bpf: convert htab map to hlist_nulls

when all map elements are pre-allocated one cpu can delete and reuse htab_elem
while another cpu is still walking the hlist. In such case the lookup may
miss the element. Convert hlist to hlist_nulls to avoid such scenario.
When bucket lock is taken there is no need to take such precautions,
so only convert map_lookup and map_get_next to nulls.
The race window is extremely small and only reproducible with explicit
udelay() inside lookup_nulls_elem_raw()

Similar to hlist add hlist_nulls_for_each_entry_safe() and
hlist_nulls_entry_safe() helpers.

Fixes: 6c9059817432 ("bpf: pre-allocate hash map elements")
Reported-by: Jonathan Perry <jonperry@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 79 insertions, 34 deletions

diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
index b01fe1009084..87ff4f58a2f0 100644
--- a/include/linux/list_nulls.h
+++ b/include/linux/list_nulls.h
@@ -29,6 +29,11 @@ struct hlist_nulls_node {
         ((ptr)->first = (struct hlist_nulls_node *) NULLS_MARKER(nulls))
 
 #define hlist_nulls_entry(ptr, type, member) container_of(ptr,type,member)
+
+#define hlist_nulls_entry_safe(ptr, type, member) \
+        ({ typeof(ptr) ____ptr = (ptr); \
+           !is_a_nulls(____ptr) ? hlist_nulls_entry(____ptr, type, member) : NULL; \
+        })
 /**
  * ptr_is_a_nulls - Test if a ptr is a nulls
  * @ptr: ptr to be tested
diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index 4ae95f7e8597..a23a33153180 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -156,5 +156,19 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
                 ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \
                 pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)))
 
+/**
+ * hlist_nulls_for_each_entry_safe -
+ *   iterate over list of given type safe against removal of list entry
+ * @tpos:       the type * to use as a loop cursor.
+ * @pos:        the &struct hlist_nulls_node to use as a loop cursor.
+ * @head:       the head for your list.
+ * @member:     the name of the hlist_nulls_node within the struct.
+ */
+#define hlist_nulls_for_each_entry_safe(tpos, pos, head, member)                \
+        for (({barrier();}),                                                    \
+             pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));            \
+                (!is_a_nulls(pos)) &&                                           \
+                ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member);        \
+                   pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)); 1; });)
 #endif
 #endif
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 63c86a7be2a1..afe5bab376c9 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -13,11 +13,12 @@
 #include <linux/bpf.h>
 #include <linux/jhash.h>
 #include <linux/filter.h>
+#include <linux/rculist_nulls.h>
 #include "percpu_freelist.h"
 #include "bpf_lru_list.h"
 
 struct bucket {
-        struct hlist_head head;
+        struct hlist_nulls_head head;
         raw_spinlock_t lock;
 };
 
@@ -44,7 +45,7 @@ enum extra_elem_state {
 /* each htab element is struct htab_elem + key + value */
 struct htab_elem {
         union {
-                struct hlist_node hash_node;
+                struct hlist_nulls_node hash_node;
                 struct {
                         void *padding;
                         union {
@@ -337,7 +338,7 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
                 goto free_htab;
 
         for (i = 0; i < htab->n_buckets; i++) {
-                INIT_HLIST_HEAD(&htab->buckets[i].head);
+                INIT_HLIST_NULLS_HEAD(&htab->buckets[i].head, i);
                 raw_spin_lock_init(&htab->buckets[i].lock);
         }
 
@@ -377,28 +378,52 @@ static inline struct bucket *__select_bucket(struct bpf_htab *htab, u32 hash)
         return &htab->buckets[hash & (htab->n_buckets - 1)];
 }
 
-static inline struct hlist_head *select_bucket(struct bpf_htab *htab, u32 hash)
+static inline struct hlist_nulls_head *select_bucket(struct bpf_htab *htab, u32 hash)
 {
         return &__select_bucket(htab, hash)->head;
 }
 
-static struct htab_elem *lookup_elem_raw(struct hlist_head *head, u32 hash,
+/* this lookup function can only be called with bucket lock taken */
+static struct htab_elem *lookup_elem_raw(struct hlist_nulls_head *head, u32 hash,
                                          void *key, u32 key_size)
 {
+        struct hlist_nulls_node *n;
         struct htab_elem *l;
 
-        hlist_for_each_entry_rcu(l, head, hash_node)
+        hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
                 if (l->hash == hash && !memcmp(&l->key, key, key_size))
                         return l;
 
         return NULL;
 }
 
+/* can be called without bucket lock. it will repeat the loop in
+ * the unlikely event when elements moved from one bucket into another
+ * while link list is being walked
+ */
+static struct htab_elem *lookup_nulls_elem_raw(struct hlist_nulls_head *head,
+                                               u32 hash, void *key,
+                                               u32 key_size, u32 n_buckets)
+{
+        struct hlist_nulls_node *n;
+        struct htab_elem *l;
+
+again:
+        hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
+                if (l->hash == hash && !memcmp(&l->key, key, key_size))
+                        return l;
+
+        if (unlikely(get_nulls_value(n) != (hash & (n_buckets - 1))))
+                goto again;
+
+        return NULL;
+}
+
 /* Called from syscall or from eBPF program */
 static void *__htab_map_lookup_elem(struct bpf_map *map, void *key)
 {
         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
-        struct hlist_head *head;
+        struct hlist_nulls_head *head;
         struct htab_elem *l;
         u32 hash, key_size;
 
@@ -411,7 +436,7 @@ static void *__htab_map_lookup_elem(struct bpf_map *map, void *key)
 
         head = select_bucket(htab, hash);
 
-        l = lookup_elem_raw(head, hash, key, key_size);
+        l = lookup_nulls_elem_raw(head, hash, key, key_size, htab->n_buckets);
 
         return l;
 }
@@ -444,8 +469,9 @@ static void *htab_lru_map_lookup_elem(struct bpf_map *map, void *key)
 static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
 {
         struct bpf_htab *htab = (struct bpf_htab *)arg;
-        struct htab_elem *l, *tgt_l;
-        struct hlist_head *head;
+        struct htab_elem *l = NULL, *tgt_l;
+        struct hlist_nulls_head *head;
+        struct hlist_nulls_node *n;
         unsigned long flags;
         struct bucket *b;
 
@@ -455,9 +481,9 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
 
         raw_spin_lock_irqsave(&b->lock, flags);
 
-        hlist_for_each_entry_rcu(l, head, hash_node)
+        hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
                 if (l == tgt_l) {
-                        hlist_del_rcu(&l->hash_node);
+                        hlist_nulls_del_rcu(&l->hash_node);
                         break;
                 }
 
@@ -470,7 +496,7 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
 static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
 {
         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
-        struct hlist_head *head;
+        struct hlist_nulls_head *head;
         struct htab_elem *l, *next_l;
         u32 hash, key_size;
         int i;
@@ -484,7 +510,7 @@ static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
         head = select_bucket(htab, hash);
 
         /* lookup the key */
-        l = lookup_elem_raw(head, hash, key, key_size);
+        l = lookup_nulls_elem_raw(head, hash, key, key_size, htab->n_buckets);
 
         if (!l) {
                 i = 0;
@@ -492,7 +518,7 @@ static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
         }
 
         /* key was found, get next key in the same bucket */
-        next_l = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(&l->hash_node)),
+        next_l = hlist_nulls_entry_safe(rcu_dereference_raw(hlist_nulls_next_rcu(&l->hash_node)),
                                   struct htab_elem, hash_node);
 
         if (next_l) {
@@ -511,7 +537,7 @@ find_first_elem:
                 head = select_bucket(htab, i);
 
                 /* pick first element in the bucket */
-                next_l = hlist_entry_safe(rcu_dereference_raw(hlist_first_rcu(head)),
+                next_l = hlist_nulls_entry_safe(rcu_dereference_raw(hlist_nulls_first_rcu(head)),
                                           struct htab_elem, hash_node);
                 if (next_l) {
                         /* if it's not empty, just return it */
@@ -676,7 +702,7 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
 {
         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
         struct htab_elem *l_new = NULL, *l_old;
-        struct hlist_head *head;
+        struct hlist_nulls_head *head;
         unsigned long flags;
         struct bucket *b;
         u32 key_size, hash;
@@ -715,9 +741,9 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
         /* add new element to the head of the list, so that
          * concurrent search will find it before old elem
          */
-        hlist_add_head_rcu(&l_new->hash_node, head);
+        hlist_nulls_add_head_rcu(&l_new->hash_node, head);
         if (l_old) {
-                hlist_del_rcu(&l_old->hash_node);
+                hlist_nulls_del_rcu(&l_old->hash_node);
                 free_htab_elem(htab, l_old);
         }
         ret = 0;
@@ -731,7 +757,7 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
 {
         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
         struct htab_elem *l_new, *l_old = NULL;
-        struct hlist_head *head;
+        struct hlist_nulls_head *head;
         unsigned long flags;
         struct bucket *b;
         u32 key_size, hash;
@@ -772,10 +798,10 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
         /* add new element to the head of the list, so that
          * concurrent search will find it before old elem
          */
-        hlist_add_head_rcu(&l_new->hash_node, head);
+        hlist_nulls_add_head_rcu(&l_new->hash_node, head);
         if (l_old) {
                 bpf_lru_node_set_ref(&l_new->lru_node);
-                hlist_del_rcu(&l_old->hash_node);
+                hlist_nulls_del_rcu(&l_old->hash_node);
         }
         ret = 0;
 
@@ -796,7 +822,7 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
 {
         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
         struct htab_elem *l_new = NULL, *l_old;
-        struct hlist_head *head;
+        struct hlist_nulls_head *head;
         unsigned long flags;
         struct bucket *b;
         u32 key_size, hash;
@@ -835,7 +861,7 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
                         ret = PTR_ERR(l_new);
                         goto err;
                 }
-                hlist_add_head_rcu(&l_new->hash_node, head);
+                hlist_nulls_add_head_rcu(&l_new->hash_node, head);
         }
         ret = 0;
 err:
@@ -849,7 +875,7 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
 {
         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
         struct htab_elem *l_new = NULL, *l_old;
-        struct hlist_head *head;
+        struct hlist_nulls_head *head;
         unsigned long flags;
         struct bucket *b;
         u32 key_size, hash;
@@ -897,7 +923,7 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
         } else {
                 pcpu_copy_value(htab, htab_elem_get_ptr(l_new, key_size),
                                 value, onallcpus);
-                hlist_add_head_rcu(&l_new->hash_node, head);
+                hlist_nulls_add_head_rcu(&l_new->hash_node, head);
                 l_new = NULL;
         }
         ret = 0;
@@ -925,7 +951,7 @@ static int htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
 static int htab_map_delete_elem(struct bpf_map *map, void *key)
 {
         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
-        struct hlist_head *head;
+        struct hlist_nulls_head *head;
         struct bucket *b;
         struct htab_elem *l;
         unsigned long flags;
@@ -945,7 +971,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
         l = lookup_elem_raw(head, hash, key, key_size);
 
         if (l) {
-                hlist_del_rcu(&l->hash_node);
+                hlist_nulls_del_rcu(&l->hash_node);
                 free_htab_elem(htab, l);
                 ret = 0;
         }
@@ -957,7 +983,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
 static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
 {
         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
-        struct hlist_head *head;
+        struct hlist_nulls_head *head;
         struct bucket *b;
         struct htab_elem *l;
         unsigned long flags;
@@ -977,7 +1003,7 @@ static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
         l = lookup_elem_raw(head, hash, key, key_size);
 
         if (l) {
-                hlist_del_rcu(&l->hash_node);
+                hlist_nulls_del_rcu(&l->hash_node);
                 ret = 0;
         }
 
@@ -992,12 +1018,12 @@ static void delete_all_elements(struct bpf_htab *htab)
         int i;
 
         for (i = 0; i < htab->n_buckets; i++) {
-                struct hlist_head *head = select_bucket(htab, i);
-                struct hlist_node *n;
+                struct hlist_nulls_head *head = select_bucket(htab, i);
+                struct hlist_nulls_node *n;
                 struct htab_elem *l;
 
-                hlist_for_each_entry_safe(l, n, head, hash_node) {
-                        hlist_del_rcu(&l->hash_node);
+                hlist_nulls_for_each_entry_safe(l, n, head, hash_node) {
+                        hlist_nulls_del_rcu(&l->hash_node);
                         if (l->state != HTAB_EXTRA_ELEM_USED)
                                 htab_elem_free(htab, l);
                 }