bpf: convert htab map to hlist_nulls

commit 4fe8435909fddc97b81472026aa954e06dd192a5 upstream.

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>
Signed-off-by: Chenbo Feng <fengc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
index b01fe10..87ff4f5 100644 (file)
--- 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 6224a0a..2720b2f 100644 (file)
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -118,5 +118,19 @@ static inline void hlist_nulls_add_head_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 f9d53ac..8648d7d 100644 (file)
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -13,10 +13,11 @@
 #include <linux/bpf.h>
 #include <linux/jhash.h>
 #include <linux/filter.h>
+#include <linux/rculist_nulls.h>
 #include "percpu_freelist.h"
 
 struct bucket {
-       struct hlist_head head;
+       struct hlist_nulls_head head;
        raw_spinlock_t lock;
 };
 
@@ -40,7 +41,7 @@ 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 {
@@ -245,7 +246,7 @@ 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);
        }
 
@@ -282,28 +283,52 @@ 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;
 
@@ -316,7 +341,7 @@ 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;
 }
@@ -335,7 +360,7 @@ static void *htab_map_lookup_elem(struct bpf_map *map, void *key)
 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 = 0;
@@ -352,13 +377,13 @@ 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)
                goto find_first_elem;
 
        /* 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) {
@@ -377,7 +402,7 @@ 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 */
@@ -534,7 +559,7 @@ 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;
@@ -573,9 +598,9 @@ 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;
@@ -590,7 +615,7 @@ 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;
@@ -642,7 +667,7 @@ 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:
@@ -660,7 +685,7 @@ static int htab_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;
@@ -680,7 +705,7 @@ 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;
        }
@@ -694,12 +719,12 @@ 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);
                }