On 2026-09-22 13:26, Ricardo Robaina wrote:
On Mon, Sep 21, 2026 at 5:07 PM Jérémy Jean
<[email protected]> wrote:
kill_rules() removes mixed AUDIT_DIR and AUDIT_EXE rules when an audit
tree is pruned. It drops entry->rule.exe before removing the rule from
the RCU-visible filter lists.
After a rule has been installed with AUDIT_ADD_RULE, which requires
CAP_AUDIT_CONTROL, removing or moving the watched directory can race
with another task that is still evaluating the rule. In that case,
fsnotify can free the executable mark before the reader reaches
audit_mark_compare(), causing a use-after-free.
KASAN reports:
BUG: KASAN: slab-use-after-free in audit_mark_compare+0x8d/0xa0
Remove the rule from the RCU-visible filter lists first, wait for a
grace
period, and only then drop the executable mark. audit_del_rule()
already
uses this ordering.
Fixes: 34d99af52ad4 ("audit: implement audit by executable")
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <[email protected]>
---
kernel/audit_tree.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
index 1ed19b7..864a58b 100644
--- a/kernel/audit_tree.c
+++ b/kernel/audit_tree.c
@@ -553,11 +553,12 @@ static void kill_rules(struct audit_context
*context, struct audit_tree *tree)
if (rule->tree) {
/* not a half-baked one */
audit_tree_log_remove_rule(context, rule);
- if (entry->rule.exe)
- audit_remove_mark(entry->rule.exe);
rule->tree = NULL;
list_del_rcu(&entry->list);
list_del(&entry->rule.list);
+ synchronize_rcu();
+ if (entry->rule.exe)
+ audit_remove_mark(entry->rule.exe);
call_rcu(&entry->rcu, audit_free_rule_rcu);
}
}
--
2.47.3
Hi Jérémy,
Thanks for troubleshooting and fixing this bug.
sashiko's review makes sense, though: synchronize_rcu() inside the
loop waits a full grace period per rule, and kill_rules() runs with
audit_filter_mutex held, so a tree with many rules stalls other audit
config operations. Below is an untested suggestion to use a single
grace period instead.
diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
index 1ed19b775912..5a3d7c7ba5ce 100644
--- a/kernel/audit_tree.c
+++ b/kernel/audit_tree.c
@@ -549,18 +549,30 @@ static void kill_rules(struct audit_context
*context, struct audit_tree *tree)
list_for_each_entry_safe(rule, next, &tree->rules, rlist) {
entry = container_of(rule, struct audit_entry, rule);
- list_del_init(&rule->rlist);
if (rule->tree) {
/* not a half-baked one */
audit_tree_log_remove_rule(context, rule);
- if (entry->rule.exe)
- audit_remove_mark(entry->rule.exe);
rule->tree = NULL;
list_del_rcu(&entry->list);
list_del(&entry->rule.list);
- call_rcu(&entry->rcu, audit_free_rule_rcu);
+ } else {
+ list_del_init(&rule->rlist);
}
}
+
+ if (list_empty(&tree->rules))
+ return;
+
+ synchronize_rcu();
+
+ list_for_each_entry_safe(rule, next, &tree->rules, rlist) {
+ entry = container_of(rule, struct audit_entry, rule);
+
+ list_del_init(&rule->rlist);
+ if (entry->rule.exe)
+ audit_remove_mark(entry->rule.exe);
+ call_rcu(&entry->rcu, audit_free_rule_rcu);
+ }
}
/*
Hello Ricardo,
Many thanks for your nice and constructive answer.
Indeed, sashiko's review makes sense and your suggested v2 is a good
fix.
I have replayed my reproducer to ensure that it indeeds fix the reported
bug.
However, I wonder whether the wait should still happen in the case where
all removed rules have rule->exe == NULL. Doesn't call_rcu() already
free (kernel/auditfilter.c:96)? If yes, could we skip synchronize_rcu()
in
that case? If this question is valid, how about this fix based on yours,
which
simply adds a bool to detect whether synchronize_rcu() should be called.
If you think this addition is good, I can send a v2 with that.
diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
index 1ed19b775912..e2cd27b17590 100644
--- a/kernel/audit_tree.c
+++ b/kernel/audit_tree.c
@@ -545,22 +545,38 @@ static void kill_rules(struct audit_context
*context, struct audit_tree *tree)
{
struct audit_krule *rule, *next;
struct audit_entry *entry;
+ bool need_sync = false;
list_for_each_entry_safe(rule, next, &tree->rules, rlist) {
entry = container_of(rule, struct audit_entry, rule);
- list_del_init(&rule->rlist);
if (rule->tree) {
/* not a half-baked one */
audit_tree_log_remove_rule(context, rule);
- if (entry->rule.exe)
- audit_remove_mark(entry->rule.exe);
rule->tree = NULL;
list_del_rcu(&entry->list);
list_del(&entry->rule.list);
- call_rcu(&entry->rcu, audit_free_rule_rcu);
+ if (rule->exe)
+ need_sync = true;
+ } else {
+ list_del_init(&rule->rlist);
}
}
+
+ if (list_empty(&tree->rules))
+ return;
+
+ if (need_sync)
+ synchronize_rcu();
+
+ list_for_each_entry_safe(rule, next, &tree->rules, rlist) {
+ entry = container_of(rule, struct audit_entry, rule);
+
+ list_del_init(&rule->rlist);
+ if (entry->rule.exe)
+ audit_remove_mark(entry->rule.exe);
+ call_rcu(&entry->rcu, audit_free_rule_rcu);
+ }
}
/*
Regards,
Jérémy
