Yama: remove locking from delete path

Instead of locking the list during a delete, mark entries as invalid
and trigger a workqueue to clean them up. This lets us easily handle
task_free from interrupt context.

Signed-off-by: Kees Cook <keescook@chromium.org>

1 files changed, 42 insertions, 7 deletions

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index 70cd85e3ba30..2663145d1197 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -17,6 +17,7 @@
 #include <linux/ptrace.h>
 #include <linux/prctl.h>
 #include <linux/ratelimit.h>
+#include <linux/workqueue.h>
 
 #define YAMA_SCOPE_DISABLED     0
 #define YAMA_SCOPE_RELATIONAL   1
@@ -29,6 +30,7 @@ static int ptrace_scope = YAMA_SCOPE_RELATIONAL;
 struct ptrace_relation {
         struct task_struct *tracer;
         struct task_struct *tracee;
+        bool invalid;
         struct list_head node;
         struct rcu_head rcu;
 };
@@ -36,6 +38,29 @@ struct ptrace_relation {
 static LIST_HEAD(ptracer_relations);
 static DEFINE_SPINLOCK(ptracer_relations_lock);
 
+static void yama_relation_cleanup(struct work_struct *work);
+static DECLARE_WORK(yama_relation_work, yama_relation_cleanup);
+
+/**
+ * yama_relation_cleanup - remove invalid entries from the relation list
+ *
+ */
+static void yama_relation_cleanup(struct work_struct *work)
+{
+        struct ptrace_relation *relation;
+
+        spin_lock(&ptracer_relations_lock);
+        rcu_read_lock();
+        list_for_each_entry_rcu(relation, &ptracer_relations, node) {
+                if (relation->invalid) {
+                        list_del_rcu(&relation->node);
+                        kfree_rcu(relation, rcu);
+                }
+        }
+        rcu_read_unlock();
+        spin_unlock(&ptracer_relations_lock);
+}
+
 /**
  * yama_ptracer_add - add/replace an exception for this tracer/tracee pair
  * @tracer: the task_struct of the process doing the ptrace
@@ -57,10 +82,13 @@ static int yama_ptracer_add(struct task_struct *tracer,
 
         added->tracee = tracee;
         added->tracer = tracer;
+        added->invalid = false;
 
-        spin_lock_bh(&ptracer_relations_lock);
+        spin_lock(&ptracer_relations_lock);
         rcu_read_lock();
         list_for_each_entry_rcu(relation, &ptracer_relations, node) {
+                if (relation->invalid)
+                        continue;
                 if (relation->tracee == tracee) {
                         list_replace_rcu(&relation->node, &added->node);
                         kfree_rcu(relation, rcu);
@@ -72,7 +100,7 @@ static int yama_ptracer_add(struct task_struct *tracer,
 
 out:
         rcu_read_unlock();
-        spin_unlock_bh(&ptracer_relations_lock);
+        spin_unlock(&ptracer_relations_lock);
         return 0;
 }
 
@@ -85,18 +113,22 @@ static void yama_ptracer_del(struct task_struct *tracer,
                              struct task_struct *tracee)
 {
         struct ptrace_relation *relation;
+        bool marked = false;
 
-        spin_lock_bh(&ptracer_relations_lock);
         rcu_read_lock();
         list_for_each_entry_rcu(relation, &ptracer_relations, node) {
+                if (relation->invalid)
+                        continue;
                 if (relation->tracee == tracee ||
                     (tracer && relation->tracer == tracer)) {
-                        list_del_rcu(&relation->node);
-                        kfree_rcu(relation, rcu);
+                        relation->invalid = true;
+                        marked = true;
                 }
         }
         rcu_read_unlock();
-        spin_unlock_bh(&ptracer_relations_lock);
+
+        if (marked)
+                schedule_work(&yama_relation_work);
 }
 
 /**
@@ -223,12 +255,15 @@ static int ptracer_exception_found(struct task_struct *tracer,
         rcu_read_lock();
         if (!thread_group_leader(tracee))
                 tracee = rcu_dereference(tracee->group_leader);
-        list_for_each_entry_rcu(relation, &ptracer_relations, node)
+        list_for_each_entry_rcu(relation, &ptracer_relations, node) {
+                if (relation->invalid)
+                        continue;
                 if (relation->tracee == tracee) {
                         parent = relation->tracer;
                         found = true;
                         break;
                 }
+        }
 
         if (found && (parent == NULL || task_is_descendant(parent, tracer)))
                 rc = 1;