bpf: Prevent deadlock from recursive bpf_task_storage_[get|delete]

BPF helpers bpf_task_storage_[get|delete] could hold two locks:
bpf_local_storage_map_bucket->lock and bpf_local_storage->lock. Calling
these helpers from fentry/fexit programs on functions in bpf_*_storage.c
may cause deadlock on either locks.

Prevent such deadlock with a per cpu counter, bpf_task_storage_busy. We
need this counter to be global, because the two locks here belong to two
different objects: bpf_local_storage_map and bpf_local_storage. If we
pick one of them as the owner of the counter, it is still possible to
trigger deadlock on the other lock. For example, if bpf_local_storage_map
owns the counters, it cannot prevent deadlock on bpf_local_storage->lock
when two maps are used.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210225234319.336131-3-songliubraving@fb.com

include/linux/bpf_local_storage.h

@@ -126,7 +126,8 @@ bpf_local_storage_lookup(struct bpf_local_storage *local_storage,
 			 struct bpf_local_storage_map *smap,
 			 bool cacheit_lockit);
 
-void bpf_local_storage_map_free(struct bpf_local_storage_map *smap);
+void bpf_local_storage_map_free(struct bpf_local_storage_map *smap,
+				int __percpu *busy_counter);
 
 int bpf_local_storage_map_check_btf(const struct bpf_map *map,
 				    const struct btf *btf,

kernel/bpf/bpf_inode_storage.c

@@ -237,7 +237,7 @@ static void inode_storage_map_free(struct bpf_map *map)
 
 	smap = (struct bpf_local_storage_map *)map;
 	bpf_local_storage_cache_idx_free(&inode_cache, smap->cache_idx);
-	bpf_local_storage_map_free(smap);
+	bpf_local_storage_map_free(smap, NULL);
 }
 
 static int inode_storage_map_btf_id;

kernel/bpf/bpf_local_storage.c

@@ -474,7 +474,8 @@ void bpf_local_storage_cache_idx_free(struct bpf_local_storage_cache *cache,
 	spin_unlock(&cache->idx_lock);
 }
 
-void bpf_local_storage_map_free(struct bpf_local_storage_map *smap)
+void bpf_local_storage_map_free(struct bpf_local_storage_map *smap,
+				int __percpu *busy_counter)
 {
 	struct bpf_local_storage_elem *selem;
 	struct bpf_local_storage_map_bucket *b;
@@ -503,7 +504,15 @@ void bpf_local_storage_map_free(struct bpf_local_storage_map *smap)
 		while ((selem = hlist_entry_safe(
 				rcu_dereference_raw(hlist_first_rcu(&b->list)),
 				struct bpf_local_storage_elem, map_node))) {
+			if (busy_counter) {
+				migrate_disable();
+				__this_cpu_inc(*busy_counter);
+			}
 			bpf_selem_unlink(selem);
+			if (busy_counter) {
+				__this_cpu_dec(*busy_counter);
+				migrate_enable();
+			}
 			cond_resched_rcu();
 		}
 		rcu_read_unlock();

kernel/bpf/bpf_task_storage.c

@@ -20,6 +20,31 @@
 
 DEFINE_BPF_STORAGE_CACHE(task_cache);
 
+DEFINE_PER_CPU(int, bpf_task_storage_busy);
+
+static void bpf_task_storage_lock(void)
+{
+	migrate_disable();
+	__this_cpu_inc(bpf_task_storage_busy);
+}
+
+static void bpf_task_storage_unlock(void)
+{
+	__this_cpu_dec(bpf_task_storage_busy);
+	migrate_enable();
+}
+
+static bool bpf_task_storage_trylock(void)
+{
+	migrate_disable();
+	if (unlikely(__this_cpu_inc_return(bpf_task_storage_busy) != 1)) {
+		__this_cpu_dec(bpf_task_storage_busy);
+		migrate_enable();
+		return false;
+	}
+	return true;
+}
+
 static struct bpf_local_storage __rcu **task_storage_ptr(void *owner)
 {
 	struct task_struct *task = owner;
@@ -67,6 +92,7 @@ void bpf_task_storage_free(struct task_struct *task)
 	 * when unlinking elem from the local_storage->list and
 	 * the map's bucket->list.
 	 */
+	bpf_task_storage_lock();
 	raw_spin_lock_irqsave(&local_storage->lock, flags);
 	hlist_for_each_entry_safe(selem, n, &local_storage->list, snode) {
 		/* Always unlink from map before unlinking from
@@ -77,6 +103,7 @@ void bpf_task_storage_free(struct task_struct *task)
 			local_storage, selem, false);
 	}
 	raw_spin_unlock_irqrestore(&local_storage->lock, flags);
+	bpf_task_storage_unlock();
 	rcu_read_unlock();
 
 	/* free_task_storage should always be true as long as
@@ -109,7 +136,9 @@ static void *bpf_pid_task_storage_lookup_elem(struct bpf_map *map, void *key)
 		goto out;
 	}
 
+	bpf_task_storage_lock();
 	sdata = task_storage_lookup(task, map, true);
+	bpf_task_storage_unlock();
 	put_pid(pid);
 	return sdata ? sdata->data : NULL;
 out:
@@ -141,8 +170,10 @@ static int bpf_pid_task_storage_update_elem(struct bpf_map *map, void *key,
 		goto out;
 	}
 
+	bpf_task_storage_lock();
 	sdata = bpf_local_storage_update(
 		task, (struct bpf_local_storage_map *)map, value, map_flags);
+	bpf_task_storage_unlock();
 
 	err = PTR_ERR_OR_ZERO(sdata);
 out:
@@ -185,7 +216,9 @@ static int bpf_pid_task_storage_delete_elem(struct bpf_map *map, void *key)
 		goto out;
 	}
 
+	bpf_task_storage_lock();
 	err = task_storage_delete(task, map);
+	bpf_task_storage_unlock();
 out:
 	put_pid(pid);
 	return err;
@@ -202,34 +235,44 @@ BPF_CALL_4(bpf_task_storage_get, struct bpf_map *, map, struct task_struct *,
 	if (!task)
 		return (unsigned long)NULL;
 
+	if (!bpf_task_storage_trylock())
+		return (unsigned long)NULL;
+
 	sdata = task_storage_lookup(task, map, true);
 	if (sdata)
-		return (unsigned long)sdata->data;
+		goto unlock;
 
 	/* only allocate new storage, when the task is refcounted */
 	if (refcount_read(&task->usage) &&
-	    (flags & BPF_LOCAL_STORAGE_GET_F_CREATE)) {
+	    (flags & BPF_LOCAL_STORAGE_GET_F_CREATE))
 		sdata = bpf_local_storage_update(
 			task, (struct bpf_local_storage_map *)map, value,
 			BPF_NOEXIST);
-		return IS_ERR(sdata) ? (unsigned long)NULL :
-					     (unsigned long)sdata->data;
-	}
 
-	return (unsigned long)NULL;
+unlock:
+	bpf_task_storage_unlock();
+	return IS_ERR_OR_NULL(sdata) ? (unsigned long)NULL :
+		(unsigned long)sdata->data;
 }
 
 BPF_CALL_2(bpf_task_storage_delete, struct bpf_map *, map, struct task_struct *,
 	   task)
 {
+	int ret;
+
 	if (!task)
 		return -EINVAL;
 
+	if (!bpf_task_storage_trylock())
+		return -EBUSY;
+
 	/* This helper must only be called from places where the lifetime of the task
 	 * is guaranteed. Either by being refcounted or by being protected
 	 * by an RCU read-side critical section.
 	 */
-	return task_storage_delete(task, map);
+	ret = task_storage_delete(task, map);
+	bpf_task_storage_unlock();
+	return ret;
 }
 
 static int notsupp_get_next_key(struct bpf_map *map, void *key, void *next_key)
@@ -255,7 +298,7 @@ static void task_storage_map_free(struct bpf_map *map)
 
 	smap = (struct bpf_local_storage_map *)map;
 	bpf_local_storage_cache_idx_free(&task_cache, smap->cache_idx);
-	bpf_local_storage_map_free(smap);
+	bpf_local_storage_map_free(smap, &bpf_task_storage_busy);
 }
 
 static int task_storage_map_btf_id;

net/core/bpf_sk_storage.c

@@ -89,7 +89,7 @@ static void bpf_sk_storage_map_free(struct bpf_map *map)
 
 	smap = (struct bpf_local_storage_map *)map;
 	bpf_local_storage_cache_idx_free(&sk_cache, smap->cache_idx);
-	bpf_local_storage_map_free(smap);
+	bpf_local_storage_map_free(smap, NULL);
 }
 
 static struct bpf_map *bpf_sk_storage_map_alloc(union bpf_attr *attr)