kernel/bpf/devmap.c

   1 /* Copyright (c) 2017 Covalent IO, Inc. http://covalent.io
   2  *
   3  * This program is free software; you can redistribute it and/or
   4  * modify it under the terms of version 2 of the GNU General Public
   5  * License as published by the Free Software Foundation.
   6  *
   7  * This program is distributed in the hope that it will be useful, but
   8  * WITHOUT ANY WARRANTY; without even the implied warranty of
   9  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  10  * General Public License for more details.
  11  */
  12
  13 /* Devmaps primary use is as a backend map for XDP BPF helper call
  14  * bpf_redirect_map(). Because XDP is mostly concerned with performance we
  15  * spent some effort to ensure the datapath with redirect maps does not use
  16  * any locking. This is a quick note on the details.
  17  *
  18  * We have three possible paths to get into the devmap control plane bpf
  19  * syscalls, bpf programs, and driver side xmit/flush operations. A bpf syscall
  20  * will invoke an update, delete, or lookup operation. To ensure updates and
  21  * deletes appear atomic from the datapath side xchg() is used to modify the
  22  * netdev_map array. Then because the datapath does a lookup into the netdev_map
  23  * array (read-only) from an RCU critical section we use call_rcu() to wait for
  24  * an rcu grace period before free'ing the old data structures. This ensures the
  25  * datapath always has a valid copy. However, the datapath does a "flush"
  26  * operation that pushes any pending packets in the driver outside the RCU
  27  * critical section. Each bpf_dtab_netdev tracks these pending operations using
  28  * an atomic per-cpu bitmap. The bpf_dtab_netdev object will not be destroyed
  29  * until all bits are cleared indicating outstanding flush operations have
  30  * completed.
  31  *
  32  * BPF syscalls may race with BPF program calls on any of the update, delete
  33  * or lookup operations. As noted above the xchg() operation also keep the
  34  * netdev_map consistent in this case. From the devmap side BPF programs
  35  * calling into these operations are the same as multiple user space threads
  36  * making system calls.
  37  *
  38  * Finally, any of the above may race with a netdev_unregister notifier. The
  39  * unregister notifier must search for net devices in the map structure that
  40  * contain a reference to the net device and remove them. This is a two step
  41  * process (a) dereference the bpf_dtab_netdev object in netdev_map and (b)
  42  * check to see if the ifindex is the same as the net_device being removed.
  43  * When removing the dev a cmpxchg() is used to ensure the correct dev is
  44  * removed, in the case of a concurrent update or delete operation it is
  45  * possible that the initially referenced dev is no longer in the map. As the
  46  * notifier hook walks the map we know that new dev references can not be
  47  * added by the user because core infrastructure ensures dev_get_by_index()
  48  * calls will fail at this point.
  49  */
  50 #include <linux/bpf.h>
  51 #include <linux/filter.h>
  52
  53 #define DEV_CREATE_FLAG_MASK \
  54         (BPF_F_NUMA_NODE | BPF_F_RDONLY | BPF_F_WRONLY)
  55
  56 struct bpf_dtab_netdev {
  57         struct net_device *dev;
  58         struct bpf_dtab *dtab;
  59         unsigned int bit;
  60         struct rcu_head rcu;
  61 };
  62
  63 struct bpf_dtab {
  64         struct bpf_map map;
  65         struct bpf_dtab_netdev **netdev_map;
  66         unsigned long __percpu *flush_needed;
  67         struct list_head list;
  68 };
  69
  70 static DEFINE_SPINLOCK(dev_map_lock);
  71 static LIST_HEAD(dev_map_list);
  72
  73 static u64 dev_map_bitmap_size(const union bpf_attr *attr)
  74 {
  75         return BITS_TO_LONGS((u64) attr->max_entries) * sizeof(unsigned long);
  76 }
  77
  78 static struct bpf_map *dev_map_alloc(union bpf_attr *attr)
  79 {
  80         struct bpf_dtab *dtab;
  81         int err = -EINVAL;
  82         u64 cost;
  83
  84         if (!capable(CAP_NET_ADMIN))
  85                 return ERR_PTR(-EPERM);
  86
  87         /* check sanity of attributes */
  88         if (attr->max_entries == 0 || attr->key_size != 4 ||
  89             attr->value_size != 4 || attr->map_flags & ~DEV_CREATE_FLAG_MASK)
  90                 return ERR_PTR(-EINVAL);
  91
  92         dtab = kzalloc(sizeof(*dtab), GFP_USER);
  93         if (!dtab)
  94                 return ERR_PTR(-ENOMEM);
  95
  96         bpf_map_init_from_attr(&dtab->map, attr);
  97
  98         /* make sure page count doesn't overflow */
  99         cost = (u64) dtab->map.max_entries * sizeof(struct bpf_dtab_netdev *);
 100         cost += dev_map_bitmap_size(attr) * num_possible_cpus();
 101         if (cost >= U32_MAX - PAGE_SIZE)
 102                 goto free_dtab;
 103
 104         dtab->map.pages = round_up(cost, PAGE_SIZE) >> PAGE_SHIFT;
 105
 106         /* if map size is larger than memlock limit, reject it early */
 107         err = bpf_map_precharge_memlock(dtab->map.pages);
 108         if (err)
 109                 goto free_dtab;
 110
 111         err = -ENOMEM;
 112
 113         /* A per cpu bitfield with a bit per possible net device */
 114         dtab->flush_needed = __alloc_percpu_gfp(dev_map_bitmap_size(attr),
 115                                                 __alignof__(unsigned long),
 116                                                 GFP_KERNEL | __GFP_NOWARN);
 117         if (!dtab->flush_needed)
 118                 goto free_dtab;
 119
 120         dtab->netdev_map = bpf_map_area_alloc(dtab->map.max_entries *
 121                                               sizeof(struct bpf_dtab_netdev *),
 122                                               dtab->map.numa_node);
 123         if (!dtab->netdev_map)
 124                 goto free_dtab;
 125
 126         spin_lock(&dev_map_lock);
 127         list_add_tail_rcu(&dtab->list, &dev_map_list);
 128         spin_unlock(&dev_map_lock);
 129
 130         return &dtab->map;
 131 free_dtab:
 132         free_percpu(dtab->flush_needed);
 133         kfree(dtab);
 134         return ERR_PTR(err);
 135 }
 136
 137 static void dev_map_free(struct bpf_map *map)
 138 {
 139         struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
 140         int i, cpu;
 141
 142         /* At this point bpf_prog->aux->refcnt == 0 and this map->refcnt == 0,
 143          * so the programs (can be more than one that used this map) were
 144          * disconnected from events. Wait for outstanding critical sections in
 145          * these programs to complete. The rcu critical section only guarantees
 146          * no further reads against netdev_map. It does __not__ ensure pending
 147          * flush operations (if any) are complete.
 148          */
 149
 150         spin_lock(&dev_map_lock);
 151         list_del_rcu(&dtab->list);
 152         spin_unlock(&dev_map_lock);
 153
 154         synchronize_rcu();
 155
 156         /* To ensure all pending flush operations have completed wait for flush
 157          * bitmap to indicate all flush_needed bits to be zero on _all_ cpus.
 158          * Because the above synchronize_rcu() ensures the map is disconnected
 159          * from the program we can assume no new bits will be set.
 160          */
 161         for_each_online_cpu(cpu) {
 162                 unsigned long *bitmap = per_cpu_ptr(dtab->flush_needed, cpu);
 163
 164                 while (!bitmap_empty(bitmap, dtab->map.max_entries))
 165                         cond_resched();
 166         }
 167
 168         for (i = 0; i < dtab->map.max_entries; i++) {
 169                 struct bpf_dtab_netdev *dev;
 170
 171                 dev = dtab->netdev_map[i];
 172                 if (!dev)
 173                         continue;
 174
 175                 dev_put(dev->dev);
 176                 kfree(dev);
 177         }
 178
 179         free_percpu(dtab->flush_needed);
 180         bpf_map_area_free(dtab->netdev_map);
 181         kfree(dtab);
 182 }
 183
 184 static int dev_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
 185 {
 186         struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
 187         u32 index = key ? *(u32 *)key : U32_MAX;
 188         u32 *next = next_key;
 189
 190         if (index >= dtab->map.max_entries) {
 191                 *next = 0;
 192                 return 0;
 193         }
 194
 195         if (index == dtab->map.max_entries - 1)
 196                 return -ENOENT;
 197         *next = index + 1;
 198         return 0;
 199 }
 200
 201 void __dev_map_insert_ctx(struct bpf_map *map, u32 bit)
 202 {
 203         struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
 204         unsigned long *bitmap = this_cpu_ptr(dtab->flush_needed);
 205
 206         __set_bit(bit, bitmap);
 207 }
 208
 209 /* __dev_map_flush is called from xdp_do_flush_map() which _must_ be signaled
 210  * from the driver before returning from its napi->poll() routine. The poll()
 211  * routine is called either from busy_poll context or net_rx_action signaled
 212  * from NET_RX_SOFTIRQ. Either way the poll routine must complete before the
 213  * net device can be torn down. On devmap tear down we ensure the ctx bitmap
 214  * is zeroed before completing to ensure all flush operations have completed.
 215  */
 216 void __dev_map_flush(struct bpf_map *map)
 217 {
 218         struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
 219         unsigned long *bitmap = this_cpu_ptr(dtab->flush_needed);
 220         u32 bit;
 221
 222         for_each_set_bit(bit, bitmap, map->max_entries) {
 223                 struct bpf_dtab_netdev *dev = READ_ONCE(dtab->netdev_map[bit]);
 224                 struct net_device *netdev;
 225
 226                 /* This is possible if the dev entry is removed by user space
 227                  * between xdp redirect and flush op.
 228                  */
 229                 if (unlikely(!dev))
 230                         continue;
 231
 232                 __clear_bit(bit, bitmap);
 233                 netdev = dev->dev;
 234                 if (likely(netdev->netdev_ops->ndo_xdp_flush))
 235                         netdev->netdev_ops->ndo_xdp_flush(netdev);
 236         }
 237 }
 238
 239 /* rcu_read_lock (from syscall and BPF contexts) ensures that if a delete and/or
 240  * update happens in parallel here a dev_put wont happen until after reading the
 241  * ifindex.
 242  */
 243 struct net_device  *__dev_map_lookup_elem(struct bpf_map *map, u32 key)
 244 {
 245         struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
 246         struct bpf_dtab_netdev *dev;
 247
 248         if (key >= map->max_entries)
 249                 return NULL;
 250
 251         dev = READ_ONCE(dtab->netdev_map[key]);
 252         return dev ? dev->dev : NULL;
 253 }
 254
 255 static void *dev_map_lookup_elem(struct bpf_map *map, void *key)
 256 {
 257         struct net_device *dev = __dev_map_lookup_elem(map, *(u32 *)key);
 258
 259         return dev ? &dev->ifindex : NULL;
 260 }
 261
 262 static void dev_map_flush_old(struct bpf_dtab_netdev *dev)
 263 {
 264         if (dev->dev->netdev_ops->ndo_xdp_flush) {
 265                 struct net_device *fl = dev->dev;
 266                 unsigned long *bitmap;
 267                 int cpu;
 268
 269                 for_each_online_cpu(cpu) {
 270                         bitmap = per_cpu_ptr(dev->dtab->flush_needed, cpu);
 271                         __clear_bit(dev->bit, bitmap);
 272
 273                         fl->netdev_ops->ndo_xdp_flush(dev->dev);
 274                 }
 275         }
 276 }
 277
 278 static void __dev_map_entry_free(struct rcu_head *rcu)
 279 {
 280         struct bpf_dtab_netdev *dev;
 281
 282         dev = container_of(rcu, struct bpf_dtab_netdev, rcu);
 283         dev_map_flush_old(dev);
 284         dev_put(dev->dev);
 285         kfree(dev);
 286 }
 287
 288 static int dev_map_delete_elem(struct bpf_map *map, void *key)
 289 {
 290         struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
 291         struct bpf_dtab_netdev *old_dev;
 292         int k = *(u32 *)key;
 293
 294         if (k >= map->max_entries)
 295                 return -EINVAL;
 296
 297         /* Use call_rcu() here to ensure any rcu critical sections have
 298          * completed, but this does not guarantee a flush has happened
 299          * yet. Because driver side rcu_read_lock/unlock only protects the
 300          * running XDP program. However, for pending flush operations the
 301          * dev and ctx are stored in another per cpu map. And additionally,
 302          * the driver tear down ensures all soft irqs are complete before
 303          * removing the net device in the case of dev_put equals zero.
 304          */
 305         old_dev = xchg(&dtab->netdev_map[k], NULL);
 306         if (old_dev)
 307                 call_rcu(&old_dev->rcu, __dev_map_entry_free);
 308         return 0;
 309 }
 310
 311 static int dev_map_update_elem(struct bpf_map *map, void *key, void *value,
 312                                 u64 map_flags)
 313 {
 314         struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
 315         struct net *net = current->nsproxy->net_ns;
 316         struct bpf_dtab_netdev *dev, *old_dev;
 317         u32 i = *(u32 *)key;
 318         u32 ifindex = *(u32 *)value;
 319
 320         if (unlikely(map_flags > BPF_EXIST))
 321                 return -EINVAL;
 322         if (unlikely(i >= dtab->map.max_entries))
 323                 return -E2BIG;
 324         if (unlikely(map_flags == BPF_NOEXIST))
 325                 return -EEXIST;
 326
 327         if (!ifindex) {
 328                 dev = NULL;
 329         } else {
 330                 dev = kmalloc_node(sizeof(*dev), GFP_ATOMIC | __GFP_NOWARN,
 331                                    map->numa_node);
 332                 if (!dev)
 333                         return -ENOMEM;
 334
 335                 dev->dev = dev_get_by_index(net, ifindex);
 336                 if (!dev->dev) {
 337                         kfree(dev);
 338                         return -EINVAL;
 339                 }
 340
 341                 dev->bit = i;
 342                 dev->dtab = dtab;
 343         }
 344
 345         /* Use call_rcu() here to ensure rcu critical sections have completed
 346          * Remembering the driver side flush operation will happen before the
 347          * net device is removed.
 348          */
 349         old_dev = xchg(&dtab->netdev_map[i], dev);
 350         if (old_dev)
 351                 call_rcu(&old_dev->rcu, __dev_map_entry_free);
 352
 353         return 0;
 354 }
 355
 356 const struct bpf_map_ops dev_map_ops = {
 357         .map_alloc = dev_map_alloc,
 358         .map_free = dev_map_free,
 359         .map_get_next_key = dev_map_get_next_key,
 360         .map_lookup_elem = dev_map_lookup_elem,
 361         .map_update_elem = dev_map_update_elem,
 362         .map_delete_elem = dev_map_delete_elem,
 363 };
 364
 365 static int dev_map_notification(struct notifier_block *notifier,
 366                                 ulong event, void *ptr)
 367 {
 368         struct net_device *netdev = netdev_notifier_info_to_dev(ptr);
 369         struct bpf_dtab *dtab;
 370         int i;
 371
 372         switch (event) {
 373         case NETDEV_UNREGISTER:
 374                 /* This rcu_read_lock/unlock pair is needed because
 375                  * dev_map_list is an RCU list AND to ensure a delete
 376                  * operation does not free a netdev_map entry while we
 377                  * are comparing it against the netdev being unregistered.
 378                  */
 379                 rcu_read_lock();
 380                 list_for_each_entry_rcu(dtab, &dev_map_list, list) {
 381                         for (i = 0; i < dtab->map.max_entries; i++) {
 382                                 struct bpf_dtab_netdev *dev, *odev;
 383
 384                                 dev = READ_ONCE(dtab->netdev_map[i]);
 385                                 if (!dev ||
 386                                     dev->dev->ifindex != netdev->ifindex)
 387                                         continue;
 388                                 odev = cmpxchg(&dtab->netdev_map[i], dev, NULL);
 389                                 if (dev == odev)
 390                                         call_rcu(&dev->rcu,
 391                                                  __dev_map_entry_free);
 392                         }
 393                 }
 394                 rcu_read_unlock();
 395                 break;
 396         default:
 397                 break;
 398         }
 399         return NOTIFY_OK;
 400 }
 401
 402 static struct notifier_block dev_map_notifier = {
 403         .notifier_call = dev_map_notification,
 404 };
 405
 406 static int __init dev_map_init(void)
 407 {
 408         register_netdevice_notifier(&dev_map_notifier);
 409         return 0;
 410 }
 411
 412 subsys_initcall(dev_map_init);