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

[Alex Williamson <alex@shazbot.org>]

On Tue, 02 Jun 2026 16:58:48 +0800
Junrui Luo <moonafterrain@outlook.com> wrote:

> 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@gmail.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
> ---
>  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];
>  

Applied to vfio next branch for v7.2.  Thanks,

Alex