[PATCH] hv: hv_balloon: validate unballoon range count

[Michael Bommarito <michael.bommarito@gmail.com>]

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.

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