[PATCH bpf 5/5] bpf, sockmap: fix sock_map_ctx_update_elem race with exist/noexist

Daniel Borkmann
2018-08-16T22:49:49Z

The current code in sock_map_ctx_update_elem() allows for BPF_EXIST
and BPF_NOEXIST map update flags. While on array-like maps this approach
is rather uncommon, e.g. bpf_fd_array_map_update_elem() and others
enforce map update flags to be BPF_ANY such that xchg() can be used
directly, the current implementation in sock map does not guarantee
that such operation with BPF_EXIST / BPF_NOEXIST is atomic.

The initial test does a READ_ONCE(stab->sock_map[i]) to fetch the
socket from the slot which is then tested for NULL / non-NULL. However
later after __sock_map_ctx_update_elem(), the actual update is done
through osock = xchg(&stab->sock_map[i], sock). Problem is that in
the meantime a different CPU could have updated / deleted a socket
on that specific slot and thus flag contraints won't hold anymore.

I've been thinking whether best would be to just break UAPI and do
an enforcement of BPF_ANY to check if someone actually complains,
however trouble is that already in BPF kselftest we use BPF_NOEXIST
for the map update, and therefore it might have been copied into
applications already. The fix to keep the current behavior intact
would be to add a map lock similar to the sock hash bucket lock only
for covering the whole map.

Fixes: 174a79ff9515 ("bpf: sockmap with sk redirect support")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
---
kernel/bpf/sockmap.c | 106 +++++++++++++++++++++++++++------------------------
1 file changed, 57 insertions(+), 49 deletions(-)

diff --git a/kernel/bpf/sockmap.c b/kernel/bpf/sockmap.c
index 921cb6b..98e621a 100644
--- a/kernel/bpf/sockmap.c
+++ b/kernel/bpf/sockmap.c
@@ -58,6 +58,7 @@ struct bpf_stab {
struct bpf_map map;
struct sock **sock_map;
struct bpf_sock_progs progs;
+ raw_spinlock_t lock;
};

struct bucket {
@@ -89,9 +90,9 @@ enum smap_psock_state {

struct smap_psock_map_entry {
struct list_head list;
+ struct bpf_map *map;
struct sock **entry;
struct htab_elem __rcu *hash_link;
- struct bpf_htab __rcu *htab;
};

struct smap_psock {
@@ -343,13 +344,18 @@ static void bpf_tcp_close(struct sock *sk, long timeout)
e = psock_map_pop(sk, psock);
while (e) {
if (e->entry) {
- osk = cmpxchg(e->entry, sk, NULL);
+ struct bpf_stab *stab = container_of(e->map, struct bpf_stab, map);
+
+ raw_spin_lock_bh(&stab->lock);
+ osk = *e->entry;
if (osk == sk) {
+ *e->entry = NULL;
smap_release_sock(psock, sk);
}
+ raw_spin_unlock_bh(&stab->lock);
} else {
struct htab_elem *link = rcu_dereference(e->hash_link);
- struct bpf_htab *htab = rcu_dereference(e->htab);
+ struct bpf_htab *htab = container_of(e->map, struct bpf_htab, map);
struct hlist_head *head;
struct htab_elem *l;
struct bucket *b;
@@ -1642,6 +1648,7 @@ static struct bpf_map *sock_map_alloc(union bpf_attr *attr)
return ERR_PTR(-ENOMEM);

bpf_map_init_from_attr(&stab->map, attr);
+ raw_spin_lock_init(&stab->lock);

/* make sure page count doesn't overflow */
cost = (u64) stab->map.max_entries * sizeof(struct sock *);
@@ -1716,14 +1723,15 @@ static void sock_map_free(struct bpf_map *map)
* and a grace period expire to ensure psock is really safe to remove.
*/
rcu_read_lock();
+ raw_spin_lock_bh(&stab->lock);
for (i = 0; i < stab->map.max_entries; i++) {
struct smap_psock *psock;
struct sock *sock;

- sock = xchg(&stab->sock_map[i], NULL);
+ sock = stab->sock_map[i];
if (!sock)
continue;
-
+ stab->sock_map[i] = NULL;
psock = smap_psock_sk(sock);
/* This check handles a racing sock event that can get the
* sk_callback_lock before this case but after xchg happens
@@ -1735,6 +1743,7 @@ static void sock_map_free(struct bpf_map *map)
smap_release_sock(psock, sock);
}
}
+ raw_spin_unlock_bh(&stab->lock);
rcu_read_unlock();

sock_map_remove_complete(stab);
@@ -1778,14 +1787,16 @@ static int sock_map_delete_elem(struct bpf_map *map, void *key)
if (k >= map->max_entries)
return -EINVAL;

- sock = xchg(&stab->sock_map[k], NULL);
+ raw_spin_lock_bh(&stab->lock);
+ sock = stab->sock_map[k];
+ stab->sock_map[k] = NULL;
+ raw_spin_unlock_bh(&stab->lock);
if (!sock)
return -EINVAL;

psock = smap_psock_sk(sock);
if (!psock)
- goto out;
-
+ return 0;
if (psock->bpf_parse) {
write_lock_bh(&sock->sk_callback_lock);
smap_stop_sock(psock, sock);
@@ -1793,7 +1804,6 @@ static int sock_map_delete_elem(struct bpf_map *map, void *key)
}
smap_list_map_remove(psock, &stab->sock_map[k]);
smap_release_sock(psock, sock);
-out:
return 0;
}

@@ -1829,11 +1839,9 @@ static int sock_map_delete_elem(struct bpf_map *map, void *key)
static int __sock_map_ctx_update_elem(struct bpf_map *map,
struct bpf_sock_progs *progs,
struct sock *sock,
- struct sock **map_link,
void *key)
{
struct bpf_prog *verdict, *parse, *tx_msg;
- struct smap_psock_map_entry *e = NULL;
struct smap_psock *psock;
bool new = false;
int err = 0;
@@ -1906,14 +1914,6 @@ static int __sock_map_ctx_update_elem(struct bpf_map *map,
new = true;
}

- if (map_link) {
- e = kzalloc(sizeof(*e), GFP_ATOMIC | __GFP_NOWARN);
- if (!e) {
- err = -ENOMEM;
- goto out_free;
- }
- }
-
/* 3. At this point we have a reference to a valid psock that is
* running. Attach any BPF programs needed.
*/
@@ -1935,17 +1935,6 @@ static int __sock_map_ctx_update_elem(struct bpf_map *map,
write_unlock_bh(&sock->sk_callback_lock);
}

- /* 4. Place psock in sockmap for use and stop any programs on
- * the old sock assuming its not the same sock we are replacing
- * it with. Because we can only have a single set of programs if
- * old_sock has a strp we can stop it.
- */
- if (map_link) {
- e->entry = map_link;
- spin_lock_bh(&psock->maps_lock);
- list_add_tail(&e->list, &psock->maps);
- spin_unlock_bh(&psock->maps_lock);
- }
return err;
out_free:
smap_release_sock(psock, sock);
@@ -1956,7 +1945,6 @@ static int __sock_map_ctx_update_elem(struct bpf_map *map,
}
if (tx_msg)
bpf_prog_put(tx_msg);
- kfree(e);
return err;
}

@@ -1966,36 +1954,57 @@ static int sock_map_ctx_update_elem(struct bpf_sock_ops_kern *skops,
{
struct bpf_stab *stab = container_of(map, struct bpf_stab, map);
struct bpf_sock_progs *progs = &stab->progs;
- struct sock *osock, *sock;
+ struct sock *osock, *sock = skops->sk;
+ struct smap_psock_map_entry *e;
+ struct smap_psock *psock;
u32 i = *(u32 *)key;
int err;

if (unlikely(flags > BPF_EXIST))
return -EINVAL;
-
if (unlikely(i >= stab->map.max_entries))
return -E2BIG;

- sock = READ_ONCE(stab->sock_map[i]);
- if (flags == BPF_EXIST && !sock)
- return -ENOENT;
- else if (flags == BPF_NOEXIST && sock)
- return -EEXIST;
+ e = kzalloc(sizeof(*e), GFP_ATOMIC | __GFP_NOWARN);
+ if (!e)
+ return -ENOMEM;

- sock = skops->sk;
- err = __sock_map_ctx_update_elem(map, progs, sock, &stab->sock_map[i],
- key);
+ err = __sock_map_ctx_update_elem(map, progs, sock, key);
if (err)
goto out;

- osock = xchg(&stab->sock_map[i], sock);
- if (osock) {
- struct smap_psock *opsock = smap_psock_sk(osock);
+ /* psock guaranteed to be present. */
+ psock = smap_psock_sk(sock);
+ raw_spin_lock_bh(&stab->lock);
+ osock = stab->sock_map[i];
+ if (osock && flags == BPF_NOEXIST) {
+ err = -EEXIST;
+ goto out_unlock;
+ }
+ if (!osock && flags == BPF_EXIST) {
+ err = -ENOENT;
+ goto out_unlock;
+ }
+
+ e->entry = &stab->sock_map[i];
+ e->map = map;
+ spin_lock_bh(&psock->maps_lock);
+ list_add_tail(&e->list, &psock->maps);
+ spin_unlock_bh(&psock->maps_lock);

- smap_list_map_remove(opsock, &stab->sock_map[i]);
- smap_release_sock(opsock, osock);
+ stab->sock_map[i] = sock;
+ if (osock) {
+ psock = smap_psock_sk(osock);
+ smap_list_map_remove(psock, &stab->sock_map[i]);
+ smap_release_sock(psock, osock);
}
+ raw_spin_unlock_bh(&stab->lock);
+ return 0;
+out_unlock:
+ smap_release_sock(psock, sock);
+ raw_spin_unlock_bh(&stab->lock);
out:
+ kfree(e);
return err;
}

@@ -2358,7 +2367,7 @@ static int sock_hash_ctx_update_elem(struct bpf_sock_ops_kern *skops,
b = __select_bucket(htab, hash);
head = &b->head;

- err = __sock_map_ctx_update_elem(map, progs, sock, NULL, key);
+ err = __sock_map_ctx_update_elem(map, progs, sock, key);
if (err)
goto err;

@@ -2384,8 +2393,7 @@ static int sock_hash_ctx_update_elem(struct bpf_sock_ops_kern *skops,
}

rcu_assign_pointer(e->hash_link, l_new);
- rcu_assign_pointer(e->htab,
- container_of(map, struct bpf_htab, map));
+ e->map = map;
spin_lock_bh(&psock->maps_lock);
list_add_tail(&e->list, &psock->maps);
spin_unlock_bh(&psock->maps_lock);
--
2.9.5