Re: [PATCH] ima: fix deadlock within "ima_match_policy" function.
flat view
From: THOBY Simon <hidden>
Date: 2021-08-24 09:50:35
Also in:
linux-integrity, lkml
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 ...."?
quoted hunk ↗ jump to hunk
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;
I haven't tested the patch myself, but the code diff looks fine to me. Thanks, Simon