[v1] IMA: pre-allocate buffer to hold keyrings string

Commit Message

Lakshmi Ramasubramanian Jan. 16, 2020, 11:46 p.m. UTC

ima_match_keyring() is called while holding rcu read lock. Since this
function executes in atmomic context, it should not call any function
that can sleep (such as kstrdup()).

This patch pre-allocates a buffer to hold the keyrings string read from
the IMA policy and uses that to match the given keyring.

Signed-off-by: Lakshmi Ramasubramanian <[e-mail]>
Fixes: e9085e0ad38a ("IMA: Add support to limit measuring keys")
---
 security/integrity/ima/ima_policy.c | 50 ++++++++++++++++++++++++-----
 1 file changed, 42 insertions(+), 8 deletions(-)

Comments

Mimi Zohar Jan. 17, 2020, 1:17 a.m. UTC | #1

Hi Lakshmi,

Trimming the Cc list. This patch is limited to IMA. 

On Thu, 2020-01-16 at 15:46 -0800, Lakshmi Ramasubramanian wrote:

>  
> @@ -949,6 +949,7 @@ static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
>  	bool uid_token;
>  	struct ima_template_desc *template_desc;
>  	int result = 0;
> +	size_t keyrings_len;
>  
>  	ab = integrity_audit_log_start(audit_context(), GFP_KERNEL,
>  				       AUDIT_INTEGRITY_POLICY_RULE);
> @@ -1114,14 +1115,47 @@ static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
>  		case Opt_keyrings:
>  			ima_log_string(ab, "keyrings", args[0].from);
>  
> +			keyrings_len = strlen(args[0].from) + 1;
> +
>  			if ((entry->keyrings) ||
>  			    (entry->action != MEASURE) ||
> -			    (entry->func != KEY_CHECK)) {
> +			    (entry->func != KEY_CHECK) ||
> +			    (keyrings_len < 2)) {
>  				result = -EINVAL;
>  				break;
>  			}
> +
> +			if (ima_keyrings) {
> +				if (keyrings_len > ima_keyrings_len) {
> +					char *tmpbuf;
> +
> +					tmpbuf = krealloc(ima_keyrings,
> +							  keyrings_len,
> +							  GFP_KERNEL);
> +					if (!tmpbuf) {
> +						result = -ENOMEM;
> +						break;
> +					}
> +
> +					ima_keyrings = tmpbuf;
> +					ima_keyrings_len = keyrings_len;
> +				}
> +			} else {
> +				ima_keyrings = kzalloc(keyrings_len,
> +						       GFP_KERNEL);
> +				if (!ima_keyrings) {
> +					result = -ENOMEM;
> +					break;
> +				}
> +
> +				ima_keyrings_len = keyrings_len;
> +			}

The first time this code is executed ima_keyrings_len is 0.  So "if
(ima_keyrings_len < keyring_len)" will be true.  There's no reason to
differentiate between the first time or subsequent times this code is
executed.  In both cases, krealloc() can be used. 

Mimi


> +
>  			entry->keyrings = kstrdup(args[0].from, GFP_KERNEL);
>  			if (!entry->keyrings) {
> +				kfree(ima_keyrings);
> +				ima_keyrings = NULL;
> +				ima_keyrings_len = 0;
>  				result = -ENOMEM;
>  				break;
>  			}

Patch

diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index 9963863d6c92..180e2069e075 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -208,6 +208,10 @@  static LIST_HEAD(ima_policy_rules);
 static LIST_HEAD(ima_temp_rules);
 static struct list_head *ima_rules;
 
+/* Pre-allocated buffer used for matching keyrings. */
+static char *ima_keyrings;
+static size_t ima_keyrings_len;
+
 static int ima_policy __initdata;
 
 static int __init default_measure_policy_setup(char *str)
@@ -369,7 +373,7 @@  int ima_lsm_policy_change(struct notifier_block *nb, unsigned long event,
 static bool ima_match_keyring(struct ima_rule_entry *rule,
 			      const char *keyring, const struct cred *cred)
 {
-	char *keyrings, *next_keyring, *keyrings_ptr;
+	char *next_keyring, *keyrings_ptr;
 	bool matched = false;
 
 	if ((rule->flags & IMA_UID) && !rule->uid_op(cred->uid, rule->uid))
@@ -381,15 +385,13 @@  static bool ima_match_keyring(struct ima_rule_entry *rule,
 	if (!keyring)
 		return false;
 
-	keyrings = kstrdup(rule->keyrings, GFP_KERNEL);
-	if (!keyrings)
-		return false;
+	strcpy(ima_keyrings, rule->keyrings);
 
 	/*
 	 * "keyrings=" is specified in the policy in the format below:
 	 * keyrings=.builtin_trusted_keys|.ima|.evm
 	 */
-	keyrings_ptr = keyrings;
+	keyrings_ptr = ima_keyrings;
 	while ((next_keyring = strsep(&keyrings_ptr, "|")) != NULL) {
 		if (!strcmp(next_keyring, keyring)) {
 			matched = true;
@@ -397,8 +399,6 @@  static bool ima_match_keyring(struct ima_rule_entry *rule,
 		}
 	}
 
-	kfree(keyrings);
-
 	return matched;
 }
 
@@ -949,6 +949,7 @@  static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
 	bool uid_token;
 	struct ima_template_desc *template_desc;
 	int result = 0;
+	size_t keyrings_len;
 
 	ab = integrity_audit_log_start(audit_context(), GFP_KERNEL,
 				       AUDIT_INTEGRITY_POLICY_RULE);
@@ -1114,14 +1115,47 @@  static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
 		case Opt_keyrings:
 			ima_log_string(ab, "keyrings", args[0].from);
 
+			keyrings_len = strlen(args[0].from) + 1;
+
 			if ((entry->keyrings) ||
 			    (entry->action != MEASURE) ||
-			    (entry->func != KEY_CHECK)) {
+			    (entry->func != KEY_CHECK) ||
+			    (keyrings_len < 2)) {
 				result = -EINVAL;
 				break;
 			}
+
+			if (ima_keyrings) {
+				if (keyrings_len > ima_keyrings_len) {
+					char *tmpbuf;
+
+					tmpbuf = krealloc(ima_keyrings,
+							  keyrings_len,
+							  GFP_KERNEL);
+					if (!tmpbuf) {
+						result = -ENOMEM;
+						break;
+					}
+
+					ima_keyrings = tmpbuf;
+					ima_keyrings_len = keyrings_len;
+				}
+			} else {
+				ima_keyrings = kzalloc(keyrings_len,
+						       GFP_KERNEL);
+				if (!ima_keyrings) {
+					result = -ENOMEM;
+					break;
+				}
+
+				ima_keyrings_len = keyrings_len;
+			}
+
 			entry->keyrings = kstrdup(args[0].from, GFP_KERNEL);
 			if (!entry->keyrings) {
+				kfree(ima_keyrings);
+				ima_keyrings = NULL;
+				ima_keyrings_len = 0;
 				result = -ENOMEM;
 				break;
 			}