KVM: arm64: Mark host bss and rodata section as shared

As the hypervisor maps the host's .bss and .rodata sections in its
stage-1, make sure to tag them as shared in hyp and host page-tables.

But since the hypervisor relies on the presence of these mappings, we
cannot let the host in complete control of the memory regions -- it
must not unshare or donate them to another entity for example. To
prevent this, let's transfer the ownership of those ranges to the
hypervisor itself, and share the pages back with the host.
Signed-off-by: Quentin Perret <qperret@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210809152448.1810400-18-qperret@google.com

arch/arm64/kvm/hyp/nvhe/setup.c

@@ -58,6 +58,7 @@ static int recreate_hyp_mappings(phys_addr_t phys, unsigned long size,
 {
 	void *start, *end, *virt = hyp_phys_to_virt(phys);
 	unsigned long pgt_size = hyp_s1_pgtable_pages() << PAGE_SHIFT;
+	enum kvm_pgtable_prot prot;
 	int ret, i;
 
 	/* Recreate the hyp page-table using the early page allocator */
@@ -83,10 +84,6 @@ static int recreate_hyp_mappings(phys_addr_t phys, unsigned long size,
 	if (ret)
 		return ret;
 
-	ret = pkvm_create_mappings(__start_rodata, __end_rodata, PAGE_HYP_RO);
-	if (ret)
-		return ret;
-
 	ret = pkvm_create_mappings(__hyp_rodata_start, __hyp_rodata_end, PAGE_HYP_RO);
 	if (ret)
 		return ret;
@@ -95,10 +92,6 @@ static int recreate_hyp_mappings(phys_addr_t phys, unsigned long size,
 	if (ret)
 		return ret;
 
-	ret = pkvm_create_mappings(__hyp_bss_end, __bss_stop, PAGE_HYP_RO);
-	if (ret)
-		return ret;
-
 	ret = pkvm_create_mappings(virt, virt + size, PAGE_HYP);
 	if (ret)
 		return ret;
@@ -117,6 +110,24 @@ static int recreate_hyp_mappings(phys_addr_t phys, unsigned long size,
 			return ret;
 	}
 
+	/*
+	 * Map the host's .bss and .rodata sections RO in the hypervisor, but
+	 * transfer the ownership from the host to the hypervisor itself to
+	 * make sure it can't be donated or shared with another entity.
+	 *
+	 * The ownership transition requires matching changes in the host
+	 * stage-2. This will be done later (see finalize_host_mappings()) once
+	 * the hyp_vmemmap is addressable.
+	 */
+	prot = pkvm_mkstate(PAGE_HYP_RO, PKVM_PAGE_SHARED_OWNED);
+	ret = pkvm_create_mappings(__start_rodata, __end_rodata, prot);
+	if (ret)
+		return ret;
+
+	ret = pkvm_create_mappings(__hyp_bss_end, __bss_stop, prot);
+	if (ret)
+		return ret;
+
 	return 0;
 }
 
@@ -148,6 +159,57 @@ static void hpool_put_page(void *addr)
 	hyp_put_page(&hpool, addr);
 }
 
+static int finalize_host_mappings_walker(u64 addr, u64 end, u32 level,
+					 kvm_pte_t *ptep,
+					 enum kvm_pgtable_walk_flags flag,
+					 void * const arg)
+{
+	enum kvm_pgtable_prot prot;
+	enum pkvm_page_state state;
+	kvm_pte_t pte = *ptep;
+	phys_addr_t phys;
+
+	if (!kvm_pte_valid(pte))
+		return 0;
+
+	if (level != (KVM_PGTABLE_MAX_LEVELS - 1))
+		return -EINVAL;
+
+	phys = kvm_pte_to_phys(pte);
+	if (!addr_is_memory(phys))
+		return 0;
+
+	/*
+	 * Adjust the host stage-2 mappings to match the ownership attributes
+	 * configured in the hypervisor stage-1.
+	 */
+	state = pkvm_getstate(kvm_pgtable_hyp_pte_prot(pte));
+	switch (state) {
+	case PKVM_PAGE_OWNED:
+		return host_stage2_set_owner_locked(phys, PAGE_SIZE, pkvm_hyp_id);
+	case PKVM_PAGE_SHARED_OWNED:
+		prot = pkvm_mkstate(PKVM_HOST_MEM_PROT, PKVM_PAGE_SHARED_BORROWED);
+		break;
+	case PKVM_PAGE_SHARED_BORROWED:
+		prot = pkvm_mkstate(PKVM_HOST_MEM_PROT, PKVM_PAGE_SHARED_OWNED);
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	return host_stage2_idmap_locked(phys, PAGE_SIZE, prot);
+}
+
+static int finalize_host_mappings(void)
+{
+	struct kvm_pgtable_walker walker = {
+		.cb	= finalize_host_mappings_walker,
+		.flags	= KVM_PGTABLE_WALK_LEAF,
+	};
+
+	return kvm_pgtable_walk(&pkvm_pgtable, 0, BIT(pkvm_pgtable.ia_bits), &walker);
+}
+
 void __noreturn __pkvm_init_finalise(void)
 {
 	struct kvm_host_data *host_data = this_cpu_ptr(&kvm_host_data);
@@ -167,6 +229,10 @@ void __noreturn __pkvm_init_finalise(void)
 	if (ret)
 		goto out;
 
+	ret = finalize_host_mappings();
+	if (ret)
+		goto out;
+
 	pkvm_pgtable_mm_ops = (struct kvm_pgtable_mm_ops) {
 		.zalloc_page = hyp_zalloc_hyp_page,
 		.phys_to_virt = hyp_phys_to_virt,