ima: fix deadlock when traversing "ima_default_rules".

[ Upstream commit eb0782bbdfd0d7c4786216659277c3fd585afc0e ]

The current IMA ruleset is identified by the variable "ima_rules"
that default to "&ima_default_rules". When loading a custom policy
for the first time, the variable is updated to "&ima_policy_rules"
instead. That update isn't RCU-safe, and deadlocks are possible.
Indeed, some functions like ima_match_policy() may loop indefinitely
when traversing "ima_default_rules" with list_for_each_entry_rcu().

When iterating over the default ruleset back to head, if the list
head is "ima_default_rules", and "ima_rules" have been updated to
"&ima_policy_rules", the loop condition (&entry->list != ima_rules)
stays always true, traversing won't terminate, causing a soft lockup
and RCU stalls.

Introduce a temporary value for "ima_rules" when iterating over
the ruleset to avoid the deadlocks.

Signed-off-by: liqiong <liqiong@nfschina.com>
Reviewed-by: THOBY Simon <Simon.THOBY@viveris.fr>
Fixes: 38d859f991f3 ("IMA: policy can now be updated multiple times")
Reported-by: kernel test robot <lkp@intel.com> (Fix sparse: incompatible types in comparison expression.)
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index 87b9b71..12e8adc 100644 (file)
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -228,7 +228,7 @@ static struct ima_rule_entry *arch_policy_entry __ro_after_init;
 static LIST_HEAD(ima_default_rules);
 static LIST_HEAD(ima_policy_rules);
 static LIST_HEAD(ima_temp_rules);
-static struct list_head *ima_rules = &ima_default_rules;
+static struct list_head __rcu *ima_rules = (struct list_head __rcu *)(&ima_default_rules);
 
 static int ima_policy __initdata;
 
@@ -675,12 +675,14 @@ int ima_match_policy(struct user_namespace *mnt_userns, struct inode *inode,
 {
        struct ima_rule_entry *entry;
        int action = 0, actmask = flags | (flags << 1);
+       struct list_head *ima_rules_tmp;
 
        if (template_desc && !*template_desc)
                *template_desc = ima_template_desc_current();
 
        rcu_read_lock();
-       list_for_each_entry_rcu(entry, ima_rules, list) {
+       ima_rules_tmp = rcu_dereference(ima_rules);
+       list_for_each_entry_rcu(entry, ima_rules_tmp, list) {
 
                if (!(entry->action & actmask))
                        continue;
@@ -741,9 +743,11 @@ void ima_update_policy_flags(void)
 {
        struct ima_rule_entry *entry;
        int new_policy_flag = 0;
+       struct list_head *ima_rules_tmp;
 
        rcu_read_lock();
-       list_for_each_entry(entry, ima_rules, list) {
+       ima_rules_tmp = rcu_dereference(ima_rules);
+       list_for_each_entry_rcu(entry, ima_rules_tmp, list) {
                /*
                 * SETXATTR_CHECK rules do not implement a full policy check
                 * because rule checking would probably have an important
@@ -968,10 +972,10 @@ void ima_update_policy(void)
 
        list_splice_tail_init_rcu(&ima_temp_rules, policy, synchronize_rcu);
 
-       if (ima_rules != policy) {
+       if (ima_rules != (struct list_head __rcu *)policy) {
                ima_policy_flag = 0;
-               ima_rules = policy;
 
+               rcu_assign_pointer(ima_rules, policy);
                /*
                 * IMA architecture specific policy rules are specified
                 * as strings and converted to an array of ima_entry_rules
@@ -1061,7 +1065,7 @@ static int ima_lsm_rule_init(struct ima_rule_entry *entry,
                pr_warn("rule for LSM \'%s\' is undefined\n",
                        entry->lsm[lsm_rule].args_p);
 
-               if (ima_rules == &ima_default_rules) {
+               if (ima_rules == (struct list_head __rcu *)(&ima_default_rules)) {
                        kfree(entry->lsm[lsm_rule].args_p);
                        entry->lsm[lsm_rule].args_p = NULL;
                        result = -EINVAL;
@@ -1768,9 +1772,11 @@ void *ima_policy_start(struct seq_file *m, loff_t *pos)
 {
        loff_t l = *pos;
        struct ima_rule_entry *entry;
+       struct list_head *ima_rules_tmp;
 
        rcu_read_lock();
-       list_for_each_entry_rcu(entry, ima_rules, list) {
+       ima_rules_tmp = rcu_dereference(ima_rules);
+       list_for_each_entry_rcu(entry, ima_rules_tmp, list) {
                if (!l--) {
                        rcu_read_unlock();
                        return entry;
@@ -1789,7 +1795,8 @@ void *ima_policy_next(struct seq_file *m, void *v, loff_t *pos)
        rcu_read_unlock();
        (*pos)++;
 
-       return (&entry->list == ima_rules) ? NULL : entry;
+       return (&entry->list == &ima_default_rules ||
+               &entry->list == &ima_policy_rules) ? NULL : entry;
 }
 
 void ima_policy_stop(struct seq_file *m, void *v)
@@ -2014,6 +2021,7 @@ bool ima_appraise_signature(enum kernel_read_file_id id)
        struct ima_rule_entry *entry;
        bool found = false;
        enum ima_hooks func;
+       struct list_head *ima_rules_tmp;
 
        if (id >= READING_MAX_ID)
                return false;
@@ -2021,7 +2029,8 @@ bool ima_appraise_signature(enum kernel_read_file_id id)
        func = read_idmap[id] ?: FILE_CHECK;
 
        rcu_read_lock();
-       list_for_each_entry_rcu(entry, ima_rules, list) {
+       ima_rules_tmp = rcu_dereference(ima_rules);
+       list_for_each_entry_rcu(entry, ima_rules_tmp, list) {
                if (entry->action != APPRAISE)
                        continue;