Re: [PATCH] ima: fix deadlock within "ima_match_policy" function.

[THOBY Simon]

Hi liqiong,

On 8/24/21 10:57 AM, liqiong wrote:
> When "ima_match_policy" is looping while "ima_update_policy" changs

Small typo: "changes"/"updates"

> the variable "ima_rules", then "ima_match_policy" may can't exit
> loop, Finally cause RCU CPU Stall Warnings: "rcu_sched detected
> stall on CPU ...".

This could perhaps be rephrased to something like:
"""
ima_match_policy() can loop on the policy ruleset while
ima_update_policy() updates the variable "ima_rules".
This can lead to a situation where ima_match_policy()
can't exit the 'list_for_each_entry_rcu' loop, causing
RCU stalls ("rcu_sched detected stall on CPU ...").
"""


> 
> The problem is limited to transitioning from the builtin policy to
> the custom policy. Eg. At boot time, systemd-services are being
> checked within "ima_match_policy", at the same time, the variable
> "ima_rules" is changed by another service.

For the second sentence, consider something in the likes of:
"This problem can happen in practice: updating the IMA policy
in the boot process while systemd-services are being checked
have been observed to trigger this issue.".


Your commit message is also supposed to explain what you are doing,
using the imperative form ((see 'Documentation/process/submitting-patches.rst'):
"""
Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
to do frotz", as if you are giving orders to the codebase to change
its behaviour.
"""

Maybe add a paragraph with something like "Fix locking by introducing ...."?


> 
> Signed-off-by: liqiong <liqiong@xxxxxxxxxxxx>
> ---
>  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;
>  
> 

I haven't tested the patch myself, but the code diff looks fine to me.

Thanks,
Simon