Re: [PATCH] ima: fix deadlock within RCU list of ima_rules
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From: liqiong <hidden>
Date: 2021-08-27 09:11:23
Also in:
linux-integrity, lkml
Subsystem:
extended verification module (evm), integrity measurement architecture (ima), security subsystem, the rest · Maintainers:
Mimi Zohar, Roberto Sassu, Dmitry Kasatkin, Paul Moore, James Morris, "Serge E. Hallyn", Linus Torvalds
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 <redacted> --- 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;
--
2.11.0
在 2021年08月27日 15:30, THOBY Simon 写道:
> Hi liqiong,
>
> a few nits but nothing significant. This is getting in good shape!
>
> On 8/27/21 8:41 AM, liqiong wrote:
>> Hi Simon,
>>
>> Thanks for you help, i may got it, here is the new commit message:
>>
>>
>> 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.
>> Because some functions like ima_match_policy() may loop indefinitely
> s/Because/Indeed,/ (in english, sentences with a subordinating conjunction
> like 'because' are usually written in two parts, and a dependent clause
> standing by itself is rarely used except for stylistic effect)
>
>> over traversing the "ima_default_rules" as list_for_each_entry_rcu().
> s/over traversing the "ima_default_rules" as list_for_each_entry_rcu()/when traversing "ima_default_rules" with list_for_each_entry_rcu()./
>
>> When iterating over the default ruleset back to head, value of
>> "&entry->list" is "&ima_default_rules", and "ima_rules" may have been
> s/value of "&entry->list" is "&ima_default_rules"/if the list head is "ima_default_rules"/
> s/may have been/have been/
>
>> updated to "&ima_policy_rules", the loop condition (&entry->list != ima_rules)
>> stay alway true, traversing doesn't terminate, cause soft lockup and
> Don't forget the 's' in "stays" (or "remains")
> Ditto for "always"
> s/traversing doesn't/traversing won't/
> Also: s/cause/causing a/
>
>> 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
>>
>>
>> Thanks
>>
>> liqiong
>>
> Thanks,
> Simon