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Subject[PATCH 5.4 26/96] bpf, sockmap: Check update requirements after locking
Date
From: Lorenz Bauer <lmb@cloudflare.com>

commit 85b8ac01a421791d66c3a458a7f83cfd173fe3fa upstream.

It's currently possible to insert sockets in unexpected states into
a sockmap, due to a TOCTTOU when updating the map from a syscall.
sock_map_update_elem checks that sk->sk_state == TCP_ESTABLISHED,
locks the socket and then calls sock_map_update_common. At this
point, the socket may have transitioned into another state, and
the earlier assumptions don't hold anymore. Crucially, it's
conceivable (though very unlikely) that a socket has become unhashed.
This breaks the sockmap's assumption that it will get a callback
via sk->sk_prot->unhash.

Fix this by checking the (fixed) sk_type and sk_protocol without the
lock, followed by a locked check of sk_state.

Unfortunately it's not possible to push the check down into
sock_(map|hash)_update_common, since BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB
run before the socket has transitioned from TCP_SYN_RECV into
TCP_ESTABLISHED.

Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20200207103713.28175-1-lmb@cloudflare.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
net/core/sock_map.c | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)

--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -417,14 +417,16 @@ static int sock_map_update_elem(struct b
ret = -EINVAL;
goto out;
}
- if (!sock_map_sk_is_suitable(sk) ||
- sk->sk_state != TCP_ESTABLISHED) {
+ if (!sock_map_sk_is_suitable(sk)) {
ret = -EOPNOTSUPP;
goto out;
}

sock_map_sk_acquire(sk);
- ret = sock_map_update_common(map, idx, sk, flags);
+ if (sk->sk_state != TCP_ESTABLISHED)
+ ret = -EOPNOTSUPP;
+ else
+ ret = sock_map_update_common(map, idx, sk, flags);
sock_map_sk_release(sk);
out:
fput(sock->file);
@@ -740,14 +742,16 @@ static int sock_hash_update_elem(struct
ret = -EINVAL;
goto out;
}
- if (!sock_map_sk_is_suitable(sk) ||
- sk->sk_state != TCP_ESTABLISHED) {
+ if (!sock_map_sk_is_suitable(sk)) {
ret = -EOPNOTSUPP;
goto out;
}

sock_map_sk_acquire(sk);
- ret = sock_hash_update_common(map, key, sk, flags);
+ if (sk->sk_state != TCP_ESTABLISHED)
+ ret = -EOPNOTSUPP;
+ else
+ ret = sock_hash_update_common(map, key, sk, flags);
sock_map_sk_release(sk);
out:
fput(sock->file);

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