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From:   Song Liu <songliubraving@fb.com>
To:     <bpf@vger.kernel.org>, <netdev@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
CC:     <ast@kernel.org>, <daniel@iogearbox.net>, <kernel-team@fb.com>,
        <peterz@infradead.org>, Song Liu <songliubraving@fb.com>
Subject: [PATCH v5 bpf-next 2/6] bpf: prevent deadlock from recursive bpf_task_storage_[get|delete]
Date:   Tue, 23 Feb 2021 14:28:41 -0800
Message-ID: <20210223222845.2866124-3-songliubraving@fb.com>
In-Reply-To: <20210223222845.2866124-1-songliubraving@fb.com>
References: <20210223222845.2866124-1-songliubraving@fb.com>
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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>
---
 kernel/bpf/bpf_task_storage.c | 57 ++++++++++++++++++++++++++++++-----
 1 file changed, 50 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_storage.=
c
index baf3566e23233..d37650a1966f4 100644
--- a/kernel/bpf/bpf_task_storage.c
+++ b/kernel/bpf/bpf_task_storage.c
@@ -20,6 +20,31 @@
=20
 DEFINE_BPF_STORAGE_CACHE(task_cache);
=20
+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) !=3D 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 =3D 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();
=20
 	/* 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;
 	}
=20
+	bpf_task_storage_lock();
 	sdata =3D 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 b=
pf_map *map, void *key,
 		goto out;
 	}
=20
+	bpf_task_storage_lock();
 	sdata =3D bpf_local_storage_update(
 		task, (struct bpf_local_storage_map *)map, value, map_flags);
+	bpf_task_storage_unlock();
=20
 	err =3D PTR_ERR_OR_ZERO(sdata);
 out:
@@ -185,7 +216,9 @@ static int bpf_pid_task_storage_delete_elem(struct bp=
f_map *map, void *key)
 		goto out;
 	}
=20
+	bpf_task_storage_lock();
 	err =3D 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;
=20
+	if (!bpf_task_storage_trylock())
+		return (unsigned long)NULL;
+
 	sdata =3D task_storage_lookup(task, map, true);
 	if (sdata)
-		return (unsigned long)sdata->data;
+		goto unlock;
=20
 	/* 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 =3D 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;
-	}
=20
-	return (unsigned long)NULL;
+unlock:
+	bpf_task_storage_unlock();
+	return IS_ERR_OR_NULL(sdata) ? (unsigned long)NULL :
+		(unsigned long)sdata->data;
 }
=20
 BPF_CALL_2(bpf_task_storage_delete, struct bpf_map *, map, struct task_s=
truct *,
 	   task)
 {
+	int ret;
+
 	if (!task)
 		return -EINVAL;
=20
+	if (!bpf_task_storage_trylock())
+		return -EBUSY;
+
 	/* This helper must only be called from places where the lifetime of th=
e 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 =3D task_storage_delete(task, map);
+	bpf_task_storage_unlock();
+	return ret;
 }
=20
 static int notsupp_get_next_key(struct bpf_map *map, void *key, void *ne=
xt_key)
--=20
2.24.1