[PATCH net v2] net: psp: check for device unregister when creating assoc

[Jakub Kicinski <kuba@kernel.org>]

psp_assoc_device_get_locked() obtains a psp_dev reference via
psp_dev_get_for_sock() (which uses psp_dev_tryget() under RCU);
it then acquires psd->lock and drops the reference. Before
the lock is taken, psp_dev_unregister() can run to completion:
take psd->lock, clear out state, unlock, drop the registration
reference.

The expectation is that the lock prevents device unregistration,
but much like with netdevs special care has to be taken when
"upgrading" a reference to a locked device. Add the missing
check if device is still alive. psp_dev_is_registered() exists
already but had no callers, which makes me wonder if I either
forgot to add this or lost the check during refactoring...

Reported-by: Yiming Qian <yimingqian591@gmail.com>
Fixes: 6b46ca260e22 ("net: psp: add socket security association code")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
---
v2:
 - Sashiko spotted that psp_dev_check_access() has to happen after
   liveness check, as well, so move the liveness check earlier
v1: https://lore.kernel.org/20260324205107.318303-1-kuba@kernel.org

CC: daniel.zahka@gmail.com
CC: willemdebruijn.kernel@gmail.com
---
 net/psp/psp_nl.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/psp/psp_nl.c b/net/psp/psp_nl.c
index 6afd7707ec12..0cc744a6e1c9 100644
--- a/net/psp/psp_nl.c
+++ b/net/psp/psp_nl.c
@@ -305,8 +305,13 @@ int psp_assoc_device_get_locked(const struct genl_split_ops *ops,
 
 	psd = psp_dev_get_for_sock(socket->sk);
 	if (psd) {
-		err = psp_dev_check_access(psd, genl_info_net(info));
-		if (err) {
+		/* Extra care needed here, psp_dev_get_for_sock() only gives
+		 * us access to struct psp_dev's memory, which is quite weak.
+		 */
+		mutex_lock(&psd->lock);
+		if (!psp_dev_is_registered(psd) ||
+		    psp_dev_check_access(psd, genl_info_net(info))) {
+			mutex_unlock(&psd->lock);
 			psp_dev_put(psd);
 			psd = NULL;
 		}
@@ -319,7 +324,6 @@ int psp_assoc_device_get_locked(const struct genl_split_ops *ops,
 
 	id = info->attrs[PSP_A_ASSOC_DEV_ID];
 	if (psd) {
-		mutex_lock(&psd->lock);
 		if (id && psd->id != nla_get_u32(id)) {
 			mutex_unlock(&psd->lock);
 			NL_SET_ERR_MSG_ATTR(info->extack, id,
-- 
2.53.0