[PATCH] vfio: prevent infinite loop in vfio_mig_get_next_state() on blocked arc

[Junrui Luo]

vfio_mig_get_next_state() walks vfio_from_fsm_table[] one step at a time,
looping to skip optional states the device does not support until
*next_fsm is supported. A blocked transition is encoded as
VFIO_DEVICE_STATE_ERROR, which the trailing return reports as -EINVAL.

The skip loop does not account for the ERROR sentinel.
state_flags_table[ERROR] is ~0U and vfio_from_fsm_table[ERROR][*] is
ERROR, so once *next_fsm becomes ERROR the loop condition stays true and
*next_fsm never changes. The blocked arcs STOP_COPY -> PRE_COPY and
STOP_COPY -> PRE_COPY_P2P map to ERROR yet pass the support check on a
precopy-capable device, causing the loop to spin forever while holding
the driver state mutex. This can result in a soft lockup, and a panic
with softlockup_panic set.

Terminate the skip loop on the ERROR sentinel so a blocked transition
falls through to the existing return and reports -EINVAL.

Fixes: 4db52602a607 ("vfio: Extend the device migration protocol with PRE_COPY")
Reported-by: Yuhao Jiang <danisjiang@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Junrui Luo <moonafterrain@xxxxxxxxxxx>
---
 drivers/vfio/vfio_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 6222376ab6ab..5e0422014523 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -858,7 +858,8 @@ int vfio_mig_get_next_state(struct vfio_device *device,
 	 * logical state, as per the above comment.
 	 */
 	*next_fsm = vfio_from_fsm_table[cur_fsm][new_fsm];
-	while ((state_flags_table[*next_fsm] & device->migration_flags) !=
+	while (*next_fsm != VFIO_DEVICE_STATE_ERROR &&
+	       (state_flags_table[*next_fsm] & device->migration_flags) !=
 			state_flags_table[*next_fsm])
 		*next_fsm = vfio_from_fsm_table[*next_fsm][new_fsm];
 

---
base-commit: e43ffb69e0438cddd72aaa30898b4dc446f664f8
change-id: 20260602-fixes-f5c6a4880594

Best regards,
-- 
Junrui Luo <moonafterrain@xxxxxxxxxxx>