Re: [PATCH net v7] net/smc: fix lgr/lnk lifetime vs diag reader race

On 04/10/26 1:18 pm, netdev-bot+sashiko@kernel.org wrote:

Thank you for your contribution! Sashiko AI review found 2 potential
issue(s) to consider.

Critical: 0 · High: 1 · Medium: 1 · Low: 0

If you believe none of the issues are legitimate, please reply to each
issue of Medium severity or higher and explain why the AI is wrong.
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- [High] The patch does not fully close the diag-reader vs. link-lifetime
  race it says it fixes.
- [Medium] The new safety gates in __smc_diag_dump() depend on a plain,
  lockless load of sk->sk_state.

--

net/smc: fix lgr/lnk lifetime vs diag reader race

This patch tries to close races between the SMC sock_diag dump and
teardown paths. The dump walks the socket hash under read_lock, while the
teardown paths free lgr, lnk or sndbuf_desc state. The patch adds sk_state
based guards for SMC_INIT and SMC_CLOSED sockets to __smc_diag_dump(), and
unhashes the socket earlier in smc_conn_kill(), smc_close_active_abort()
and smc_close_passive_work().

quoted

diff --git a/net/smc/smc_diag.c b/net/smc/smc_diag.c
index bf0beaa23bdb6..b29bd34acf7b5 100644
--- a/net/smc/smc_diag.c
+++ b/net/smc/smc_diag.c

@@ -90,7 +90,8 @@ static int __smc_diag_dump(struct sock *sk, struct sk_buff *skb,
 	r->diag_state = sk->sk_state;
 	if (smc->use_fallback)
 		r->diag_mode = SMC_DIAG_MODE_FALLBACK_TCP;
-	else if (smc_conn_lgr_valid(&smc->conn) && smc->conn.lgr->is_smcd)
+	else if (sk->sk_state != SMC_INIT &&
+		 smc_conn_lgr_valid(&smc->conn) && smc->conn.lgr->is_smcd)
 		r->diag_mode = SMC_DIAG_MODE_SMCD;
 	else
 		r->diag_mode = SMC_DIAG_MODE_SMCR;

[Severity: Medium]
Is a plain load of sk->sk_state enough to order the later loads of
conn->alert_token_local, conn->lgr->is_smcd and conn->sndbuf_desc->len?

This check, and the SMC_INIT/SMC_CLOSED guard added below, share no lock
with the writer. Nothing sits between the sk_state load and the conn field
loads: no READ_ONCE(), no smp_load_acquire(), no smp_rmb(). Only a control
dependency links them, and that doesn't order load->load on weakly ordered
CPUs.

On the early abort path the writer runs under lock_sock():

smc_conn_abort()
  smc_conn_free()
    smc_lgr_unregister_conn()    /* alert_token_local = 0 */
  smc_lgr_cleanup_early()
    __smc_lgr_terminate()
      smc_lgr_free()             /* lgr and buffers freed, conn->lgr and
                                    conn->sndbuf_desc left set */
smc_switch_to_fallback()
...
sk->sk_state = SMC_ACTIVE

On arm64, could the reader see sk_state == SMC_ACTIVE together with a
stale non-zero alert_token_local? It would then read lgr->is_smcd or
sndbuf_desc->len from freed memory and copy the value into the netlink
reply.

This can't happen on x86 or s390, which keep load-load order, and the
writer's window is long. The Linux kernel memory model still allows it.

Would READ_ONCE() or smp_load_acquire() on sk_state (or an smp_rmb()
before the conn field loads), paired with release-ordered state changes,
be needed here?

Agreed. Under the LKMM, a control dependency does not order load-load on
weakly ordered architectures (such as ARM64).

In v8, I will address this with acquire-release ordering:
- In __smc_diag_dump(), I use smp_load_acquire(&sk->sk_state) and reuse
  the acquired state (in r->diag_state) across the subsequent mode/state
  checks.
- In af_smc.c, I paired it with smp_store_release(&sk->sk_state,
SMC_ACTIVE) when transitioning out of SMC_INIT.

quoted

@@ -103,6 +104,9 @@ static int __smc_diag_dump(struct sock *sk, struct sk_buff *skb,
 	if (nla_put(skb, SMC_DIAG_FALLBACK, sizeof(fallback), &fallback) < 0)
 		goto errout;
 
+	if (sk->sk_state == SMC_INIT || sk->sk_state == SMC_CLOSED)
+		goto out;
+

[Severity: High]
Does this leave the conn->lnk replacement during SMC-R link failover still
racing with the diag reader?

For SMC_ACTIVE and closing-state sockets, the LGRINFO block further down
still does this with only read_lock(&smc_hash->lock) held:

net/smc/smc_diag.c:__smc_diag_dump() {
    ...
		struct smc_link *link = smc->conn.lnk;
		...
		memcpy(linfo.lnk[0].ibname, link->smcibdev->ibdev->name,
		       sizeof(link->smcibdev->ibdev->name));
    ...
}

The failover side doesn't take the hash lock and doesn't unhash the
socket. For example, when the peer sends DELETE_LINK:

smc_llc_process_cli_delete_link()
  smc_switch_conns(lgr, lnk_del, false)
    smc_switch_link_and_count(conn, to_lnk)
      smcr_link_put(conn->lnk);
      conn->lnk = to_lnk;
  smcr_link_clear(lnk_del, true)
    ...
    __smcr_link_clear()
      put_device(&lnk->smcibdev->ibdev->dev);
      smcibdev = lnk->smcibdev;
      memset(lnk, 0, sizeof(struct smc_link));
      ...
      atomic_dec_return(&smcibdev->lnk_cnt)

A local RNIC or port going down reaches smc_switch_conns() the same way,
through smcr_link_down().

smc_switch_link_and_count() runs only under lgr->conns_lock and
conn->send_lock. The diag reader takes neither lock.

Suppose the reader loaded the old conn->lnk before the switch and was
delayed before the later loads. Could it then dereference a NULL
link->smcibdev after the memset()? Could it instead read a freed smcibdev
or ibdev and copy those bytes to userspace?

An unprivileged SOCK_DIAG dump that asks for SMC_DIAG_LGRINFO (for
example smcss -R) can reach this.

The socket stays hashed in SMC_ACTIVE or a closing state. So neither this
guard nor the !list_empty(&smc->conn.lgr->list) check from 9dbe086c69b8
filters it out

Good catch. When smcr_link_clear() drops the final base refcount after
smc_switch_conns(), __smcr_link_clear() can execute memset() and
put_device() while a concurrent diag reader holds a pointer to the old link.

To eliminate the pointer dereference and external device lifetime
dependency entirely, in v8 I switched from chasing
link->smcibdev->ibdev->name to copying directly from the cached
link->ibname (which is embedded in struct smc_link and initialized once
at link creation). Since struct smc_link is embedded in the link group
(pinned while the socket is hashed), this avoids the NULL dereference
and UAF on smcibdev/ibdev without introducing extra locking on the
critical TX path.

I'll post v8 shortly with these 2 changes.

pw-bot: cr