bpf: Add poke dependency tracking for prog array maps

This work adds program tracking to prog array maps. This is needed such
that upon prog array updates/deletions we can fix up all programs which
make use of this tail call map. We add ops->map_poke_{un,}track()
helpers to maps to maintain the list of programs and ops->map_poke_run()
for triggering the actual update.

bpf_array_aux is extended to contain the list head and poke_mutex in
order to serialize program patching during updates/deletions.
bpf_free_used_maps() will untrack the program shortly before dropping
the reference to the map. For clearing out the prog array once all urefs
are dropped we need to use schedule_work() to have a sleepable context.

The prog_array_map_poke_run() is triggered during updates/deletions and
walks the maintained prog list. It checks in their poke_tabs whether the
map and key is matching and runs the actual bpf_arch_text_poke() for
patching in the nop or new jmp location. Depending on the type of update,
we use one of BPF_MOD_{NOP_TO_JUMP,JUMP_TO_NOP,JUMP_TO_JUMP}.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/1fb364bb3c565b3e415d5ea348f036ff379e779d.1574452833.git.daniel@iogearbox.net

1 files changed, 179 insertions, 4 deletions

diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index 57da950ee55b..58bdf5fd24cc 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -586,10 +586,17 @@ int bpf_fd_array_map_update_elem(struct bpf_map *map, struct file *map_file,
 	if (IS_ERR(new_ptr))
 		return PTR_ERR(new_ptr);
 
-	old_ptr = xchg(array->ptrs + index, new_ptr);
+	if (map->ops->map_poke_run) {
+		mutex_lock(&array->aux->poke_mutex);
+		old_ptr = xchg(array->ptrs + index, new_ptr);
+		map->ops->map_poke_run(map, index, old_ptr, new_ptr);
+		mutex_unlock(&array->aux->poke_mutex);
+	} else {
+		old_ptr = xchg(array->ptrs + index, new_ptr);
+	}
+
 	if (old_ptr)
 		map->ops->map_fd_put_ptr(old_ptr);
-
 	return 0;
 }
 
@@ -602,7 +609,15 @@ static int fd_array_map_delete_elem(struct bpf_map *map, void *key)
 	if (index >= array->map.max_entries)
 		return -E2BIG;
 
-	old_ptr = xchg(array->ptrs + index, NULL);
+	if (map->ops->map_poke_run) {
+		mutex_lock(&array->aux->poke_mutex);
+		old_ptr = xchg(array->ptrs + index, NULL);
+		map->ops->map_poke_run(map, index, old_ptr, NULL);
+		mutex_unlock(&array->aux->poke_mutex);
+	} else {
+		old_ptr = xchg(array->ptrs + index, NULL);
+	}
+
 	if (old_ptr) {
 		map->ops->map_fd_put_ptr(old_ptr);
 		return 0;
@@ -671,6 +686,152 @@ static void prog_array_map_seq_show_elem(struct bpf_map *map, void *key,
 	rcu_read_unlock();
 }
 
+struct prog_poke_elem {
+	struct list_head list;
+	struct bpf_prog_aux *aux;
+};
+
+static int prog_array_map_poke_track(struct bpf_map *map,
+				     struct bpf_prog_aux *prog_aux)
+{
+	struct prog_poke_elem *elem;
+	struct bpf_array_aux *aux;
+	int ret = 0;
+
+	aux = container_of(map, struct bpf_array, map)->aux;
+	mutex_lock(&aux->poke_mutex);
+	list_for_each_entry(elem, &aux->poke_progs, list) {
+		if (elem->aux == prog_aux)
+			goto out;
+	}
+
+	elem = kmalloc(sizeof(*elem), GFP_KERNEL);
+	if (!elem) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	INIT_LIST_HEAD(&elem->list);
+	/* We must track the program's aux info at this point in time
+	 * since the program pointer itself may not be stable yet, see
+	 * also comment in prog_array_map_poke_run().
+	 */
+	elem->aux = prog_aux;
+
+	list_add_tail(&elem->list, &aux->poke_progs);
+out:
+	mutex_unlock(&aux->poke_mutex);
+	return ret;
+}
+
+static void prog_array_map_poke_untrack(struct bpf_map *map,
+					struct bpf_prog_aux *prog_aux)
+{
+	struct prog_poke_elem *elem, *tmp;
+	struct bpf_array_aux *aux;
+
+	aux = container_of(map, struct bpf_array, map)->aux;
+	mutex_lock(&aux->poke_mutex);
+	list_for_each_entry_safe(elem, tmp, &aux->poke_progs, list) {
+		if (elem->aux == prog_aux) {
+			list_del_init(&elem->list);
+			kfree(elem);
+			break;
+		}
+	}
+	mutex_unlock(&aux->poke_mutex);
+}
+
+static void prog_array_map_poke_run(struct bpf_map *map, u32 key,
+				    struct bpf_prog *old,
+				    struct bpf_prog *new)
+{
+	enum bpf_text_poke_type type;
+	struct prog_poke_elem *elem;
+	struct bpf_array_aux *aux;
+
+	if (!old && new)
+		type = BPF_MOD_NOP_TO_JUMP;
+	else if (old && !new)
+		type = BPF_MOD_JUMP_TO_NOP;
+	else if (old && new)
+		type = BPF_MOD_JUMP_TO_JUMP;
+	else
+		return;
+
+	aux = container_of(map, struct bpf_array, map)->aux;
+	WARN_ON_ONCE(!mutex_is_locked(&aux->poke_mutex));
+
+	list_for_each_entry(elem, &aux->poke_progs, list) {
+		struct bpf_jit_poke_descriptor *poke;
+		int i, ret;
+
+		for (i = 0; i < elem->aux->size_poke_tab; i++) {
+			poke = &elem->aux->poke_tab[i];
+
+			/* Few things to be aware of:
+			 *
+			 * 1) We can only ever access aux in this context, but
+			 *    not aux->prog since it might not be stable yet and
+			 *    there could be danger of use after free otherwise.
+			 * 2) Initially when we start tracking aux, the program
+			 *    is not JITed yet and also does not have a kallsyms
+			 *    entry. We skip these as poke->ip_stable is not
+			 *    active yet. The JIT will do the final fixup before
+			 *    setting it stable. The various poke->ip_stable are
+			 *    successively activated, so tail call updates can
+			 *    arrive from here while JIT is still finishing its
+			 *    final fixup for non-activated poke entries.
+			 * 3) On program teardown, the program's kallsym entry gets
+			 *    removed out of RCU callback, but we can only untrack
+			 *    from sleepable context, therefore bpf_arch_text_poke()
+			 *    might not see that this is in BPF text section and
+			 *    bails out with -EINVAL. As these are unreachable since
+			 *    RCU grace period already passed, we simply skip them.
+			 * 4) Also programs reaching refcount of zero while patching
+			 *    is in progress is okay since we're protected under
+			 *    poke_mutex and untrack the programs before the JIT
+			 *    buffer is freed. When we're still in the middle of
+			 *    patching and suddenly kallsyms entry of the program
+			 *    gets evicted, we just skip the rest which is fine due
+			 *    to point 3).
+			 * 5) Any other error happening below from bpf_arch_text_poke()
+			 *    is a unexpected bug.
+			 */
+			if (!READ_ONCE(poke->ip_stable))
+				continue;
+			if (poke->reason != BPF_POKE_REASON_TAIL_CALL)
+				continue;
+			if (poke->tail_call.map != map ||
+			    poke->tail_call.key != key)
+				continue;
+
+			ret = bpf_arch_text_poke(poke->ip, type,
+						 old ? (u8 *)old->bpf_func +
+						 poke->adj_off : NULL,
+						 new ? (u8 *)new->bpf_func +
+						 poke->adj_off : NULL);
+			BUG_ON(ret < 0 && ret != -EINVAL);
+		}
+	}
+}
+
+static void prog_array_map_clear_deferred(struct work_struct *work)
+{
+	struct bpf_map *map = container_of(work, struct bpf_array_aux,
+					   work)->map;
+	bpf_fd_array_map_clear(map);
+	bpf_map_put(map);
+}
+
+static void prog_array_map_clear(struct bpf_map *map)
+{
+	struct bpf_array_aux *aux = container_of(map, struct bpf_array,
+						 map)->aux;
+	bpf_map_inc(map);
+	schedule_work(&aux->work);
+}
+
 static struct bpf_map *prog_array_map_alloc(union bpf_attr *attr)
 {
 	struct bpf_array_aux *aux;
@@ -680,6 +841,10 @@ static struct bpf_map *prog_array_map_alloc(union bpf_attr *attr)
 	if (!aux)
 		return ERR_PTR(-ENOMEM);
 
+	INIT_WORK(&aux->work, prog_array_map_clear_deferred);
+	INIT_LIST_HEAD(&aux->poke_progs);
+	mutex_init(&aux->poke_mutex);
+
 	map = array_map_alloc(attr);
 	if (IS_ERR(map)) {
 		kfree(aux);
@@ -687,14 +852,21 @@ static struct bpf_map *prog_array_map_alloc(union bpf_attr *attr)
 	}
 
 	container_of(map, struct bpf_array, map)->aux = aux;
+	aux->map = map;
+
 	return map;
 }
 
 static void prog_array_map_free(struct bpf_map *map)
 {
+	struct prog_poke_elem *elem, *tmp;
 	struct bpf_array_aux *aux;
 
 	aux = container_of(map, struct bpf_array, map)->aux;
+	list_for_each_entry_safe(elem, tmp, &aux->poke_progs, list) {
+		list_del_init(&elem->list);
+		kfree(elem);
+	}
 	kfree(aux);
 	fd_array_map_free(map);
 }
@@ -703,13 +875,16 @@ const struct bpf_map_ops prog_array_map_ops = {
 	.map_alloc_check = fd_array_map_alloc_check,
 	.map_alloc = prog_array_map_alloc,
 	.map_free = prog_array_map_free,
+	.map_poke_track = prog_array_map_poke_track,
+	.map_poke_untrack = prog_array_map_poke_untrack,
+	.map_poke_run = prog_array_map_poke_run,
 	.map_get_next_key = array_map_get_next_key,
 	.map_lookup_elem = fd_array_map_lookup_elem,
 	.map_delete_elem = fd_array_map_delete_elem,
 	.map_fd_get_ptr = prog_fd_array_get_ptr,
 	.map_fd_put_ptr = prog_fd_array_put_ptr,
 	.map_fd_sys_lookup_elem = prog_fd_array_sys_lookup_elem,
-	.map_release_uref = bpf_fd_array_map_clear,
+	.map_release_uref = prog_array_map_clear,
 	.map_seq_show_elem = prog_array_map_seq_show_elem,
 };