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

liqiong
2021-08-24T12:10:19Z

Hi Simon :

ima: fix deadlock within RCU list of ima_rules.

ima_match_policy() is looping 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 ...").

This problem can happen in practice: updating the IMA policy
in the boot process while systemd-services are being checked.

In addition to ima_match_policy(), other function with
"list_for_each_entry_rcu" should happen too. Fix locking by
introducing a duplicate of "ima_rules" for each
"list_for_each_entry_rcu".

How about this commit message ?

I have tested this patch in lab, we can reproduced this error case,
have done reboot test many times. This patch should work.

在 2021年08月24日 17:50, THOBY Simon 写道:
> 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@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;
>>
>>
> I haven't tested the patch myself, but the code diff looks fine to me.
>
> Thanks,
> Simon