1 files changed, 124 insertions, 50 deletions

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index a1468e3f5af2..d541c8486c95 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -27,9 +27,62 @@
 	.map_delete_batch =			\
 	generic_map_delete_batch
 
+/*
+ * The bucket lock has two protection scopes:
+ *
+ * 1) Serializing concurrent operations from BPF programs on differrent
+ *    CPUs
+ *
+ * 2) Serializing concurrent operations from BPF programs and sys_bpf()
+ *
+ * BPF programs can execute in any context including perf, kprobes and
+ * tracing. As there are almost no limits where perf, kprobes and tracing
+ * can be invoked from the lock operations need to be protected against
+ * deadlocks. Deadlocks can be caused by recursion and by an invocation in
+ * the lock held section when functions which acquire this lock are invoked
+ * from sys_bpf(). BPF recursion is prevented by incrementing the per CPU
+ * variable bpf_prog_active, which prevents BPF programs attached to perf
+ * events, kprobes and tracing to be invoked before the prior invocation
+ * from one of these contexts completed. sys_bpf() uses the same mechanism
+ * by pinning the task to the current CPU and incrementing the recursion
+ * protection accross the map operation.
+ *
+ * This has subtle implications on PREEMPT_RT. PREEMPT_RT forbids certain
+ * operations like memory allocations (even with GFP_ATOMIC) from atomic
+ * contexts. This is required because even with GFP_ATOMIC the memory
+ * allocator calls into code pathes which acquire locks with long held lock
+ * sections. To ensure the deterministic behaviour these locks are regular
+ * spinlocks, which are converted to 'sleepable' spinlocks on RT. The only
+ * true atomic contexts on an RT kernel are the low level hardware
+ * handling, scheduling, low level interrupt handling, NMIs etc. None of
+ * these contexts should ever do memory allocations.
+ *
+ * As regular device interrupt handlers and soft interrupts are forced into
+ * thread context, the existing code which does
+ *   spin_lock*(); alloc(GPF_ATOMIC); spin_unlock*();
+ * just works.
+ *
+ * In theory the BPF locks could be converted to regular spinlocks as well,
+ * but the bucket locks and percpu_freelist locks can be taken from
+ * arbitrary contexts (perf, kprobes, tracepoints) which are required to be
+ * atomic contexts even on RT. These mechanisms require preallocated maps,
+ * so there is no need to invoke memory allocations within the lock held
+ * sections.
+ *
+ * BPF maps which need dynamic allocation are only used from (forced)
+ * thread context on RT and can therefore use regular spinlocks which in
+ * turn allows to invoke memory allocations from the lock held section.
+ *
+ * On a non RT kernel this distinction is neither possible nor required.
+ * spinlock maps to raw_spinlock and the extra code is optimized out by the
+ * compiler.
+ */
 struct bucket {
 	struct hlist_nulls_head head;
-	raw_spinlock_t lock;
+	union {
+		raw_spinlock_t raw_lock;
+		spinlock_t     lock;
+	};
 };
 
 struct bpf_htab {
@@ -65,9 +118,54 @@ struct htab_elem {
 		struct bpf_lru_node lru_node;
 	};
 	u32 hash;
-	char key[0] __aligned(8);
+	char key[] __aligned(8);
 };
 
+static inline bool htab_is_prealloc(const struct bpf_htab *htab)
+{
+	return !(htab->map.map_flags & BPF_F_NO_PREALLOC);
+}
+
+static inline bool htab_use_raw_lock(const struct bpf_htab *htab)
+{
+	return (!IS_ENABLED(CONFIG_PREEMPT_RT) || htab_is_prealloc(htab));
+}
+
+static void htab_init_buckets(struct bpf_htab *htab)
+{
+	unsigned i;
+
+	for (i = 0; i < htab->n_buckets; i++) {
+		INIT_HLIST_NULLS_HEAD(&htab->buckets[i].head, i);
+		if (htab_use_raw_lock(htab))
+			raw_spin_lock_init(&htab->buckets[i].raw_lock);
+		else
+			spin_lock_init(&htab->buckets[i].lock);
+	}
+}
+
+static inline unsigned long htab_lock_bucket(const struct bpf_htab *htab,
+					     struct bucket *b)
+{
+	unsigned long flags;
+
+	if (htab_use_raw_lock(htab))
+		raw_spin_lock_irqsave(&b->raw_lock, flags);
+	else
+		spin_lock_irqsave(&b->lock, flags);
+	return flags;
+}
+
+static inline void htab_unlock_bucket(const struct bpf_htab *htab,
+				      struct bucket *b,
+				      unsigned long flags)
+{
+	if (htab_use_raw_lock(htab))
+		raw_spin_unlock_irqrestore(&b->raw_lock, flags);
+	else
+		spin_unlock_irqrestore(&b->lock, flags);
+}
+
 static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node);
 
 static bool htab_is_lru(const struct bpf_htab *htab)
@@ -82,11 +180,6 @@ static bool htab_is_percpu(const struct bpf_htab *htab)
 		htab->map.map_type == BPF_MAP_TYPE_LRU_PERCPU_HASH;
 }
 
-static bool htab_is_prealloc(const struct bpf_htab *htab)
-{
-	return !(htab->map.map_flags & BPF_F_NO_PREALLOC);
-}
-
 static inline void htab_elem_set_ptr(struct htab_elem *l, u32 key_size,
 				     void __percpu *pptr)
 {
@@ -328,8 +421,8 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
 	bool percpu_lru = (attr->map_flags & BPF_F_NO_COMMON_LRU);
 	bool prealloc = !(attr->map_flags & BPF_F_NO_PREALLOC);
 	struct bpf_htab *htab;
-	int err, i;
 	u64 cost;
+	int err;
 
 	htab = kzalloc(sizeof(*htab), GFP_USER);
 	if (!htab)
@@ -391,10 +484,7 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
 	else
 		htab->hashrnd = get_random_int();
 
-	for (i = 0; i < htab->n_buckets; i++) {
-		INIT_HLIST_NULLS_HEAD(&htab->buckets[i].head, i);
-		raw_spin_lock_init(&htab->buckets[i].lock);
-	}
+	htab_init_buckets(htab);
 
 	if (prealloc) {
 		err = prealloc_init(htab);
@@ -602,7 +692,7 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
 	b = __select_bucket(htab, tgt_l->hash);
 	head = &b->head;
 
-	raw_spin_lock_irqsave(&b->lock, flags);
+	flags = htab_lock_bucket(htab, b);
 
 	hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
 		if (l == tgt_l) {
@@ -610,7 +700,7 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
 			break;
 		}
 
-	raw_spin_unlock_irqrestore(&b->lock, flags);
+	htab_unlock_bucket(htab, b, flags);
 
 	return l == tgt_l;
 }
@@ -686,15 +776,7 @@ static void htab_elem_free_rcu(struct rcu_head *head)
 	struct htab_elem *l = container_of(head, struct htab_elem, rcu);
 	struct bpf_htab *htab = l->htab;
 
-	/* must increment bpf_prog_active to avoid kprobe+bpf triggering while
-	 * we're calling kfree, otherwise deadlock is possible if kprobes
-	 * are placed somewhere inside of slub
-	 */
-	preempt_disable();
-	__this_cpu_inc(bpf_prog_active);
 	htab_elem_free(htab, l);
-	__this_cpu_dec(bpf_prog_active);
-	preempt_enable();
 }
 
 static void free_htab_elem(struct bpf_htab *htab, struct htab_elem *l)
@@ -884,8 +966,7 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
 		 */
 	}
 
-	/* bpf_map_update_elem() can be called in_irq() */
-	raw_spin_lock_irqsave(&b->lock, flags);
+	flags = htab_lock_bucket(htab, b);
 
 	l_old = lookup_elem_raw(head, hash, key, key_size);
 
@@ -926,7 +1007,7 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
 	}
 	ret = 0;
 err:
-	raw_spin_unlock_irqrestore(&b->lock, flags);
+	htab_unlock_bucket(htab, b, flags);
 	return ret;
 }
 
@@ -964,8 +1045,7 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
 		return -ENOMEM;
 	memcpy(l_new->key + round_up(map->key_size, 8), value, map->value_size);
 
-	/* bpf_map_update_elem() can be called in_irq() */
-	raw_spin_lock_irqsave(&b->lock, flags);
+	flags = htab_lock_bucket(htab, b);
 
 	l_old = lookup_elem_raw(head, hash, key, key_size);
 
@@ -984,7 +1064,7 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
 	ret = 0;
 
 err:
-	raw_spin_unlock_irqrestore(&b->lock, flags);
+	htab_unlock_bucket(htab, b, flags);
 
 	if (ret)
 		bpf_lru_push_free(&htab->lru, &l_new->lru_node);
@@ -1019,8 +1099,7 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
 	b = __select_bucket(htab, hash);
 	head = &b->head;
 
-	/* bpf_map_update_elem() can be called in_irq() */
-	raw_spin_lock_irqsave(&b->lock, flags);
+	flags = htab_lock_bucket(htab, b);
 
 	l_old = lookup_elem_raw(head, hash, key, key_size);
 
@@ -1043,7 +1122,7 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
 	}
 	ret = 0;
 err:
-	raw_spin_unlock_irqrestore(&b->lock, flags);
+	htab_unlock_bucket(htab, b, flags);
 	return ret;
 }
 
@@ -1083,8 +1162,7 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
 			return -ENOMEM;
 	}
 
-	/* bpf_map_update_elem() can be called in_irq() */
-	raw_spin_lock_irqsave(&b->lock, flags);
+	flags = htab_lock_bucket(htab, b);
 
 	l_old = lookup_elem_raw(head, hash, key, key_size);
 
@@ -1106,7 +1184,7 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
 	}
 	ret = 0;
 err:
-	raw_spin_unlock_irqrestore(&b->lock, flags);
+	htab_unlock_bucket(htab, b, flags);
 	if (l_new)
 		bpf_lru_push_free(&htab->lru, &l_new->lru_node);
 	return ret;
@@ -1144,7 +1222,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
 	b = __select_bucket(htab, hash);
 	head = &b->head;
 
-	raw_spin_lock_irqsave(&b->lock, flags);
+	flags = htab_lock_bucket(htab, b);
 
 	l = lookup_elem_raw(head, hash, key, key_size);
 
@@ -1154,7 +1232,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
 		ret = 0;
 	}
 
-	raw_spin_unlock_irqrestore(&b->lock, flags);
+	htab_unlock_bucket(htab, b, flags);
 	return ret;
 }
 
@@ -1176,7 +1254,7 @@ static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
 	b = __select_bucket(htab, hash);
 	head = &b->head;
 
-	raw_spin_lock_irqsave(&b->lock, flags);
+	flags = htab_lock_bucket(htab, b);
 
 	l = lookup_elem_raw(head, hash, key, key_size);
 
@@ -1185,7 +1263,7 @@ static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
 		ret = 0;
 	}
 
-	raw_spin_unlock_irqrestore(&b->lock, flags);
+	htab_unlock_bucket(htab, b, flags);
 	if (l)
 		bpf_lru_push_free(&htab->lru, &l->lru_node);
 	return ret;
@@ -1325,8 +1403,7 @@ alloc:
 	}
 
 again:
-	preempt_disable();
-	this_cpu_inc(bpf_prog_active);
+	bpf_disable_instrumentation();
 	rcu_read_lock();
 again_nocopy:
 	dst_key = keys;
@@ -1335,7 +1412,7 @@ again_nocopy:
 	head = &b->head;
 	/* do not grab the lock unless need it (bucket_cnt > 0). */
 	if (locked)
-		raw_spin_lock_irqsave(&b->lock, flags);
+		flags = htab_lock_bucket(htab, b);
 
 	bucket_cnt = 0;
 	hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
@@ -1352,10 +1429,9 @@ again_nocopy:
 		/* Note that since bucket_cnt > 0 here, it is implicit
 		 * that the locked was grabbed, so release it.
 		 */
-		raw_spin_unlock_irqrestore(&b->lock, flags);
+		htab_unlock_bucket(htab, b, flags);
 		rcu_read_unlock();
-		this_cpu_dec(bpf_prog_active);
-		preempt_enable();
+		bpf_enable_instrumentation();
 		goto after_loop;
 	}
 
@@ -1364,10 +1440,9 @@ again_nocopy:
 		/* Note that since bucket_cnt > 0 here, it is implicit
 		 * that the locked was grabbed, so release it.
 		 */
-		raw_spin_unlock_irqrestore(&b->lock, flags);
+		htab_unlock_bucket(htab, b, flags);
 		rcu_read_unlock();
-		this_cpu_dec(bpf_prog_active);
-		preempt_enable();
+		bpf_enable_instrumentation();
 		kvfree(keys);
 		kvfree(values);
 		goto alloc;
@@ -1418,7 +1493,7 @@ again_nocopy:
 		dst_val += value_size;
 	}
 
-	raw_spin_unlock_irqrestore(&b->lock, flags);
+	htab_unlock_bucket(htab, b, flags);
 	locked = false;
 
 	while (node_to_free) {
@@ -1437,8 +1512,7 @@ next_batch:
 	}
 
 	rcu_read_unlock();
-	this_cpu_dec(bpf_prog_active);
-	preempt_enable();
+	bpf_enable_instrumentation();
 	if (bucket_cnt && (copy_to_user(ukeys + total * key_size, keys,
 	    key_size * bucket_cnt) ||
 	    copy_to_user(uvalues + total * value_size, values,