Re: [PATCH] vhost: Add a defensive check in vhost_commit against wrong deallocation

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

On Wed, Sep 13, 2023 at 09:46:57AM +0200, Eric Auger wrote:
> In vhost_commit(), it may happen that dev->mem_sections and
> dev->tmp_sections are equal,

Could you please explain a bit more how this can happen?
I don't see how.

> in which case, unconditionally
> freeing old_sections at the end of the function will also free
> dev->mem_sections used on subsequent call leading to a segmentation
> fault.
> 
> Check this situation before deallocating memory.
> 
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> Fixes: c44317efecb2 ("vhost: Build temporary section list and deref
> after commit")
> CC: QEMU Stable <qemu-stable@nongnu.org>
> 
> ---
> 
> This SIGSEV condition can be reproduced with
> https://lore.kernel.org/all/20230904080451.424731-1-eric.auger@redhat.com/#r
> This is most probably happening in a situation where the memory API is
> used in a wrong manner but well.

sounds like misusing the memory API can lead to all kind of mischief.

> ---
>  hw/virtio/vhost.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index e2f6ffb446..c02c599ef0 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -545,6 +545,11 @@ static void vhost_commit(MemoryListener *listener)
>      dev->mem_sections = dev->tmp_sections;
>      dev->n_mem_sections = dev->n_tmp_sections;
>  
> +    if (old_sections == dev->mem_sections) {
> +        assert(n_old_sections ==  dev->n_mem_sections);
> +        return;
> +    }
> +
>      if (dev->n_mem_sections != n_old_sections) {
>          changed = true;
>      } else {
> -- 
> 2.41.0