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 <mjg59@google.com>
Reviewed-by: James Morris <james.morris@microsoft.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

4 files changed, 188 insertions, 4 deletions

diff --git a/security/integrity/evm/Kconfig b/security/integrity/evm/Kconfig
index e825e0ae78e7..d593346d0bba 100644
--- a/security/integrity/evm/Kconfig
+++ b/security/integrity/evm/Kconfig
@@ -42,6 +42,17 @@ config EVM_EXTRA_SMACK_XATTRS
 	  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
diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
index caeea20670cc..494da5fcc092 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -208,7 +208,7 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
 		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)
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 09582d4fc4a8..f9eff5041e4c 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -35,7 +35,7 @@ static const char * const integrity_status_msg[] = {
 };
 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
@@ -101,7 +101,7 @@ static int evm_find_protected_xattrs(struct dentry *dentry)
 	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)
@@ -228,7 +228,7 @@ static int evm_protected_xattr(const char *req_xattr_name)
 	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;
diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
index e44380f0cb45..a7a0a1acae99 100644
--- a/security/integrity/evm/evm_secfs.c
+++ b/security/integrity/evm/evm_secfs.c
@@ -15,14 +15,22 @@
 
 #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
  *
@@ -109,6 +117,166 @@ static const struct file_operations evm_key_ops = {
 	.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;
@@ -131,6 +299,11 @@ int __init evm_init_secfs(void)
 		goto out;
 	}
 
+	if (evm_init_xattrs() != 0) {
+		error = -EFAULT;
+		goto out;
+	}
+
 	return 0;
 out:
 	securityfs_remove(evm_symlink);