Re: [RFCv2 1/9] tcp: authopt: Initial support and key management

[Dmitry Safonov <0x7f454c46@gmail.com>]

Hi Leonard,

On Tue, 10 Aug 2021 at 02:50, Leonard Crestez <cdleonard@gmail.com> wrote:
[..]
> +/* Representation of a Master Key Tuple as per RFC5925 */
> +struct tcp_authopt_key_info {
> +       struct hlist_node node;
> +       /* Local identifier */
> +       u32 local_id;

There is no local_id in RFC5925, what's that?
An MKT is identified by (send_id, recv_id), together with
(src_addr/src_port, dst_addr/dst_port).
Why introducing something new to already complicated RFC?

> +       u32 flags;
> +       /* Wire identifiers */
> +       u8 send_id, recv_id;
> +       u8 alg_id;
> +       u8 keylen;
> +       u8 key[TCP_AUTHOPT_MAXKEYLEN];
> +       struct rcu_head rcu;

This is unaligned and will add padding.
I wonder if it's also worth saving some bytes by something like
struct tcp_ao_key *key;

With
struct tcp_ao_key {
      u8 keylen;
      u8 key[0];
};

Hmm?

> +       struct sockaddr_storage addr;
> +};
> +
> +/* Per-socket information regarding tcp_authopt */
> +struct tcp_authopt_info {
> +       /* List of tcp_authopt_key_info */
> +       struct hlist_head head;
> +       u32 flags;
> +       u32 src_isn;
> +       u32 dst_isn;
> +       struct rcu_head rcu;

Ditto, adds padding on 64-bit.

[..]
> +++ b/include/uapi/linux/tcp.h
> @@ -126,10 +126,12 @@ enum {
>  #define TCP_INQ                        36      /* Notify bytes available to read as a cmsg on read */
>
>  #define TCP_CM_INQ             TCP_INQ
>
>  #define TCP_TX_DELAY           37      /* delay outgoing packets by XX usec */
> +#define TCP_AUTHOPT                    38      /* TCP Authentication Option (RFC2385) */
> +#define TCP_AUTHOPT_KEY                39      /* TCP Authentication Option update key (RFC2385) */

RFC2385 is md5 one.
Also, should there be TCP_AUTHOPT_ADD_KEY, TCP_AUTHOPT_DELTE_KEY
instead of using flags inside setsocketopt()? (no hard feelings)

[..]
> +/**
> + * enum tcp_authopt_flag - flags for `tcp_authopt.flags`
> + */
> +enum tcp_authopt_flag {
> +       /**
> +        * @TCP_AUTHOPT_FLAG_REJECT_UNEXPECTED:
> +        *      Configure behavior of segments with TCP-AO coming from hosts for which no
> +        *      key is configured. The default recommended by RFC is to silently accept
> +        *      such connections.
> +        */
> +       TCP_AUTHOPT_FLAG_REJECT_UNEXPECTED = (1 << 2),
> +};
> +
> +/**
> + * struct tcp_authopt - Per-socket options related to TCP Authentication Option
> + */
> +struct tcp_authopt {
> +       /** @flags: Combination of &enum tcp_authopt_flag */
> +       __u32   flags;
> +};

I'm not sure what's the use of enum here, probably:
#define TCP_AUTHOPT_FLAG_REJECT_UNEXPECTED BIT(2)

[..]
> +struct tcp_authopt_key {
> +       /** @flags: Combination of &enum tcp_authopt_key_flag */
> +       __u32   flags;
> +       /** @local_id: Local identifier */
> +       __u32   local_id;
> +       /** @send_id: keyid value for send */
> +       __u8    send_id;
> +       /** @recv_id: keyid value for receive */
> +       __u8    recv_id;
> +       /** @alg: One of &enum tcp_authopt_alg */
> +       __u8    alg;
> +       /** @keylen: Length of the key buffer */
> +       __u8    keylen;
> +       /** @key: Secret key */
> +       __u8    key[TCP_AUTHOPT_MAXKEYLEN];
> +       /**
> +        * @addr: Key is only valid for this address
> +        *
> +        * Ignored unless TCP_AUTHOPT_KEY_ADDR_BIND flag is set
> +        */
> +       struct __kernel_sockaddr_storage addr;
> +};

It'll be an ABI if this is accepted. As it is - it can't support RFC5925 fully.
Extending syscall ABI is painful. I think, even the initial ABI *must* support
all possible features of the RFC.
In other words, there must be src_addr, src_port, dst_addr and dst_port.
I can see from docs you've written you don't want to support a mix of different
addr/port MKTs, so you can return -EINVAL or -ENOTSUPP for any value
but zero.
This will create an ABI that can be later extended to fully support RFC.

The same is about key: I don't think you need to define/use tcp_authopt_alg.
Just use algo name - that way TCP-AO will automatically be able to use
any algo supported by crypto engine.
See how xfrm does it, e.g.:
struct xfrm_algo_auth {
    char        alg_name[64];
    unsigned int    alg_key_len;    /* in bits */
    unsigned int    alg_trunc_len;  /* in bits */
    char        alg_key[0];
};

So you can let a user chose maclen instead of hard-coding it.
Much more extendable than what you propose.

[..]
> +#ifdef CONFIG_TCP_AUTHOPT
> +       case TCP_AUTHOPT: {
> +               struct tcp_authopt info;
> +
> +               if (get_user(len, optlen))
> +                       return -EFAULT;
> +
> +               lock_sock(sk);
> +               tcp_get_authopt_val(sk, &info);
> +               release_sock(sk);
> +
> +               len = min_t(unsigned int, len, sizeof(info));
> +               if (put_user(len, optlen))
> +                       return -EFAULT;
> +               if (copy_to_user(optval, &info, len))
> +                       return -EFAULT;
> +               return 0;
> +       }

I'm pretty sure it's not a good choice to write partly tcp_authopt.
And a user has no way to check what's the correct len on this kernel.
Instead of len = min_t(unsigned int, len, sizeof(info)), it should be
if (len != sizeof(info))
    return -EINVAL;

[..]
> +int tcp_set_authopt(struct sock *sk, sockptr_t optval, unsigned int optlen)
> +{
> +       struct tcp_authopt opt;
> +       struct tcp_authopt_info *info;
> +
> +       WARN_ON(!lockdep_sock_is_held(sk));

sock_owned_by_me(sk)

> +
> +       /* If userspace optlen is too short fill the rest with zeros */
> +       if (optlen > sizeof(opt))
> +               return -EINVAL;
> +       memset(&opt, 0, sizeof(opt));

it's 4 bytes, why not just (optlen != sizeof(opt))?

[..]
> +int tcp_get_authopt_val(struct sock *sk, struct tcp_authopt *opt)
> +{
> +       struct tcp_sock *tp = tcp_sk(sk);
> +       struct tcp_authopt_info *info;
> +
> +       WARN_ON(!lockdep_sock_is_held(sk));

sock_owned_by_me(sk)

[..]
> +int tcp_set_authopt_key(struct sock *sk, sockptr_t optval, unsigned int optlen)
> +{
> +       struct tcp_authopt_key opt;
> +       struct tcp_authopt_info *info;
> +       struct tcp_authopt_key_info *key_info;
> +
> +       /* If userspace optlen is too short fill the rest with zeros */
> +       if (optlen > sizeof(opt))
> +               return -EINVAL;
> +       memset(&opt, 0, sizeof(opt));
> +       if (copy_from_sockptr(&opt, optval, optlen))
> +               return -EFAULT;

Again, not a good practice to zero-extend struct. Enforce proper size
with -EINVAL.

[..]
> +       /* Initialize tcp_authopt_info if not already set */
> +       info = __tcp_authopt_info_get_or_create(sk);
> +       if (IS_ERR(info))
> +               return PTR_ERR(info);
> +
> +       /* check key family */
> +       if (opt.flags & TCP_AUTHOPT_KEY_ADDR_BIND) {
> +               if (sk->sk_family != opt.addr.ss_family)
> +                       return -EINVAL;
> +       }

Probably, can be in the reverse order, so that
__tcp_authopt_info_get_or_create()
won't allocate memory.

> +       /* If an old value exists for same local_id it is deleted */
> +       key_info = __tcp_authopt_key_info_lookup(sk, info, opt.local_id);
> +       if (key_info)
> +               tcp_authopt_key_del(sk, info, key_info);
> +       key_info = sock_kmalloc(sk, sizeof(*key_info), GFP_KERNEL | __GFP_ZERO);
> +       if (!key_info)
> +               return -ENOMEM;

1. You don't need sock_kmalloc() together with tcp_authopt_key_del().
    It just frees the memory and allocates it back straight away - no
sense in doing that.
2. I think RFC says you must not allow a user to change an existing key:
> MKT parameters are not changed. Instead, new MKTs can be installed, and a connection
> can change which MKT it uses.

IIUC, it means that one can't just change an existing MKT, but one can
remove and later
add MKT with the same (send_id, recv_id, src_addr/port, dst_addr/port).

So, a reasonable thing to do:
if (key_info)
    return -EEXIST.

Thanks,
             Dmitry