[PATCH net] pds_core: fix auxiliary device add/del races

["Nikhil P. Rao" <nikhil.rao@amd.com>]

Two paths add or delete the same slot (pf->vfs[vf_id].padev): a VF's
pdsc_reset_done() and the PF's devlink enable_vnet/disable_vnet handler.
They serialize on config_lock, but neither guards the slot under it
correctly.

add() registers and stores a new auxiliary device without first checking
the slot, so a second add of an already-populated slot leaks the first
device. del() makes that check outside config_lock, so two concurrent
dels can both pass it; the first clears the slot, and the second
dereferences a NULL pointer.

Check and update the slot under config_lock in both paths.

Fixes: b699bdc720c0 ("pds_core: specify auxiliary_device to be created")
Reported-by: sashiko-bot@kernel.org # Running on a local machine
Signed-off-by: Nikhil P. Rao <nikhil.rao@amd.com>
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
---
 drivers/net/ethernet/amd/pds_core/auxbus.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/amd/pds_core/auxbus.c b/drivers/net/ethernet/amd/pds_core/auxbus.c
index 73b3481220b1..3acafe10a6d5 100644
--- a/drivers/net/ethernet/amd/pds_core/auxbus.c
+++ b/drivers/net/ethernet/amd/pds_core/auxbus.c
@@ -177,17 +177,21 @@ void pdsc_auxbus_dev_del(struct pdsc *cf, struct pdsc *pf,
 {
 	struct pds_auxiliary_dev *padev;
 
-	if (!*pd_ptr)
-		return;
-
 	mutex_lock(&pf->config_lock);
 
+	/* A concurrent del may have already torn this device down and
+	 * cleared it.
+	 */
 	padev = *pd_ptr;
+	if (!padev)
+		goto out_unlock;
+
 	pds_client_unregister(pf, padev->client_id);
 	auxiliary_device_delete(&padev->aux_dev);
 	auxiliary_device_uninit(&padev->aux_dev);
 	*pd_ptr = NULL;
 
+out_unlock:
 	mutex_unlock(&pf->config_lock);
 }
 
@@ -210,6 +214,13 @@ int pdsc_auxbus_dev_add(struct pdsc *cf, struct pdsc *pf,
 
 	mutex_lock(&pf->config_lock);
 
+	/* Nothing to do if the aux device is already present.  This also
+	 * guards against a second add overwriting *pd_ptr and leaking the
+	 * first, symmetric with the check in pdsc_auxbus_dev_del().
+	 */
+	if (*pd_ptr)
+		goto out_unlock;
+
 	mask = BIT_ULL(PDSC_S_FW_DEAD) |
 	       BIT_ULL(PDSC_S_STOPPING_DRIVER);
 	if (cf->state & mask) {
-- 
2.43.0