpm;
 static struct dentry *evm_symlink;
+
+#ifdef CONFIG_EVM_ADD_XATTRS
+static struct dentry *evm_xattrs;
+static DEFINE_MUTEX(xattr_list_mutex);
+static int evm_xattrs_locked;
+#endif
 
 /**
  * evm_read_key - read() for <securityfs>/evm
···
 	.write		= evm_write_key,
 };
 
+#ifdef CONFIG_EVM_ADD_XATTRS
+/**
+ * evm_read_xattrs - read() for <securityfs>/evm_xattrs
+ *
+ * @filp: file pointer, not actually used
+ * @buf: where to put the result
+ * @count: maximum to send along
+ * @ppos: where to start
+ *
+ * Returns number of bytes read or error code, as appropriate
+ */
+static ssize_t evm_read_xattrs(struct file *filp, char __user *buf,
+			       size_t count, loff_t *ppos)
+{
+	char *temp;
+	int offset = 0;
+	ssize_t rc, size = 0;
+	struct xattr_list *xattr;
+
+	if (*ppos != 0)
+		return 0;
+
+	rc = mutex_lock_interruptible(&xattr_list_mutex);
+	if (rc)
+		return -ERESTARTSYS;
+
+	list_for_each_entry(xattr, &evm_config_xattrnames, list)
+		size += strlen(xattr->name) + 1;
+
+	temp = kmalloc(size + 1, GFP_KERNEL);
+	if (!temp)
+		return -ENOMEM;
+
+	list_for_each_entry(xattr, &evm_config_xattrnames, list) {
+		sprintf(temp + offset, "%s\n", xattr->name);
+		offset += strlen(xattr->name) + 1;
+	}
+
+	mutex_unlock(&xattr_list_mutex);
+	rc = simple_read_from_buffer(buf, count, ppos, temp, strlen(temp));
+
+	return rc;
+}
+
+/**
+ * evm_write_xattrs - write() for <securityfs>/evm_xattrs
+ * @file: file pointer, not actually used
+ * @buf: where to get the data from
+ * @count: bytes sent
+ * @ppos: where to start
+ *
+ * Returns number of bytes written or error code, as appropriate
+ */
+static ssize_t evm_write_xattrs(struct file *file, const char __user *buf,
+				size_t count, loff_t *ppos)
+{
+	int len, err;
+	struct xattr_list *xattr, *tmp;
+	struct audit_buffer *ab;
+	struct iattr newattrs;
+	struct inode *inode;
+
+	if (!capable(CAP_SYS_ADMIN) || evm_xattrs_locked)
+		return -EPERM;
+
+	if (*ppos != 0)
+		return -EINVAL;
+
+	if (count > XATTR_NAME_MAX)
+		return -E2BIG;
+
+	ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_INTEGRITY_EVM_XATTR);
+	if (IS_ERR(ab))
+		return PTR_ERR(ab);
+
+	xattr = kmalloc(sizeof(struct xattr_list), GFP_KERNEL);
+	if (!xattr) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	xattr->name = memdup_user_nul(buf, count);
+	if (IS_ERR(xattr->name)) {
+		err = PTR_ERR(xattr->name);
+		xattr->name = NULL;
+		goto out;
+	}
+
+	/* Remove any trailing newline */
+	len = strlen(xattr->name);
+	if (xattr->name[len-1] == '\n')
+		xattr->name[len-1] = '\0';
+
+	if (strcmp(xattr->name, ".") == 0) {
+		evm_xattrs_locked = 1;
+		newattrs.ia_mode = S_IFREG | 0440;
+		newattrs.ia_valid = ATTR_MODE;
+		inode = evm_xattrs->d_inode;
+		inode_lock(inode);
+		err = simple_setattr(evm_xattrs, &newattrs);
+		inode_unlock(inode);
+		audit_log_format(ab, "locked");
+		if (!err)
+			err = count;
+		goto out;
+	}
+
+	audit_log_format(ab, "xattr=");
+	audit_log_untrustedstring(ab, xattr->name);
+
+	if (strncmp(xattr->name, XATTR_SECURITY_PREFIX,
+		    XATTR_SECURITY_PREFIX_LEN) != 0) {
+		err = -EINVAL;
+		goto out;
+	}
+
+	/* Guard against races in evm_read_xattrs */
+	mutex_lock(&xattr_list_mutex);
+	list_for_each_entry(tmp, &evm_config_xattrnames, list) {
+		if (strcmp(xattr->name, tmp->name) == 0) {
+			err = -EEXIST;
+			mutex_unlock(&xattr_list_mutex);
+			goto out;
+		}
+	}
+	list_add_tail_rcu(&xattr->list, &evm_config_xattrnames);
+	mutex_unlock(&xattr_list_mutex);
+
+	audit_log_format(ab, " res=0");
+	audit_log_end(ab);
+	return count;
+out:
+	audit_log_format(ab, " res=%d", err);
+	audit_log_end(ab);
+	kfree(xattr->name);
+	kfree(xattr);
+	return err;
+}
+
+static const struct file_operations evm_xattr_ops = {
+	.read		= evm_read_xattrs,
+	.write		= evm_write_xattrs,
+};
+
+static int evm_init_xattrs(void)
+{
+	evm_xattrs = securityfs_create_file("evm_xattrs", 0660, evm_dir, NULL,
+					    &evm_xattr_ops);
+	if (!evm_xattrs || IS_ERR(evm_xattrs))
+		return -EFAULT;
+
+	return 0;
+}
+#else
+static int evm_init_xattrs(void)
+{
+	return 0;
+}
+#endif
+
 int __init evm_init_secfs(void)
 {
 	int error = 0;
···
 	evm_symlink = securityfs_create_symlink("evm", NULL,
 						"integrity/evm/evm", NULL);
 	if (!evm_symlink || IS_ERR(evm_symlink)) {
+		error = -EFAULT;
+		goto out;
+	}
+
+	if (evm_init_xattrs() != 0) {
 		error = -EFAULT;
 		goto out;
 	}