Subject: Re: [regression] TCP_MD5SIG on established sockets

From: Mathieu Desnoyers <[email-masked]>
Date: Tue, 30 Jun 2020 19:44:21 -0400 (EDT)

----- On Jun 30, 2020, at 6:38 PM, Eric Dumazet [email-masked] wrote:
[...]
> 
> For updates of keys, it seems existing code lacks some RCU care.
> 
> MD5 keys use RCU for lookups/hashes, but the replacement of a key does
> not allocate a new piece of memory.

How is that RCU-safe ?

Based on what I see here:

tcp_md5_do_add() has a comment stating:

"/* This can be called on a newly created socket, from other files */"

which appears to be untrue if this can indeed be called on a live socket.

The path for pre-existing keys does:

        key = tcp_md5_do_lookup_exact(sk, addr, family, prefixlen, l3index);
        if (key) {
                /* Pre-existing entry - just update that one. */
                memcpy(key->key, newkey, newkeylen);
                key->keylen = newkeylen;
                return 0;
        }

AFAIU, this works only if you assume there are no concurrent readers
accessing key->key, else they can see a corrupted key.

The change you are proposing adds smp_wmb/smp_rmb to pair stores
to key before key_len with loads of key_len before key. I'm not sure
what this is trying to achieve, and how it prevents the readers from
observing a corrupted state if the key is updated on a live socket ?

Based on my understanding, this path which deals with pre-existing keys
in-place should only ever be used when there are no concurrent readers,
else a new memory allocation would be needed to guarantee that readers
always observe a valid copy.

Thanks,

Mathieu

> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index
> 810cc164f795f8e1e8ca747ed5df51bb20fec8a2..ecc0e3fabce8b03bef823cbfc5c1b0a9e24df124
> 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -4034,9 +4034,12 @@ EXPORT_SYMBOL(tcp_md5_hash_skb_data);
> int tcp_md5_hash_key(struct tcp_md5sig_pool *hp, const struct
> tcp_md5sig_key *key)
> {
>        struct scatterlist sg;
> +       u8 keylen = key->keylen;
> 
> -       sg_init_one(&sg, key->key, key->keylen);
> -       ahash_request_set_crypt(hp->md5_req, &sg, NULL, key->keylen);
> +       smp_rmb(); /* paired with smp_wmb() in tcp_md5_do_add() */
> +
> +       sg_init_one(&sg, key->key, keylen);
> +       ahash_request_set_crypt(hp->md5_req, &sg, NULL, keylen);
>        return crypto_ahash_update(hp->md5_req);
> }
> EXPORT_SYMBOL(tcp_md5_hash_key);
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index
> ad6435ba6d72ffd8caf783bb25cad7ec151d6909..99916fcc15ca0be12c2c133ff40516f79e6fdf7f
> 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -1113,6 +1113,9 @@ int tcp_md5_do_add(struct sock *sk, const union
> tcp_md5_addr *addr,
>        if (key) {
>                /* Pre-existing entry - just update that one. */
>                memcpy(key->key, newkey, newkeylen);
> +
> +               smp_wmb(); /* pairs with smp_rmb() in tcp_md5_hash_key() */
> +
>                key->keylen = newkeylen;
>                return 0;
>         }

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com