RE: [PATCH] hv: hv_balloon: validate unballoon range count

[Michael Kelley <mhklinux@outlook.com>]

From: Michael Bommarito <michael.bommarito@gmail.com> Sent: Thursday, July 9, 2026 7:29 PM
> 
> The Hyper-V dynamic memory host supplies DM_UNBALLOON_REQUEST messages
> with a header size and a range_count field. balloon_down() trusts
> range_count and walks req->range_array without checking that the received
> message contains that many ranges.
> 
> A malformed host or backend message can therefore make the guest read
> past the received VMBus packet while freeing balloon ranges. Validate the
> received message size and reject range_count values that exceed the
> present range array before walking it.

Same comment applies here as I wrote for your proposed validations for
the Hyper-V mouse driver. The balloon driver also has .allowed_in_isolated
set to false, so it isn't loaded in a CoCo VM and it hasn't been hardened
for the "untrusted host" threat model.

Michael

> 
> Impact: A malicious Hyper-V host or backend can crash a guest by sending
> a short unballoon request with an oversized range_count.
> 
> Fixes: 9aa8b50b2b3d ("Drivers: hv: Add Hyper-V balloon driver")
> Cc: stable@vger.kernel.org
> Assisted-by: Codex:gpt-5-5-xhigh
> Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
> ---
>  drivers/hv/hv_balloon.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index a848400a59a2d..f5bc8c9fea7b9 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -1337,8 +1337,23 @@ static void balloon_up(struct work_struct *dummy)
>  	}
>  }
> 
> +static bool unballoon_request_valid(struct dm_unballoon_request *req,
> +				    u32 msg_size)
> +{
> +	u32 max_ranges;
> +
> +	if (msg_size < sizeof(*req) || req->hdr.size < sizeof(*req) ||
> +	    req->hdr.size > msg_size)
> +		return false;
> +
> +	max_ranges = (req->hdr.size - sizeof(*req)) /
> +		     sizeof(req->range_array[0]);
> +
> +	return req->range_count <= max_ranges;
> +}
> +
>  static void balloon_down(struct hv_dynmem_device *dm,
> -			 struct dm_unballoon_request *req)
> +			 struct dm_unballoon_request *req, u32 msg_size)
>  {
>  	union dm_mem_page_range *range_array = req->range_array;
>  	int range_count = req->range_count;
> @@ -1346,6 +1361,12 @@ static void balloon_down(struct hv_dynmem_device *dm,
>  	int i;
>  	unsigned int prev_pages_ballooned = dm->num_pages_ballooned;
> 
> +	if (!unballoon_request_valid(req, msg_size)) {
> +		pr_warn_ratelimited("Invalid unballoon request: size %u, header size
> %u, range count %u\n",
> +				    msg_size, req->hdr.size, req->range_count);
> +		return;
> +	}
> +
>  	for (i = 0; i < range_count; i++) {
>  		free_balloon_pages(dm, &range_array[i]);
>  		complete(&dm_device.config_event);
> @@ -1527,7 +1548,8 @@ static void balloon_onchannelcallback(void *context)
> 
>  			dm->state = DM_BALLOON_DOWN;
>  			balloon_down(dm,
> -				     (struct dm_unballoon_request *)recv_buffer);
> +				     (struct dm_unballoon_request *)recv_buffer,
> +				     recvlen);
>  			break;
> 
>  		case DM_MEM_HOT_ADD_REQUEST:
> --
> 2.53.0