Re: [PATCH v1] virtio-mem: fix division by zero in virtio_mem_activate_memslots_to_plug()

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 23, 2023 at 01:13:41PM +0200, David Hildenbrand wrote:
> When running with "dynamic-memslots=off", we enter
> virtio_mem_activate_memslots_to_plug() to return immediately again
> because "vmem->dynamic_memslots == false". However, the compiler might
> not optimize out calculating start_idx+end_idx, where we divide by
> vmem->memslot_size. In such a configuration, the memslot size is 0 and
> we'll get a division by zero:
> 
>     (qemu) qom-set vmem0 requested-size 3G
>     (qemu) q35.sh: line 38: 622940 Floating point exception(core dumped)
> 
> The same is true for virtio_mem_deactivate_unplugged_memslots(), however
> we never really reach that code without a prior
> virtio_mem_activate_memslots_to_plug() call.
> 
> Let's fix it by simply calling these functions only with
> "dynamic-memslots=on".
> 
> This was found when using a debug build of QEMU.
> 
> Reprted-by: Mario Casquero <mcasquer@redhat.com>
> Fixes: 177f9b1ee464 ("virtio-mem: Expose device memory dynamically via multiple memslots if enabled")
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>


Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  hw/virtio/virtio-mem.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> index 9dc3c61b5a..be4b0b364f 100644
> --- a/hw/virtio/virtio-mem.c
> +++ b/hw/virtio/virtio-mem.c
> @@ -525,9 +525,7 @@ static void virtio_mem_activate_memslots_to_plug(VirtIOMEM *vmem,
>                                   vmem->memslot_size;
>      unsigned int idx;
>  
> -    if (!vmem->dynamic_memslots) {
> -        return;
> -    }
> +    assert(vmem->dynamic_memslots);
>  
>      /* Activate all involved memslots in a single transaction. */
>      memory_region_transaction_begin();
> @@ -547,9 +545,7 @@ static void virtio_mem_deactivate_unplugged_memslots(VirtIOMEM *vmem,
>                                   vmem->memslot_size;
>      unsigned int idx;
>  
> -    if (!vmem->dynamic_memslots) {
> -        return;
> -    }
> +    assert(vmem->dynamic_memslots);
>  
>      /* Deactivate all memslots with unplugged blocks in a single transaction. */
>      memory_region_transaction_begin();
> @@ -598,7 +594,9 @@ static int virtio_mem_set_block_state(VirtIOMEM *vmem, uint64_t start_gpa,
>          virtio_mem_notify_unplug(vmem, offset, size);
>          virtio_mem_set_range_unplugged(vmem, start_gpa, size);
>          /* Deactivate completely unplugged memslots after updating the state. */
> -        virtio_mem_deactivate_unplugged_memslots(vmem, offset, size);
> +        if (vmem->dynamic_memslots) {
> +            virtio_mem_deactivate_unplugged_memslots(vmem, offset, size);
> +        }
>          return 0;
>      }
>  
> @@ -635,9 +633,11 @@ static int virtio_mem_set_block_state(VirtIOMEM *vmem, uint64_t start_gpa,
>           * blocks we are plugging here. The following notification will inform
>           * registered listeners about the blocks we're plugging.
>           */
> -        virtio_mem_activate_memslots_to_plug(vmem, offset, size);
> +        if (vmem->dynamic_memslots) {
> +            virtio_mem_activate_memslots_to_plug(vmem, offset, size);
> +        }
>          ret = virtio_mem_notify_plug(vmem, offset, size);
> -        if (ret) {
> +        if (ret && vmem->dynamic_memslots) {
>              virtio_mem_deactivate_unplugged_memslots(vmem, offset, size);
>          }
>      }
> @@ -749,7 +749,9 @@ static int virtio_mem_unplug_all(VirtIOMEM *vmem)
>          notifier_list_notify(&vmem->size_change_notifiers, &vmem->size);
>  
>          /* Deactivate all memslots after updating the state. */
> -        virtio_mem_deactivate_unplugged_memslots(vmem, 0, region_size);
> +        if (vmem->dynamic_memslots) {
> +            virtio_mem_deactivate_unplugged_memslots(vmem, 0, region_size);
> +        }
>      }
>  
>      trace_virtio_mem_unplugged_all();
> -- 
> 2.41.0