diff --git a/repos/ports/src/virtualbox6/nem.cc b/repos/ports/src/virtualbox6/nem.cc
index 77a9b2b0d0..4697d35808 100644
--- a/repos/ports/src/virtualbox6/nem.cc
+++ b/repos/ports/src/virtualbox6/nem.cc
@@ -28,6 +28,9 @@
 #include <VBox/vmm/em.h>
 #include <VBox/err.h>
 
+/* Genode includes */
+#include <base/mutex.h>
+
 /* local includes */
 #include <stub_macros.h>
 #include <sup.h>
@@ -104,10 +107,11 @@ struct Sup::Nem
 		}
 	};
 
+	Mutex mutex       { };
 	Range host_range  { };
 	Range guest_range { };
 
-	void commit_range()
+	void commit_range_unsynchronized()
 	{
 		/* ignore commit of invalid ranges */
 		if (!host_range.valid())
@@ -124,8 +128,17 @@ struct Sup::Nem
 		guest_range = { };
 	}
 
+	void commit_range()
+	{
+		Mutex::Guard guard(mutex);
+
+		commit_range_unsynchronized();
+	}
+
 	void map_to_guest(addr_t host_addr, addr_t guest_addr, size_t size, Protection prot)
 	{
+		Mutex::Guard guard(mutex);
+
 		Range new_host_range  { host_addr,  host_addr  + (size - 1), prot };
 		Range new_guest_range { guest_addr, guest_addr + (size - 1), prot };
 
@@ -144,7 +157,7 @@ struct Sup::Nem
 		}
 
 		/* new page starts a new range */
-		commit_range();
+		commit_range_unsynchronized();
 
 		/* start over with new page */
 		host_range  = { host_addr,  host_addr  + (size - 1), prot };
diff --git a/repos/ports/src/virtualbox6/sup_gmm.cc b/repos/ports/src/virtualbox6/sup_gmm.cc
index 5aab1427f7..ae223c5063 100644
--- a/repos/ports/src/virtualbox6/sup_gmm.cc
+++ b/repos/ports/src/virtualbox6/sup_gmm.cc
@@ -95,6 +95,8 @@ Sup::Gmm::Vmm_addr Sup::Gmm::_alloc_pages(Pages pages)
 
 void Sup::Gmm::reservation_pages(Pages pages)
 {
+	Mutex::Guard guard(_mutex);
+
 	_reservation_pages = pages;
 
 	_update_pool_size();
@@ -103,6 +105,8 @@ void Sup::Gmm::reservation_pages(Pages pages)
 
 Sup::Gmm::Vmm_addr Sup::Gmm::alloc_ex(Pages pages)
 {
+	Mutex::Guard guard(_mutex);
+
 	_alloc_ex_pages = { _alloc_ex_pages.value + pages.value };
 
 	_update_pool_size();
@@ -113,12 +117,16 @@ Sup::Gmm::Vmm_addr Sup::Gmm::alloc_ex(Pages pages)
 
 Sup::Gmm::Vmm_addr Sup::Gmm::alloc_from_reservation(Pages pages)
 {
+	Mutex::Guard guard(_mutex);
+
 	return _alloc_pages(pages);
 }
 
 
 Sup::Gmm::Page_id Sup::Gmm::page_id(Vmm_addr addr)
 {
+	Mutex::Guard guard(_mutex);
+
 	bool const inside_map = (addr.value >= _map.base.value)
 	                     && (addr.value <= _map.end.value);
 
@@ -143,6 +151,8 @@ uint32_t Sup::Gmm::page_id_as_uint32(Page_id page_id)
 
 Sup::Gmm::Vmm_addr Sup::Gmm::vmm_addr(Page_id page_id)
 {
+	Mutex::Guard guard(_mutex);
+
 	/* NIL_GMM_CHUNKID kept unused - so offset 0 is chunk ID 1 */
 	addr_t const addr = _map.base.value
 	                  + ((page_id.value - (1u << GMM_CHUNKID_SHIFT)) << PAGE_SHIFT);
@@ -159,6 +169,8 @@ Sup::Gmm::Vmm_addr Sup::Gmm::vmm_addr(Page_id page_id)
 
 void Sup::Gmm::map_to_guest(Vmm_addr from, Guest_addr to, Pages pages, Protection prot)
 {
+	Mutex::Guard guard(_mutex);
+
 	/* revoke existing mappings to avoid overmap */
 	_vm_connection.detach(to.value, pages.value << PAGE_SHIFT);
 
diff --git a/repos/ports/src/virtualbox6/sup_gmm.h b/repos/ports/src/virtualbox6/sup_gmm.h
index 337aa8a613..12cb754cac 100644
--- a/repos/ports/src/virtualbox6/sup_gmm.h
+++ b/repos/ports/src/virtualbox6/sup_gmm.h
@@ -18,6 +18,7 @@
 /* Genode includes */
 #include <base/allocator_avl.h>
 #include <base/attached_dataspace.h>
+#include <base/mutex.h>
 #include <rm_session/connection.h>
 #include <vm_session/connection.h>
 #include <region_map/client.h>
@@ -93,6 +94,8 @@ class Sup::Gmm
 		static constexpr Bytes _map_size   { 32*1024*1024*1024ul };
 		static constexpr auto  _num_slices { _map_size.value / _slice_size.value };
 
+		Mutex _mutex { };
+
 		Dataspace_capability _slices[_num_slices];
 
 		Pages _size_pages        { 0 }; /* current backing-store allocations */