Yama: add RCU to drop read locking

Stop using spinlocks in the read path. Add RCU list to handle the readers.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Acked-by: John Johansen <john.johansen@canonical.com>

1 files changed, 24 insertions, 23 deletions

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index b4c29848b49d..70cd85e3ba30 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -30,6 +30,7 @@ struct ptrace_relation {
         struct task_struct *tracer;
         struct task_struct *tracee;
         struct list_head node;
+        struct rcu_head rcu;
 };
 
 static LIST_HEAD(ptracer_relations);
@@ -48,32 +49,31 @@ static DEFINE_SPINLOCK(ptracer_relations_lock);
 static int yama_ptracer_add(struct task_struct *tracer,
                             struct task_struct *tracee)
 {
-        int rc = 0;
-        struct ptrace_relation *added;
-        struct ptrace_relation *entry, *relation = NULL;
+        struct ptrace_relation *relation, *added;
 
         added = kmalloc(sizeof(*added), GFP_KERNEL);
         if (!added)
                 return -ENOMEM;
 
+        added->tracee = tracee;
+        added->tracer = tracer;
+
         spin_lock_bh(&ptracer_relations_lock);
-        list_for_each_entry(entry, &ptracer_relations, node)
-                if (entry->tracee == tracee) {
-                        relation = entry;
-                        break;
+        rcu_read_lock();
+        list_for_each_entry_rcu(relation, &ptracer_relations, node) {
+                if (relation->tracee == tracee) {
+                        list_replace_rcu(&relation->node, &added->node);
+                        kfree_rcu(relation, rcu);
+                        goto out;
                 }
-        if (!relation) {
-                relation = added;
-                relation->tracee = tracee;
-                list_add(&relation->node, &ptracer_relations);
         }
-        relation->tracer = tracer;
 
-        spin_unlock_bh(&ptracer_relations_lock);
-        if (added != relation)
-                kfree(added);
+        list_add_rcu(&added->node, &ptracer_relations);
 
-        return rc;
+out:
+        rcu_read_unlock();
+        spin_unlock_bh(&ptracer_relations_lock);
+        return 0;
 }
 
 /**
@@ -84,15 +84,18 @@ static int yama_ptracer_add(struct task_struct *tracer,
 static void yama_ptracer_del(struct task_struct *tracer,
                              struct task_struct *tracee)
 {
-        struct ptrace_relation *relation, *safe;
+        struct ptrace_relation *relation;
 
         spin_lock_bh(&ptracer_relations_lock);
-        list_for_each_entry_safe(relation, safe, &ptracer_relations, node)
+        rcu_read_lock();
+        list_for_each_entry_rcu(relation, &ptracer_relations, node) {
                 if (relation->tracee == tracee ||
                     (tracer && relation->tracer == tracer)) {
-                        list_del(&relation->node);
-                        kfree(relation);
+                        list_del_rcu(&relation->node);
+                        kfree_rcu(relation, rcu);
                 }
+        }
+        rcu_read_unlock();
         spin_unlock_bh(&ptracer_relations_lock);
 }
 
@@ -217,11 +220,10 @@ static int ptracer_exception_found(struct task_struct *tracer,
         struct task_struct *parent = NULL;
         bool found = false;
 
-        spin_lock_bh(&ptracer_relations_lock);
         rcu_read_lock();
         if (!thread_group_leader(tracee))
                 tracee = rcu_dereference(tracee->group_leader);
-        list_for_each_entry(relation, &ptracer_relations, node)
+        list_for_each_entry_rcu(relation, &ptracer_relations, node)
                 if (relation->tracee == tracee) {
                         parent = relation->tracer;
                         found = true;
@@ -231,7 +233,6 @@ static int ptracer_exception_found(struct task_struct *tracer,
         if (found && (parent == NULL || task_is_descendant(parent, tracer)))
                 rc = 1;
         rcu_read_unlock();
-        spin_unlock_bh(&ptracer_relations_lock);
 
         return rc;
 }