[PATCH bpf-next v4 1/4] bpf: Protect the struct_ops association with a spinlock

[Qiliang Yuan <odys.yuan@gmail.com>]

bpf_prog_assoc_struct_ops() and bpf_prog_disassoc_struct_ops() update
prog->aux->st_ops_assoc under st_ops_assoc_mutex, and the latter puts
the map of a non-struct_ops prog while holding it.

A struct_ops map is about to dissociate its progs from
map_free_pre_rcu(), which bpf_map_put() calls in whatever context drops
the last reference, including softirq, e.g. bpf_struct_ops_put() from
tcp_cleanup_congestion_control(). A mutex can't be taken there, and the
put of the last reference under the lock of one prog would take the
lock of other progs.

Replace the mutex with a spinlock taken with interrupts disabled, and
put the map after dropping it.

Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
---
 include/linux/bpf.h         |  2 +-
 kernel/bpf/bpf_struct_ops.c | 19 ++++++++++---------
 kernel/bpf/core.c           |  3 +--
 3 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 54144372281cb..2a51acd34f9a9 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1960,7 +1960,7 @@ struct bpf_prog_aux {
 		struct rcu_head	rcu;
 	};
 	struct bpf_stream *stream[2];
-	struct mutex st_ops_assoc_mutex;
+	spinlock_t st_ops_assoc_lock; /* protects st_ops_assoc */
 	struct bpf_map __rcu *st_ops_assoc;
 };
 
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 1178acd72296e..a1ae30b145a03 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -1541,10 +1541,10 @@ int bpf_prog_assoc_struct_ops(struct bpf_prog *prog, struct bpf_map *map)
 {
 	struct bpf_map *st_ops_assoc;
 
-	guard(mutex)(&prog->aux->st_ops_assoc_mutex);
+	guard(spinlock_irqsave)(&prog->aux->st_ops_assoc_lock);
 
 	st_ops_assoc = rcu_dereference_protected(prog->aux->st_ops_assoc,
-						 lockdep_is_held(&prog->aux->st_ops_assoc_mutex));
+						 lockdep_is_held(&prog->aux->st_ops_assoc_lock));
 	if (st_ops_assoc && st_ops_assoc == map)
 		return 0;
 
@@ -1569,19 +1569,20 @@ int bpf_prog_assoc_struct_ops(struct bpf_prog *prog, struct bpf_map *map)
 
 void bpf_prog_disassoc_struct_ops(struct bpf_prog *prog)
 {
+	struct bpf_prog_aux *aux = prog->aux;
 	struct bpf_map *st_ops_assoc;
 
-	guard(mutex)(&prog->aux->st_ops_assoc_mutex);
+	scoped_guard(spinlock_irqsave, &aux->st_ops_assoc_lock) {
+		st_ops_assoc = rcu_dereference_protected(aux->st_ops_assoc,
+							 lockdep_is_held(&aux->st_ops_assoc_lock));
+		if (!st_ops_assoc || st_ops_assoc == BPF_PTR_POISON)
+			return;
 
-	st_ops_assoc = rcu_dereference_protected(prog->aux->st_ops_assoc,
-						 lockdep_is_held(&prog->aux->st_ops_assoc_mutex));
-	if (!st_ops_assoc || st_ops_assoc == BPF_PTR_POISON)
-		return;
+		RCU_INIT_POINTER(aux->st_ops_assoc, NULL);
+	}
 
 	if (prog->type != BPF_PROG_TYPE_STRUCT_OPS)
 		bpf_map_put(st_ops_assoc);
-
-	RCU_INIT_POINTER(prog->aux->st_ops_assoc, NULL);
 }
 
 /*
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 05c89396119ad..fd4e4dc4acc8c 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -141,7 +141,7 @@ struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flag
 	mutex_init(&fp->aux->used_maps_mutex);
 	mutex_init(&fp->aux->ext_mutex);
 	mutex_init(&fp->aux->dst_mutex);
-	mutex_init(&fp->aux->st_ops_assoc_mutex);
+	spin_lock_init(&fp->aux->st_ops_assoc_lock);
 
 	return fp;
 }
@@ -291,7 +291,6 @@ void __bpf_prog_free(struct bpf_prog *fp)
 #endif
 		mutex_destroy(&fp->aux->used_maps_mutex);
 		mutex_destroy(&fp->aux->dst_mutex);
-		mutex_destroy(&fp->aux->st_ops_assoc_mutex);
 		kfree(fp->aux->poke_tab);
 		kfree(fp->aux);
 	}

-- 
2.43.0