Re: [PATCH] ima: fix deadlock within RCU list of ima_rules

[THOBY Simon <Simon.THOBY@viveris.fr>]

On 8/27/21 11:10 AM, liqiong wrote:
> Hi Simon,
> 
> Thanks for your patient, i learn a lot. If the commit message
> does work, i would resubmit the patch.  Here is the whole patch:
> 
> 
> 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>
> ---
>  security/integrity/ima/ima_policy.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index fd5d46e511f1..e92b197bfd3c 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -662,12 +662,14 @@ 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;
> @@ -919,8 +921,8 @@ void ima_update_policy(void)
>  
>  	if (ima_rules != 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
> @@ -1649,9 +1651,11 @@ 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;
> @@ -1670,7 +1674,8 @@ 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)
> @@ -1872,6 +1877,7 @@ 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;
> @@ -1879,7 +1885,8 @@ 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;
>  
> 

Reviewed-By: THOBY Simon <Simon.THOBY@viveris.fr>