Re: [RFC PATCH 1/3] Fixed Error Handling in vhost_user_set_mem_table_postcopy

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

On Mon, Dec 09, 2019 at 02:00:45AM -0500, Raphael Norwitz wrote:
> The current vhost_user_set_mem_table_postcopy() implementation
> populates each region of the VHOST_USER_SET_MEM_TABLE
> message without first checking if there are more than
> VHOST_MEMORY_MAX_NREGIONS already populated. This can
> cause memory corruption and potentially a crash if too many
> regions are added to the message during the postcopy step.
> 
> Additionally, after populating each region, the current
> implementation asserts that the current region index is less than
> VHOST_MEMORY_MAX_NREGIONS. Thus, even if the aforementioned
> bug is fixed by moving the existing assert up, too many hot-adds
> during the postcopy step will bring down qemu instead of
> gracefully propogating up the error as in
> vhost_user_set_mem_table().
> 
> This change cleans up error handling in
> vhost_user_set_mem_table_postcopy() such that it handles
> an unsupported number of memory hot-adds like
> vhost_user_set_mem_table(), gracefully propogating an error
> up instead of corrupting memory and crashing qemu.
> 
> Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>

Unfortunately all the nice error handling is mostly
futile since vhost_commit does:

        r = dev->vhost_ops->vhost_set_mem_table(dev, dev->mem);
        if (r < 0) {
            VHOST_OPS_DEBUG("vhost_set_mem_table failed");
        }
        goto out;

so the reason there's an assert is that we really must never
hit this path at all.  So moving assert up seems cleaner.

> ---
>  hw/virtio/vhost-user.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index 02a9b25..f74ff3b 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -441,6 +441,10 @@ static int vhost_user_set_mem_table_postcopy(struct vhost_dev *dev,
>                                       &offset);
>          fd = memory_region_get_fd(mr);
>          if (fd > 0) {
> +            if (fd_num == VHOST_MEMORY_MAX_NREGIONS) {
> +                error_report("Failed preparing vhost-user memory table msg");
> +                return -1;
> +            }
>              trace_vhost_user_set_mem_table_withfd(fd_num, mr->name,
>                                                    reg->memory_size,
>                                                    reg->guest_phys_addr,
> @@ -453,7 +457,6 @@ static int vhost_user_set_mem_table_postcopy(struct vhost_dev *dev,
>              msg.payload.memory.regions[fd_num].guest_phys_addr =
>                  reg->guest_phys_addr;
>              msg.payload.memory.regions[fd_num].mmap_offset = offset;
> -            assert(fd_num < VHOST_MEMORY_MAX_NREGIONS);
>              fds[fd_num++] = fd;
>          } else {
>              u->region_rb_offset[i] = 0;
> -- 
> 1.8.3.1