Re: [PATCH bpf 1/2] bpf: sockmap, af_unix sockets need to hold ref for pair sock

[Jakub Sitnicki <jakub@cloudflare.com>]

On Mon, Oct 16, 2023 at 12:08 PM -07, John Fastabend wrote:
> AF_UNIX sockets are a paired socket. So sending on one of the pairs
> will lookup the paired socket as part of the send operation. It is
> possible however to put just one of the pairs in a BPF map. This
> currently increments the refcnt on the sock in the sockmap to
> ensure it is not free'd by the stack before sockmap cleans up its
> state and stops any skbs being sent/recv'd to that socket.
>
> But we missed a case. If the peer socket is closed it will be
> free'd by the stack. However, the paired socket can still be
> referenced from BPF sockmap side because we hold a reference
> there. Then if we are sending traffic through BPF sockmap to
> that socket it will try to dereference the free'd pair in its
> send logic creating a use after free.  And following splat,
>
>    [59.900375] BUG: KASAN: slab-use-after-free in sk_wake_async+0x31/0x1b0
>    [59.901211] Read of size 8 at addr ffff88811acbf060 by task kworker/1:2/954
>    [...]
>    [59.905468] Call Trace:
>    [59.905787]  <TASK>
>    [59.906066]  dump_stack_lvl+0x130/0x1d0
>    [59.908877]  print_report+0x16f/0x740
>    [59.910629]  kasan_report+0x118/0x160
>    [59.912576]  sk_wake_async+0x31/0x1b0
>    [59.913554]  sock_def_readable+0x156/0x2a0
>    [59.914060]  unix_stream_sendmsg+0x3f9/0x12a0
>    [59.916398]  sock_sendmsg+0x20e/0x250
>    [59.916854]  skb_send_sock+0x236/0xac0
>    [59.920527]  sk_psock_backlog+0x287/0xaa0
>
> To fix let BPF sockmap hold a refcnt on both the socket in the
> sockmap and its paired socket.  It wasn't obvious how to contain
> the fix to bpf_unix logic. The primarily problem with keeping this
> logic in bpf_unix was: In the sock close() we could handle the
> deref by having a close handler. But, when we are destroying the
> psock through a map delete operation we wouldn't have gotten any
> signal thorugh the proto struct other than it being replaced.
> If we do the deref from the proto replace its too early because
> we need to deref the skpair after the backlog worker has been
> stopped.
>
> Given all this it seems best to just cache it at the end of the
> psock and eat 8B for the af_unix and vsock users.
>
> Fixes: 94531cfcbe79 ("af_unix: Add unix_stream_proto for sockmap")
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> ---
>  include/linux/skmsg.h |  1 +
>  include/net/af_unix.h |  1 +
>  net/core/skmsg.c      |  2 ++
>  net/unix/af_unix.c    |  2 --
>  net/unix/unix_bpf.c   | 10 ++++++++++
>  5 files changed, 14 insertions(+), 2 deletions(-)
>

[...]

> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 3e8a04a13668..87dd723aacf9 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -212,8 +212,6 @@ static inline bool unix_secdata_eq(struct scm_cookie *scm, struct sk_buff *skb)
>  }
>  #endif /* CONFIG_SECURITY_NETWORK */
>  
> -#define unix_peer(sk) (unix_sk(sk)->peer)
> -
>  static inline int unix_our_peer(struct sock *sk, struct sock *osk)
>  {
>  	return unix_peer(osk) == sk;
> diff --git a/net/unix/unix_bpf.c b/net/unix/unix_bpf.c
> index 2f9d8271c6ec..705eeed10be3 100644
> --- a/net/unix/unix_bpf.c
> +++ b/net/unix/unix_bpf.c
> @@ -143,6 +143,8 @@ static void unix_stream_bpf_check_needs_rebuild(struct proto *ops)
>  
>  int unix_dgram_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool restore)
>  {
> +	struct sock *skpair;
> +
>  	if (sk->sk_type != SOCK_DGRAM)
>  		return -EOPNOTSUPP;
>  
> @@ -152,6 +154,9 @@ int unix_dgram_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool re
>  		return 0;
>  	}
>  
> +	skpair = unix_peer(sk);
> +	sock_hold(skpair);
> +	psock->skpair = skpair;
>  	unix_dgram_bpf_check_needs_rebuild(psock->sk_proto);
>  	sock_replace_proto(sk, &unix_dgram_bpf_prot);
>  	return 0;

unix_dgram should not need this, since it grabs a ref on each sendmsg.

I'm not able to reproduce this bug for unix_dgram.

Have you seen any KASAN reports for unix_dgram from syzcaller?

> @@ -159,12 +164,17 @@ int unix_dgram_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool re
>  
>  int unix_stream_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool restore)
>  {
> +	struct sock *skpair = unix_peer(sk);
> +
>  	if (restore) {
>  		sk->sk_write_space = psock->saved_write_space;
>  		sock_replace_proto(sk, psock->sk_proto);
>  		return 0;
>  	}
>  
> +	skpair = unix_peer(sk);
> +	sock_hold(skpair);
> +	psock->skpair = skpair;
>  	unix_stream_bpf_check_needs_rebuild(psock->sk_proto);
>  	sock_replace_proto(sk, &unix_stream_bpf_prot);
>  	return 0;