mshv: Don't free overlay pages after a failed unmap

On L1VH partitions where the VMM can provide overlay GPFNs, the kernel allocates the VP state and stats pages and supplies them to the hypervisor as overlay locations through hv_map_vp_state_page() and hv_map_stats_page(). The hypervisor may write intercept messages, register state, and statistics to these pages. Only a successful HVCALL_UNMAP_VP_STATE_PAGE or HVCALL_UNMAP_STATS_PAGE confirms that the corresponding overlay has been revoked.

hv_unmap_vp_state_page() and hv_unmap_stats_page() free the overlay page regardless of the hypercall result. If the unmap fails, revocation is unconfirmed, but dropping the driver's page reference may still allow the page to be returned to the allocator and reused. A subsequent hypervisor write through a retained overlay could then corrupt an unrelated kernel or user allocation.

Free an overlay page only after the unmap hypercall succeeds. On failure, leak the page rather than return memory that the hypervisor may still be able to access. This matches how the rest of drivers/hv handles pages whose state is unknown after a failed hypervisor operation; for example, hv_free_page() leaks a page that could not be re-encrypted. Log the leaked page in both helpers, as some of their callers discard the return value.

---
v2:
- Also log a failed stats page unmap before leaking the page (Wei Liu).
- Rebased on v7.4-rc1.

Compile-tested with x86_64 defconfig, CONFIG_HYPERV=y, CONFIG_MSHV_ROOT=m, and W=1. Not tested on an L1VH system.

 drivers/hv/mshv_root_hv_call.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

index b8d199f95299..e761addda50e 100644
@@ -643,8 +643,18 @@ int hv_unmap_vp_state_page(u64 partition_id, u32 vp_index, u32 type,
 {
 	int ret = hv_call_unmap_vp_state_page(partition_id, vp_index, type, input_vtl);
 
-	if (mshv_use_overlay_gpfn() && state_page)
-		__free_page(state_page);
+	if (mshv_use_overlay_gpfn() && state_page) {
+		/*
+		 * If the unmap failed, the hypervisor may still be using the
+		 * page as an overlay. Leak it rather than handing it back to
+		 * the page allocator.
+		 */
+		if (ret)
+			pr_err("%s: failed to unmap partition %llu vp %u state page type %u, leaking page: %d\n",
+			       __func__, partition_id, vp_index, type, ret);
+		else
+			__free_page(state_page);
+	}
 
 	return ret;
 }

@@ -995,8 +1005,18 @@ int hv_unmap_stats_page(enum hv_stats_object_type type,
 
 	ret = hv_call_unmap_stats_page(type, identity);
 
-	if (mshv_use_overlay_gpfn() && page_addr)
-		__free_page(virt_to_page(page_addr));
+	if (mshv_use_overlay_gpfn() && page_addr) {
+		/*
+		 * If the unmap failed, the hypervisor may still be updating the
+		 * overlay page. Leak it rather than handing it back to the page
+		 * allocator.
+		 */
+		if (ret)
+			pr_err("%s: failed to unmap stats page type %u, leaking page: %d\n",
+			       __func__, type, ret);
+		else
+			__free_page(virt_to_page(page_addr));
+	}
 
 	return ret;
 }

2.43.0

Applied. Thanks.