evm: re-release

EVM protects a file's security extended attributes(xattrs) against integrity
attacks.  This patchset provides the framework and an initial method.  The
initial method maintains an HMAC-sha1 value across the security extended
attributes, storing the HMAC value as the extended attribute 'security.evm'.
Other methods of validating the integrity of a file's metadata will be posted
separately (eg. EVM-digital-signatures).

While this patchset does authenticate the security xattrs, and
cryptographically binds them to the inode, coming extensions will bind other
directory and inode metadata for more complete protection.  To help simplify
the review and upstreaming process, each extension will be posted separately
(eg. IMA-appraisal, IMA-appraisal-directory).  For a general overview of the
proposed Linux integrity subsystem, refer to Dave Safford's whitepaper:
http://downloads.sf.net/project/linux-ima/linux-ima/Integrity_overview.pdf.

EVM depends on the Kernel Key Retention System to provide it with a
trusted/encrypted key for the HMAC-sha1 operation. The key is loaded onto the
root's keyring using keyctl.  Until EVM receives notification that the key has
been successfully loaded onto the keyring (echo 1 > <securityfs>/evm), EVM can
not create or validate the 'security.evm' xattr, but returns INTEGRITY_UNKNOWN.
Loading the key and signaling EVM should be done as early as possible. Normally
this is done in the initramfs, which has already been measured as part of the
trusted boot.  For more information on creating and loading existing
trusted/encrypted keys, refer to Documentation/keys-trusted-encrypted.txt.  A
sample dracut patch, which loads the trusted/encrypted key and enables EVM, is
available from http://linux-ima.sourceforge.net/#EVM.

Based on the LSMs enabled, the set of EVM protected security xattrs is defined
at compile.  EVM adds the following three calls to the existing security hooks:
evm_inode_setxattr(), evm_inode_post_setxattr(), and evm_inode_removexattr.  To
initialize and update the 'security.evm' extended attribute, EVM defines three
calls: evm_inode_post_init(), evm_inode_post_setattr() and
evm_inode_post_removexattr() hooks.  To verify the integrity of a security
xattr, EVM exports evm_verifyxattr().

Changelog v7:
- Fixed URL in EVM ABI documentation

Changelog v6: (based on Serge Hallyn's review)
- fix URL in patch description
- remove evm_hmac_size definition
- use SHA1_DIGEST_SIZE (removed both MAX_DIGEST_SIZE and evm_hmac_size)
- moved linux include before other includes
- test for crypto_hash_setkey failure
- fail earlier for invalid key
- clear entire encrypted key, even on failure
- check xattr name length before comparing xattr names

Changelog:
- locking based on i_mutex, remove evm_mutex
- using trusted/encrypted keys for storing the EVM key used in the HMAC-sha1
  operation.
- replaced crypto hash with shash (Dmitry Kasatkin)
- support for additional methods of verifying the security xattrs
  (Dmitry Kasatkin)
- iint not allocated for all regular files, but only for those appraised
- Use cap_sys_admin in lieu of cap_mac_admin
- Use __vfs_setxattr_noperm(), without permission checks, from EVM

Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

1 files changed, 183 insertions, 0 deletions

diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
new file mode 100644
index 000000000000..d49bb002f3da
--- /dev/null
+++ b/security/integrity/evm/evm_crypto.c
@@ -0,0 +1,183 @@
+/*
+ * Copyright (C) 2005-2010 IBM Corporation
+ *
+ * Authors:
+ * Mimi Zohar <zohar@us.ibm.com>
+ * Kylene Hall <kjhall@us.ibm.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 2 of the License.
+ *
+ * File: evm_crypto.c
+ *       Using root's kernel master key (kmk), calculate the HMAC
+ */
+
+#include <linux/module.h>
+#include <linux/crypto.h>
+#include <linux/xattr.h>
+#include <linux/scatterlist.h>
+#include <keys/encrypted-type.h>
+#include "evm.h"
+
+#define EVMKEY "evm-key"
+#define MAX_KEY_SIZE 128
+static unsigned char evmkey[MAX_KEY_SIZE];
+static int evmkey_len = MAX_KEY_SIZE;
+
+static int init_desc(struct hash_desc *desc)
+{
+        int rc;
+
+        desc->tfm = crypto_alloc_hash(evm_hmac, 0, CRYPTO_ALG_ASYNC);
+        if (IS_ERR(desc->tfm)) {
+                pr_info("Can not allocate %s (reason: %ld)\n",
+                        evm_hmac, PTR_ERR(desc->tfm));
+                rc = PTR_ERR(desc->tfm);
+                return rc;
+        }
+        desc->flags = 0;
+        rc = crypto_hash_setkey(desc->tfm, evmkey, evmkey_len);
+        if (rc)
+                goto out;
+        rc = crypto_hash_init(desc);
+out:
+        if (rc)
+                crypto_free_hash(desc->tfm);
+        return rc;
+}
+
+/* Protect against 'cutting & pasting' security.evm xattr, include inode
+ * specific info.
+ *
+ * (Additional directory/file metadata needs to be added for more complete
+ * protection.)
+ */
+static void hmac_add_misc(struct hash_desc *desc, struct inode *inode,
+                          char *digest)
+{
+        struct h_misc {
+                unsigned long ino;
+                __u32 generation;
+                uid_t uid;
+                gid_t gid;
+                umode_t mode;
+        } hmac_misc;
+        struct scatterlist sg[1];
+
+        memset(&hmac_misc, 0, sizeof hmac_misc);
+        hmac_misc.ino = inode->i_ino;
+        hmac_misc.generation = inode->i_generation;
+        hmac_misc.uid = inode->i_uid;
+        hmac_misc.gid = inode->i_gid;
+        hmac_misc.mode = inode->i_mode;
+        sg_init_one(sg, &hmac_misc, sizeof hmac_misc);
+        crypto_hash_update(desc, sg, sizeof hmac_misc);
+        crypto_hash_final(desc, digest);
+}
+
+/*
+ * Calculate the HMAC value across the set of protected security xattrs.
+ *
+ * Instead of retrieving the requested xattr, for performance, calculate
+ * the hmac using the requested xattr value. Don't alloc/free memory for
+ * each xattr, but attempt to re-use the previously allocated memory.
+ */
+int evm_calc_hmac(struct dentry *dentry, const char *req_xattr_name,
+                  const char *req_xattr_value, size_t req_xattr_value_len,
+                  char *digest)
+{
+        struct inode *inode = dentry->d_inode;
+        struct hash_desc desc;
+        struct scatterlist sg[1];
+        char **xattrname;
+        size_t xattr_size = 0;
+        char *xattr_value = NULL;
+        int error;
+        int size;
+
+        if (!inode->i_op || !inode->i_op->getxattr)
+                return -EOPNOTSUPP;
+        error = init_desc(&desc);
+        if (error)
+                return error;
+
+        error = -ENODATA;
+        for (xattrname = evm_config_xattrnames; *xattrname != NULL; xattrname++) {
+                if ((req_xattr_name && req_xattr_value)
+                    && !strcmp(*xattrname, req_xattr_name)) {
+                        error = 0;
+                        sg_init_one(sg, req_xattr_value, req_xattr_value_len);
+                        crypto_hash_update(&desc, sg, req_xattr_value_len);
+                        continue;
+                }
+                size = vfs_getxattr_alloc(dentry, *xattrname,
+                                          &xattr_value, xattr_size, GFP_NOFS);
+                if (size == -ENOMEM) {
+                        error = -ENOMEM;
+                        goto out;
+                }
+                if (size < 0)
+                        continue;
+
+                error = 0;
+                xattr_size = size;
+                sg_init_one(sg, xattr_value, xattr_size);
+                crypto_hash_update(&desc, sg, xattr_size);
+        }
+        hmac_add_misc(&desc, inode, digest);
+        kfree(xattr_value);
+out:
+        crypto_free_hash(desc.tfm);
+        return error;
+}
+
+/*
+ * Calculate the hmac and update security.evm xattr
+ *
+ * Expects to be called with i_mutex locked.
+ */
+int evm_update_evmxattr(struct dentry *dentry, const char *xattr_name,
+                        const char *xattr_value, size_t xattr_value_len)
+{
+        struct inode *inode = dentry->d_inode;
+        u8 hmac[SHA1_DIGEST_SIZE];
+        int rc = 0;
+
+        rc = evm_calc_hmac(dentry, xattr_name, xattr_value,
+                           xattr_value_len, hmac);
+        if (rc == 0)
+                rc = __vfs_setxattr_noperm(dentry, XATTR_NAME_EVM,
+                                           hmac, SHA1_DIGEST_SIZE, 0);
+        else if (rc == -ENODATA)
+                rc = inode->i_op->removexattr(dentry, XATTR_NAME_EVM);
+        return rc;
+}
+
+/*
+ * Get the key from the TPM for the SHA1-HMAC
+ */
+int evm_init_key(void)
+{
+        struct key *evm_key;
+        struct encrypted_key_payload *ekp;
+        int rc = 0;
+
+        evm_key = request_key(&key_type_encrypted, EVMKEY, NULL);
+        if (IS_ERR(evm_key))
+                return -ENOENT;
+
+        down_read(&evm_key->sem);
+        ekp = evm_key->payload.data;
+        if (ekp->decrypted_datalen > MAX_KEY_SIZE) {
+                rc = -EINVAL;
+                goto out;
+        }
+        memcpy(evmkey, ekp->decrypted_data, ekp->decrypted_datalen);
+out:
+        /* burn the original key contents */
+        memset(ekp->decrypted_data, 0, ekp->decrypted_datalen);
+        up_read(&evm_key->sem);
+        key_put(evm_key);
+        return rc;
+}