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linux/security/integrity/ima/ima_appraise.c
THOBY Simon 4f2946aa0c IMA: introduce a new policy option func=SETXATTR_CHECK 
While users can restrict the accepted hash algorithms for the
security.ima xattr file signature when appraising said file, users
cannot restrict the algorithms that can be set on that attribute:
any algorithm built in the kernel is accepted on a write.

Define a new value for the ima policy option 'func' that restricts
globally the hash algorithms accepted when writing the security.ima
xattr.

When a policy contains a rule of the form
	appraise func=SETXATTR_CHECK appraise_algos=sha256,sha384,sha512
only values corresponding to one of these three digest algorithms
will be accepted for writing the security.ima xattr.  Attempting to
write the attribute using another algorithm (or "free-form" data)
will be denied with an audit log message.  In the absence of such a
policy rule, the default is still to only accept hash algorithms
built in the kernel (with all the limitations that entails).

Signed-off-by: THOBY Simon <Simon.THOBY@viveris.fr>
Reviewed-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
2021-08-16 17:35:35 -04:00

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// SPDX-License-Identifier: GPL-2.0-only
/*
* Copyright (C) 2011 IBM Corporation
*
* Author:
* Mimi Zohar <zohar@us.ibm.com>
*/
#include <linux/module.h>
#include <linux/init.h>
#include <linux/file.h>
#include <linux/fs.h>
#include <linux/xattr.h>
#include <linux/magic.h>
#include <linux/ima.h>
#include <linux/evm.h>
#include <keys/system_keyring.h>
#include "ima.h"
#ifdef CONFIG_IMA_APPRAISE_BOOTPARAM
static char *ima_appraise_cmdline_default __initdata;
core_param(ima_appraise, ima_appraise_cmdline_default, charp, 0);
void __init ima_appraise_parse_cmdline(void)
{
const char *str = ima_appraise_cmdline_default;
bool sb_state = arch_ima_get_secureboot();
int appraisal_state = ima_appraise;
if (!str)
return;
if (strncmp(str, "off", 3) == 0)
appraisal_state = 0;
else if (strncmp(str, "log", 3) == 0)
appraisal_state = IMA_APPRAISE_LOG;
else if (strncmp(str, "fix", 3) == 0)
appraisal_state = IMA_APPRAISE_FIX;
else if (strncmp(str, "enforce", 7) == 0)
appraisal_state = IMA_APPRAISE_ENFORCE;
else
pr_err("invalid \"%s\" appraise option", str);
/* If appraisal state was changed, but secure boot is enabled,
* keep its default */
if (sb_state) {
if (!(appraisal_state & IMA_APPRAISE_ENFORCE))
pr_info("Secure boot enabled: ignoring ima_appraise=%s option",
str);
} else {
ima_appraise = appraisal_state;
}
}
#endif
/*
* is_ima_appraise_enabled - return appraise status
*
* Only return enabled, if not in ima_appraise="fix" or "log" modes.
*/
bool is_ima_appraise_enabled(void)
{
return ima_appraise & IMA_APPRAISE_ENFORCE;
}
/*
* ima_must_appraise - set appraise flag
*
* Return 1 to appraise or hash
*/
int ima_must_appraise(struct user_namespace *mnt_userns, struct inode *inode,
int mask, enum ima_hooks func)
{
u32 secid;
if (!ima_appraise)
return 0;
security_task_getsecid_subj(current, &secid);
return ima_match_policy(mnt_userns, inode, current_cred(), secid,
func, mask, IMA_APPRAISE | IMA_HASH, NULL,
NULL, NULL, NULL);
}
static int ima_fix_xattr(struct dentry *dentry,
struct integrity_iint_cache *iint)
{
int rc, offset;
u8 algo = iint->ima_hash->algo;
if (algo <= HASH_ALGO_SHA1) {
offset = 1;
iint->ima_hash->xattr.sha1.type = IMA_XATTR_DIGEST;
} else {
offset = 0;
iint->ima_hash->xattr.ng.type = IMA_XATTR_DIGEST_NG;
iint->ima_hash->xattr.ng.algo = algo;
}
rc = __vfs_setxattr_noperm(&init_user_ns, dentry, XATTR_NAME_IMA,
&iint->ima_hash->xattr.data[offset],
(sizeof(iint->ima_hash->xattr) - offset) +
iint->ima_hash->length, 0);
return rc;
}
/* Return specific func appraised cached result */
enum integrity_status ima_get_cache_status(struct integrity_iint_cache *iint,
enum ima_hooks func)
{
switch (func) {
case MMAP_CHECK:
return iint->ima_mmap_status;
case BPRM_CHECK:
return iint->ima_bprm_status;
case CREDS_CHECK:
return iint->ima_creds_status;
case FILE_CHECK:
case POST_SETATTR:
return iint->ima_file_status;
case MODULE_CHECK ... MAX_CHECK - 1:
default:
return iint->ima_read_status;
}
}
static void ima_set_cache_status(struct integrity_iint_cache *iint,
enum ima_hooks func,
enum integrity_status status)
{
switch (func) {
case MMAP_CHECK:
iint->ima_mmap_status = status;
break;
case BPRM_CHECK:
iint->ima_bprm_status = status;
break;
case CREDS_CHECK:
iint->ima_creds_status = status;
break;
case FILE_CHECK:
case POST_SETATTR:
iint->ima_file_status = status;
break;
case MODULE_CHECK ... MAX_CHECK - 1:
default:
iint->ima_read_status = status;
break;
}
}
static void ima_cache_flags(struct integrity_iint_cache *iint,
enum ima_hooks func)
{
switch (func) {
case MMAP_CHECK:
iint->flags |= (IMA_MMAP_APPRAISED | IMA_APPRAISED);
break;
case BPRM_CHECK:
iint->flags |= (IMA_BPRM_APPRAISED | IMA_APPRAISED);
break;
case CREDS_CHECK:
iint->flags |= (IMA_CREDS_APPRAISED | IMA_APPRAISED);
break;
case FILE_CHECK:
case POST_SETATTR:
iint->flags |= (IMA_FILE_APPRAISED | IMA_APPRAISED);
break;
case MODULE_CHECK ... MAX_CHECK - 1:
default:
iint->flags |= (IMA_READ_APPRAISED | IMA_APPRAISED);
break;
}
}
enum hash_algo ima_get_hash_algo(const struct evm_ima_xattr_data *xattr_value,
int xattr_len)
{
struct signature_v2_hdr *sig;
enum hash_algo ret;
if (!xattr_value || xattr_len < 2)
/* return default hash algo */
return ima_hash_algo;
switch (xattr_value->type) {
case EVM_IMA_XATTR_DIGSIG:
sig = (typeof(sig))xattr_value;
if (sig->version != 2 || xattr_len <= sizeof(*sig))
return ima_hash_algo;
return sig->hash_algo;
break;
case IMA_XATTR_DIGEST_NG:
/* first byte contains algorithm id */
ret = xattr_value->data[0];
if (ret < HASH_ALGO__LAST)
return ret;
break;
case IMA_XATTR_DIGEST:
/* this is for backward compatibility */
if (xattr_len == 21) {
unsigned int zero = 0;
if (!memcmp(&xattr_value->data[16], &zero, 4))
return HASH_ALGO_MD5;
else
return HASH_ALGO_SHA1;
} else if (xattr_len == 17)
return HASH_ALGO_MD5;
break;
}
/* return default hash algo */
return ima_hash_algo;
}
int ima_read_xattr(struct dentry *dentry,
struct evm_ima_xattr_data **xattr_value)
{
ssize_t ret;
ret = vfs_getxattr_alloc(&init_user_ns, dentry, XATTR_NAME_IMA,
(char **)xattr_value, 0, GFP_NOFS);
if (ret == -EOPNOTSUPP)
ret = 0;
return ret;
}
/*
* xattr_verify - verify xattr digest or signature
*
* Verify whether the hash or signature matches the file contents.
*
* Return 0 on success, error code otherwise.
*/
static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
struct evm_ima_xattr_data *xattr_value, int xattr_len,
enum integrity_status *status, const char **cause)
{
int rc = -EINVAL, hash_start = 0;
switch (xattr_value->type) {
case IMA_XATTR_DIGEST_NG:
/* first byte contains algorithm id */
hash_start = 1;
fallthrough;
case IMA_XATTR_DIGEST:
if (*status != INTEGRITY_PASS_IMMUTABLE) {
if (iint->flags & IMA_DIGSIG_REQUIRED) {
*cause = "IMA-signature-required";
*status = INTEGRITY_FAIL;
break;
}
clear_bit(IMA_DIGSIG, &iint->atomic_flags);
} else {
set_bit(IMA_DIGSIG, &iint->atomic_flags);
}
if (xattr_len - sizeof(xattr_value->type) - hash_start >=
iint->ima_hash->length)
/*
* xattr length may be longer. md5 hash in previous
* version occupied 20 bytes in xattr, instead of 16
*/
rc = memcmp(&xattr_value->data[hash_start],
iint->ima_hash->digest,
iint->ima_hash->length);
else
rc = -EINVAL;
if (rc) {
*cause = "invalid-hash";
*status = INTEGRITY_FAIL;
break;
}
*status = INTEGRITY_PASS;
break;
case EVM_IMA_XATTR_DIGSIG:
set_bit(IMA_DIGSIG, &iint->atomic_flags);
rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
(const char *)xattr_value,
xattr_len,
iint->ima_hash->digest,
iint->ima_hash->length);
if (rc == -EOPNOTSUPP) {
*status = INTEGRITY_UNKNOWN;
break;
}
if (IS_ENABLED(CONFIG_INTEGRITY_PLATFORM_KEYRING) && rc &&
func == KEXEC_KERNEL_CHECK)
rc = integrity_digsig_verify(INTEGRITY_KEYRING_PLATFORM,
(const char *)xattr_value,
xattr_len,
iint->ima_hash->digest,
iint->ima_hash->length);
if (rc) {
*cause = "invalid-signature";
*status = INTEGRITY_FAIL;
} else {
*status = INTEGRITY_PASS;
}
break;
default:
*status = INTEGRITY_UNKNOWN;
*cause = "unknown-ima-data";
break;
}
return rc;
}
/*
* modsig_verify - verify modsig signature
*
* Verify whether the signature matches the file contents.
*
* Return 0 on success, error code otherwise.
*/
static int modsig_verify(enum ima_hooks func, const struct modsig *modsig,
enum integrity_status *status, const char **cause)
{
int rc;
rc = integrity_modsig_verify(INTEGRITY_KEYRING_IMA, modsig);
if (IS_ENABLED(CONFIG_INTEGRITY_PLATFORM_KEYRING) && rc &&
func == KEXEC_KERNEL_CHECK)
rc = integrity_modsig_verify(INTEGRITY_KEYRING_PLATFORM,
modsig);
if (rc) {
*cause = "invalid-signature";
*status = INTEGRITY_FAIL;
} else {
*status = INTEGRITY_PASS;
}
return rc;
}
/*
* ima_check_blacklist - determine if the binary is blacklisted.
*
* Add the hash of the blacklisted binary to the measurement list, based
* on policy.
*
* Returns -EPERM if the hash is blacklisted.
*/
int ima_check_blacklist(struct integrity_iint_cache *iint,
const struct modsig *modsig, int pcr)
{
enum hash_algo hash_algo;
const u8 *digest = NULL;
u32 digestsize = 0;
int rc = 0;
if (!(iint->flags & IMA_CHECK_BLACKLIST))
return 0;
if (iint->flags & IMA_MODSIG_ALLOWED && modsig) {
ima_get_modsig_digest(modsig, &hash_algo, &digest, &digestsize);
rc = is_binary_blacklisted(digest, digestsize);
if ((rc == -EPERM) && (iint->flags & IMA_MEASURE))
process_buffer_measurement(&init_user_ns, NULL, digest, digestsize,
"blacklisted-hash", NONE,
pcr, NULL, false, NULL, 0);
}
return rc;
}
/*
* ima_appraise_measurement - appraise file measurement
*
* Call evm_verifyxattr() to verify the integrity of 'security.ima'.
* Assuming success, compare the xattr hash with the collected measurement.
*
* Return 0 on success, error code otherwise
*/
int ima_appraise_measurement(enum ima_hooks func,
struct integrity_iint_cache *iint,
struct file *file, const unsigned char *filename,
struct evm_ima_xattr_data *xattr_value,
int xattr_len, const struct modsig *modsig)
{
static const char op[] = "appraise_data";
const char *cause = "unknown";
struct dentry *dentry = file_dentry(file);
struct inode *inode = d_backing_inode(dentry);
enum integrity_status status = INTEGRITY_UNKNOWN;
int rc = xattr_len;
bool try_modsig = iint->flags & IMA_MODSIG_ALLOWED && modsig;
/* If not appraising a modsig, we need an xattr. */
if (!(inode->i_opflags & IOP_XATTR) && !try_modsig)
return INTEGRITY_UNKNOWN;
/* If reading the xattr failed and there's no modsig, error out. */
if (rc <= 0 && !try_modsig) {
if (rc && rc != -ENODATA)
goto out;
cause = iint->flags & IMA_DIGSIG_REQUIRED ?
"IMA-signature-required" : "missing-hash";
status = INTEGRITY_NOLABEL;
if (file->f_mode & FMODE_CREATED)
iint->flags |= IMA_NEW_FILE;
if ((iint->flags & IMA_NEW_FILE) &&
(!(iint->flags & IMA_DIGSIG_REQUIRED) ||
(inode->i_size == 0)))
status = INTEGRITY_PASS;
goto out;
}
status = evm_verifyxattr(dentry, XATTR_NAME_IMA, xattr_value, rc, iint);
switch (status) {
case INTEGRITY_PASS:
case INTEGRITY_PASS_IMMUTABLE:
case INTEGRITY_UNKNOWN:
break;
case INTEGRITY_NOXATTRS: /* No EVM protected xattrs. */
/* It's fine not to have xattrs when using a modsig. */
if (try_modsig)
break;
fallthrough;
case INTEGRITY_NOLABEL: /* No security.evm xattr. */
cause = "missing-HMAC";
goto out;
case INTEGRITY_FAIL_IMMUTABLE:
set_bit(IMA_DIGSIG, &iint->atomic_flags);
cause = "invalid-fail-immutable";
goto out;
case INTEGRITY_FAIL: /* Invalid HMAC/signature. */
cause = "invalid-HMAC";
goto out;
default:
WARN_ONCE(true, "Unexpected integrity status %d\n", status);
}
if (xattr_value)
rc = xattr_verify(func, iint, xattr_value, xattr_len, &status,
&cause);
/*
* If we have a modsig and either no imasig or the imasig's key isn't
* known, then try verifying the modsig.
*/
if (try_modsig &&
(!xattr_value || xattr_value->type == IMA_XATTR_DIGEST_NG ||
rc == -ENOKEY))
rc = modsig_verify(func, modsig, &status, &cause);
out:
/*
* File signatures on some filesystems can not be properly verified.
* When such filesystems are mounted by an untrusted mounter or on a
* system not willing to accept such a risk, fail the file signature
* verification.
*/
if ((inode->i_sb->s_iflags & SB_I_IMA_UNVERIFIABLE_SIGNATURE) &&
((inode->i_sb->s_iflags & SB_I_UNTRUSTED_MOUNTER) ||
(iint->flags & IMA_FAIL_UNVERIFIABLE_SIGS))) {
status = INTEGRITY_FAIL;
cause = "unverifiable-signature";
integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
op, cause, rc, 0);
} else if (status != INTEGRITY_PASS) {
/* Fix mode, but don't replace file signatures. */
if ((ima_appraise & IMA_APPRAISE_FIX) && !try_modsig &&
(!xattr_value ||
xattr_value->type != EVM_IMA_XATTR_DIGSIG)) {
if (!ima_fix_xattr(dentry, iint))
status = INTEGRITY_PASS;
}
/*
* Permit new files with file/EVM portable signatures, but
* without data.
*/
if (inode->i_size == 0 && iint->flags & IMA_NEW_FILE &&
test_bit(IMA_DIGSIG, &iint->atomic_flags)) {
status = INTEGRITY_PASS;
}
integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
op, cause, rc, 0);
} else {
ima_cache_flags(iint, func);
}
ima_set_cache_status(iint, func, status);
return status;
}
/*
* ima_update_xattr - update 'security.ima' hash value
*/
void ima_update_xattr(struct integrity_iint_cache *iint, struct file *file)
{
struct dentry *dentry = file_dentry(file);
int rc = 0;
/* do not collect and update hash for digital signatures */
if (test_bit(IMA_DIGSIG, &iint->atomic_flags))
return;
if ((iint->ima_file_status != INTEGRITY_PASS) &&
!(iint->flags & IMA_HASH))
return;
rc = ima_collect_measurement(iint, file, NULL, 0, ima_hash_algo, NULL);
if (rc < 0)
return;
inode_lock(file_inode(file));
ima_fix_xattr(dentry, iint);
inode_unlock(file_inode(file));
}
/**
* ima_inode_post_setattr - reflect file metadata changes
* @mnt_userns: user namespace of the mount the inode was found from
* @dentry: pointer to the affected dentry
*
* Changes to a dentry's metadata might result in needing to appraise.
*
* This function is called from notify_change(), which expects the caller
* to lock the inode's i_mutex.
*/
void ima_inode_post_setattr(struct user_namespace *mnt_userns,
struct dentry *dentry)
{
struct inode *inode = d_backing_inode(dentry);
struct integrity_iint_cache *iint;
int action;
if (!(ima_policy_flag & IMA_APPRAISE) || !S_ISREG(inode->i_mode)
|| !(inode->i_opflags & IOP_XATTR))
return;
action = ima_must_appraise(mnt_userns, inode, MAY_ACCESS, POST_SETATTR);
iint = integrity_iint_find(inode);
if (iint) {
set_bit(IMA_CHANGE_ATTR, &iint->atomic_flags);
if (!action)
clear_bit(IMA_UPDATE_XATTR, &iint->atomic_flags);
}
}
/*
* ima_protect_xattr - protect 'security.ima'
*
* Ensure that not just anyone can modify or remove 'security.ima'.
*/
static int ima_protect_xattr(struct dentry *dentry, const char *xattr_name,
const void *xattr_value, size_t xattr_value_len)
{
if (strcmp(xattr_name, XATTR_NAME_IMA) == 0) {
if (!capable(CAP_SYS_ADMIN))
return -EPERM;
return 1;
}
return 0;
}
static void ima_reset_appraise_flags(struct inode *inode, int digsig)
{
struct integrity_iint_cache *iint;
if (!(ima_policy_flag & IMA_APPRAISE) || !S_ISREG(inode->i_mode))
return;
iint = integrity_iint_find(inode);
if (!iint)
return;
iint->measured_pcrs = 0;
set_bit(IMA_CHANGE_XATTR, &iint->atomic_flags);
if (digsig)
set_bit(IMA_DIGSIG, &iint->atomic_flags);
else
clear_bit(IMA_DIGSIG, &iint->atomic_flags);
}
/**
* validate_hash_algo() - Block setxattr with unsupported hash algorithms
* @dentry: object of the setxattr()
* @xattr_value: userland supplied xattr value
* @xattr_value_len: length of xattr_value
*
* The xattr value is mapped to its hash algorithm, and this algorithm
* must be built in the kernel for the setxattr to be allowed.
*
* Emit an audit message when the algorithm is invalid.
*
* Return: 0 on success, else an error.
*/
static int validate_hash_algo(struct dentry *dentry,
const struct evm_ima_xattr_data *xattr_value,
size_t xattr_value_len)
{
char *path = NULL, *pathbuf = NULL;
enum hash_algo xattr_hash_algo;
const char *errmsg = "unavailable-hash-algorithm";
unsigned int allowed_hashes;
xattr_hash_algo = ima_get_hash_algo(xattr_value, xattr_value_len);
allowed_hashes = atomic_read(&ima_setxattr_allowed_hash_algorithms);
if (allowed_hashes) {
/* success if the algorithm is allowed in the ima policy */
if (allowed_hashes & (1U << xattr_hash_algo))
return 0;
/*
* We use a different audit message when the hash algorithm
* is denied by a policy rule, instead of not being built
* in the kernel image
*/
errmsg = "denied-hash-algorithm";
} else {
if (likely(xattr_hash_algo == ima_hash_algo))
return 0;
/* allow any xattr using an algorithm built in the kernel */
if (crypto_has_alg(hash_algo_name[xattr_hash_algo], 0, 0))
return 0;
}
pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
if (!pathbuf)
return -EACCES;
path = dentry_path(dentry, pathbuf, PATH_MAX);
integrity_audit_msg(AUDIT_INTEGRITY_DATA, d_inode(dentry), path,
"set_data", errmsg, -EACCES, 0);
kfree(pathbuf);
return -EACCES;
}
int ima_inode_setxattr(struct dentry *dentry, const char *xattr_name,
const void *xattr_value, size_t xattr_value_len)
{
const struct evm_ima_xattr_data *xvalue = xattr_value;
int digsig = 0;
int result;
result = ima_protect_xattr(dentry, xattr_name, xattr_value,
xattr_value_len);
if (result == 1) {
if (!xattr_value_len || (xvalue->type >= IMA_XATTR_LAST))
return -EINVAL;
digsig = (xvalue->type == EVM_IMA_XATTR_DIGSIG);
} else if (!strcmp(xattr_name, XATTR_NAME_EVM) && xattr_value_len > 0) {
digsig = (xvalue->type == EVM_XATTR_PORTABLE_DIGSIG);
}
if (result == 1 || evm_revalidate_status(xattr_name)) {
result = validate_hash_algo(dentry, xvalue, xattr_value_len);
if (result)
return result;
ima_reset_appraise_flags(d_backing_inode(dentry), digsig);
}
return result;
}
int ima_inode_removexattr(struct dentry *dentry, const char *xattr_name)
{
int result;
result = ima_protect_xattr(dentry, xattr_name, NULL, 0);
if (result == 1 || evm_revalidate_status(xattr_name)) {
ima_reset_appraise_flags(d_backing_inode(dentry), 0);
if (result == 1)
result = 0;
}
return result;
}
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