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_hash=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).
On policy update, the latest SETXATTR_CHECK rule is the only one
that apply, and other SETXATTR_CHECK rules are deleted.
Signed-off-by: Simon Thoby <simon.thoby@xxxxxxxxxx>
---
Documentation/ABI/testing/ima_policy | 9 +++-
security/integrity/ima/ima.h | 4 ++
security/integrity/ima/ima_appraise.c | 33 ++++++++++--
security/integrity/ima/ima_policy.c | 75 +++++++++++++++++++++++++++
4 files changed, 115 insertions(+), 6 deletions(-)
diff --git a/Documentation/ABI/testing/ima_policy b/Documentation/ABI/testing/ima_policy
index aeb622698047..537be0e1720e 100644
--- a/Documentation/ABI/testing/ima_policy
+++ b/Documentation/ABI/testing/ima_policy
@@ -30,9 +30,10 @@ Description:
[appraise_flag=] [appraise_hash=] [keyrings=]
base:
func:= [BPRM_CHECK][MMAP_CHECK][CREDS_CHECK][FILE_CHECK][MODULE_CHECK]
- [FIRMWARE_CHECK]
+ [FIRMWARE_CHECK]
[KEXEC_KERNEL_CHECK] [KEXEC_INITRAMFS_CHECK]
[KEXEC_CMDLINE] [KEY_CHECK] [CRITICAL_DATA]
+ [SETXATTR_CHECK]
mask:= [[^]MAY_READ] [[^]MAY_WRITE] [[^]MAY_APPEND]
[[^]MAY_EXEC]
fsmagic:= hex value
@@ -138,3 +139,9 @@ Description:
keys added to .builtin_trusted_keys or .ima keyring:
measure func=KEY_CHECK keyrings=.builtin_trusted_keys|.ima
+
+ Example of the special SETXATTR_CHECK appraise rule, that
+ restricts the hash algorithms allowed when writing to the
+ security.ima xattr of a file:
+
+ appraise func=SETXATTR_CHECK appraise_hash=sha256,sha384,sha512
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index 7ef1b214d358..aeb3bf30c0f9 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -46,6 +46,9 @@ enum tpm_pcrs { TPM_PCR0 = 0, TPM_PCR8 = 8, TPM_PCR10 = 10 };
/* current content of the policy */
extern int ima_policy_flag;
+/* bitset of digests algorithms allowed in the setxattr hook */
+extern atomic_t ima_setxattr_allowed_hash_algorithms;
+
/* set during initialization */
extern int ima_hash_algo __ro_after_init;
extern int ima_sha1_idx __ro_after_init;
@@ -198,6 +201,7 @@ static inline unsigned int ima_hash_key(u8 *digest)
hook(KEXEC_CMDLINE, kexec_cmdline) \
hook(KEY_CHECK, key) \
hook(CRITICAL_DATA, critical_data) \
+ hook(SETXATTR_CHECK, setxattr_check) \
hook(MAX_CHECK, none)
#define __ima_hook_enumify(ENUM, str) ENUM,
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index f751410930a5..b938a053366c 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -596,23 +596,46 @@ int ima_setxattr_validate_hash_alg(struct dentry *dentry,
{
int res = -EACCES;
char *path = NULL, *pathbuf = NULL;
+ const char *errmsg = "unavailable-hash-algorithm";
enum hash_algo dentry_hash;
+ unsigned int allowed_hashes;
dentry_hash = ima_get_hash_algo((struct evm_ima_xattr_data *)xattr_value,
xattr_value_len);
- if (likely(dentry_hash == ima_hash_algo
- || crypto_has_alg(hash_algo_name[dentry_hash], 0, 0)))
- return 0;
+ allowed_hashes = atomic_read(&ima_setxattr_allowed_hash_algorithms);
+
+ if (allowed_hashes) {
+ /* success if the algorithm is whitelisted in the ima policy */
+ if (allowed_hashes & (1U << dentry_hash))
+ 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(dentry_hash == ima_hash_algo))
+ return 0;
+
+ /* allow any xattr using an algorithm built in the kernel */
+ if (crypto_has_alg(hash_algo_name[dentry_hash], 0, 0))
+ return 0;
+ }
pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
/* no memory available ? no file path for you */
if (pathbuf)
path = dentry_path(dentry, pathbuf, PATH_MAX);
- /* disallow xattr writes with algorithms not built in the kernel */
+ /*
+ * disallow xattr writes with algorithms not built in the kernel or
+ * denied by policy
+ */
integrity_audit_msg(AUDIT_INTEGRITY_DATA, d_inode(dentry),
- path, "collect_data", "unavailable-hash-algorithm", res, 0);
+ path, "collect_data", errmsg, res, 0);
kfree(pathbuf);
diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index aadd95753229..a74b78e1746e 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -53,6 +53,8 @@ int ima_policy_flag;
static int temp_ima_appraise;
static int build_ima_appraise __ro_after_init;
+atomic_t ima_setxattr_allowed_hash_algorithms;
+
#define MAX_LSM_RULES 6
enum lsm_rule_types { LSM_OBJ_USER, LSM_OBJ_ROLE, LSM_OBJ_TYPE,
LSM_SUBJ_USER, LSM_SUBJ_ROLE, LSM_SUBJ_TYPE
@@ -87,6 +89,7 @@ struct ima_rule_entry {
int type; /* audit type */
} lsm[MAX_LSM_RULES];
char *fsname;
+ struct rcu_head rcu;
struct ima_rule_opt_list *keyrings; /* Measure keys added to these keyrings */
struct ima_rule_opt_list *label; /* Measure data grouped under this label */
struct ima_template_desc *template;
@@ -368,6 +371,13 @@ static void ima_free_rule(struct ima_rule_entry *entry)
kfree(entry);
}
+static void ima_free_rule_rcu(struct rcu_head *rcu)
+{
+ struct ima_rule_entry *entry = container_of(rcu, struct ima_rule_entry, rcu);
+
+ ima_free_rule(entry);
+}
+
static struct ima_rule_entry *ima_lsm_copy_rule(struct ima_rule_entry *entry)
{
struct ima_rule_entry *nentry;
@@ -903,6 +913,8 @@ void __init ima_init_policy(void)
ARRAY_SIZE(critical_data_rules),
IMA_DEFAULT_POLICY);
+ atomic_xchg(&ima_setxattr_allowed_hash_algorithms, 0);
+
ima_update_policy_flag();
}
@@ -914,6 +926,52 @@ int ima_check_policy(void)
return 0;
}
+/** ima_update_setxattr_allowed_hash_algorithms - cleanup SETXATTR_CHECK rules
+ * in the new ruleset
+ * @policy: the list of ima_rules_entry to clean
+ *
+ * Context: called when updating the IMA policy. Delete non-applicable
+ * rules with 'func' set to SETXATTR_CHECK and update the atomic variable
+ * to hold the list of allowed hash algorithms for the security.ima xattr.
+ *
+ * SETXATTR_CHECK rules do not implement a full policy check because of
+ * the performance impact performing rules checking on setxattr() would
+ * have. The consequence is that only one SETXATTR_CHECK can be active at
+ * a time. To prevent confusion, on policy updates, if a new SETXATTR_CHECK
+ * is defined, other SETXATTR_CHECK rules are remove from the ruleset.
+ */
+void ima_update_setxattr_allowed_hash_algorithms(struct list_head *policy)
+{
+ struct ima_rule_entry *entry, *tmp;
+ bool setxattr_check_already_defined = false;
+
+ rcu_read_lock();
+ list_for_each_entry_safe_reverse(entry, tmp, policy, list) {
+ if (entry->func != SETXATTR_CHECK)
+ continue;
+
+ if (setxattr_check_already_defined) {
+ /*
+ * delete old SETXATTR_CHECK entries when a newer
+ * one already exists
+ */
+ list_del_rcu(&entry->list);
+ call_rcu(&entry->rcu, ima_free_rule_rcu);
+ } else {
+ /*
+ * only the last entry with the SETXATTR_CHECK func
+ * apply: this allows runtime upgrades of the
+ * digest algorithm policy, unlike the other IMA
+ * rules
+ */
+ atomic_xchg(&ima_setxattr_allowed_hash_algorithms,
+ entry->allowed_hashes);
+ setxattr_check_already_defined = true;
+ }
+ }
+ rcu_read_unlock();
+}
+
/**
* ima_update_policy - update default_rules with new measure rules
*
@@ -931,6 +989,8 @@ void ima_update_policy(void)
list_splice_tail_init_rcu(&ima_temp_rules, policy, synchronize_rcu);
+ ima_update_setxattr_allowed_hash_algorithms(policy);
+
if (ima_rules != policy) {
ima_policy_flag = 0;
ima_rules = policy;
@@ -1176,6 +1236,19 @@ static bool ima_validate_rule(struct ima_rule_entry *entry)
if (ima_rule_contains_lsm_cond(entry))
return false;
+ break;
+ case SETXATTR_CHECK:
+ /* any action other than APPRAISE is unsupported */
+ if (entry->action != APPRAISE)
+ return false;
+
+ /*
+ * full policies are not supported, they would have too
+ * much of a performance impact
+ */
+ if (entry->flags & ~(IMA_FUNC | IMA_VALIDATE_HASH))
+ return false;
+
break;
default:
return false;
@@ -1333,6 +1406,8 @@ static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
entry->func = KEY_CHECK;
else if (strcmp(args[0].from, "CRITICAL_DATA") == 0)
entry->func = CRITICAL_DATA;
+ else if (strcmp(args[0].from, "SETXATTR_CHECK") == 0)
+ entry->func = SETXATTR_CHECK;
else
result = -EINVAL;
if (!result)
--
2.31.1
