Re: [PATCH net] net/smc: serialize sndbuf descriptor release with diagnostic dumps

[netdev-bot+sashiko@kernel.org]

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- [High] The fix is incomplete: smc_buf_create() still frees
  conn->sndbuf_desc without serializing against __smc_diag_dump().
- [High] The SMC-R release path smc_conn_free() -> smc_buf_unuse() ->
  smcr_buf_unuse() also frees descriptors that the diag reader can still…

Pre-existing issues:
- [High] __smc_diag_dump() checks its guards without any lock:
  smc->conn.alert_token_local (smc_diag.c:107-108) and…

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net/smc: serialize sndbuf descriptor release with diagnostic dumps

This patch takes the SMC hash write lock in smcd_buf_detach() while it
clears conn->sndbuf_desc. The goal is to stop __smc_diag_dump(), which
reads the descriptor under read_lock(&h->lock), from seeing a freed
descriptor. The descriptor is freed once the lock has been dropped.

> diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
> index 9974149659c2f..f32fc1bd5bc85 100644
> --- a/net/smc/smc_core.c
> +++ b/net/smc/smc_core.c
> @@ -1207,6 +1207,8 @@ static void smcr_buf_unuse(struct smc_buf_desc *buf_desc, bool is_rmb,
>  
>  static void smcd_buf_detach(struct smc_connection *conn)
>  {
> +	struct smc_sock *smc = container_of(conn, struct smc_sock, conn);
> +	struct smc_hashinfo *h = smc->sk.sk_prot->h.smc_hash;
>  	struct smcd_dev *smcd = conn->lgr->smcd;
>  	u64 peer_token = conn->peer_token;
>  	struct smc_buf_desc *buf_desc;
> @@ -1216,8 +1218,10 @@ static void smcd_buf_detach(struct smc_connection *conn)
>  
>  	smc_ism_detach_dmb(smcd, peer_token);
>  
> +	write_lock_bh(&h->lock);
>  	buf_desc = conn->sndbuf_desc;
>  	conn->sndbuf_desc = NULL;
> +	write_unlock_bh(&h->lock);
>  	kfree(buf_desc);
>  }

[Severity: High]
The subject says sndbuf descriptor release is now serialized with diag
dumps. Is smcd_buf_detach() the only place that frees conn->sndbuf_desc
while the socket is visible to __smc_diag_dump()?

smc_buf_create() has an error path for when the RMB fails after the
sndbuf was already created:

net/smc/smc_core.c:smc_buf_create() {
    ...
	rc = __smc_buf_create(smc, is_smcd, true);
	if (rc && smc->conn.sndbuf_desc) {
		down_write(&smc->conn.lgr->sndbufs_lock);
		smc_lgr_buf_list_del(smc->conn.lgr, false,
				     smc->conn.sndbuf_desc);
		up_write(&smc->conn.lgr->sndbufs_lock);
		smc_buf_free(smc->conn.lgr, false, smc->conn.sndbuf_desc);
		smc->conn.sndbuf_desc = NULL;
	}
    ...
}

smc_buf_free() goes to smcr_buf_free() or smcd_buf_free() and kfree()s
the descriptor. The pointer is only cleared after that, and h->lock is
never taken.

At this point smc_sk_init() has already hashed the socket via
sk->sk_prot->hash(sk). smc_conn_create()->smc_lgr_register_conn() has
also set conn->alert_token_local. A dump that holds only
read_lock(&h->lock) can pass the alert_token_local check and then
evaluate this on the freed descriptor:

net/smc/smc_diag.c:__smc_diag_dump() {
    ...
			.sndbuf_size = conn->sndbuf_desc ?
				conn->sndbuf_desc->len : 0,
    ...
}

Could this cause the same slab-use-after-free the commit message
describes? It would affect SMC-R, and SMC-D devices without DMB nocopy.
Four callers can reach it when RMB allocation fails (-ENOMEM, or DMB
exhaustion on SMC-D): smc_connect_rdma(), smc_connect_ism(),
smc_listen_rdma_init() and smc_listen_ism_init(). The value is copied
to userspace as cinfo.sndbuf_size, and the SOCK_DIAG dump has no
capability check.

[Severity: High]
Is there a similar unserialized release on SMC-R through
smc_conn_free()->smc_buf_unuse()->smcr_buf_unuse()?

net/smc/smc_core.c:smcr_buf_unuse() {
    ...
	if (buf_desc->is_reg_err) {
		/* buf registration failed, reuse not possible */
		...
		smc_buf_free(lgr, is_rmb, buf_desc);
    ...
}

This kfree()s the descriptor, but conn->sndbuf_desc (for a vzalloc'ed
sndbuf) or conn->rmb_desc still points at it.

In smc_conn_free() this happens before smc_lgr_unregister_conn(), and
alert_token_local is only cleared inside the unregister:

	if (!list_empty(&lgr->list)) {
		smc_buf_unuse(conn, lgr); /* allow buffer reuse */
		smc_lgr_unregister_conn(conn);
	}

smcr_link_reg_buf() sets is_reg_err when smc_wr_reg_send() fails.
smc_connect_rdma() then jumps to connect_abort, which calls
smc_conn_abort()->smc_conn_free() on a socket that is hashed and still
has its token set. The listen side follows the same pattern.

Would a dump that runs between smc_buf_unuse() and
smc_lgr_unregister_conn() read sndbuf_desc->len or rmb_desc->len from
freed memory? Should this release site also be serialized with h->lock?

[Severity: High]
This is a pre-existing issue, not one introduced by this patch. The new
h->lock section ends before the rest of the teardown in smc_conn_kill():

net/smc/smc_core.c:smc_conn_kill() {
    ...
	smc_lgr_unregister_conn(conn);
	smc_close_active_abort(smc);
}

__smc_diag_dump() checks smc->conn.alert_token_local, smc_conn_lgr_valid()
and list_empty(&lgr->list), and nothing on the teardown side excludes
those checks. After them it dereferences conn->sndbuf_desc->len,
conn->rmb_desc->len, conn->rmb_desc->token and
conn->lgr->smcd->dibs->gid.

The socket can still be hashed in PEERCLOSEWAIT1/2, PEERFINCLOSEWAIT,
PROCESSABORT or APPFINCLOSEWAIT, for example after shutdown(). In that
case smc_close_active_abort() calls smc_conn_free()->smc_lgr_put(), and
the link group can then be freed through __smc_lgr_free():

	smc_lgr_free_bufs(lgr);
	...
	kfree(lgr);

conn->sndbuf_desc, conn->rmb_desc and conn->lgr are not cleared on this
path.

Suppose a dump passes the checks before smc_lgr_unregister_conn() and is
then delayed before the dereferences. read_lock() is preemptible on
PREEMPT_RT, and IRQ or vCPU preemption can also cause this. Can the dump
then read freed descriptors, or load smcd from a freed lgr and
dereference smcd->dibs?

This also affects the commit's own smc_smcd_terminate_all() scenario.
The sndbuf_desc->len read is still exposed on SMC-D devices without
nocopy, and the rmb_desc->len read on the next line is still exposed on
nocopy devices.

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