[PATCH v2 1/3] KVM: arm64: Check PGD alignment when creating a pVM

[Fuad Tabba <fuad.tabba@linux.dev>]

From: Quentin Perret <qperret@google.com>

Martijn reported a hypervisor crash when providing pKVM with an
undersized PGD allocation. Indeed, although the size of the PGD
allocation at EL2 is not under host control, a smaller host-side
allocation can lead to providing pKVM with a misaligned PGD, which will
cause the guest stage-2 init to fail in a bad way. Specifically,
guest_s2_zalloc_pages_exact() expects a successful allocation from
hyp_alloc_pages(), which can only happen if the pool has been pre-filled
with a physically aligned high-order page.

In order to guarantee allocation success in this path, check the
host-provided PGD alignment early on.

Fixes: 60dfe093ec13 ("KVM: arm64: Instantiate guest stage-2 page-tables at EL2")
Reported-by: Martijn Bogaard <martijnbogaard@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
[Fuad: Returned -EINVAL and moved the check ahead of hyp_alloc(), with
 the other input checks in __pkvm_init_vm(). Cast pgd_hva for its __kern
 type, updated the comment, and pointed Fixes: at the commit that
 initialises the guest's pool from the PGD.]
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/kvm/hyp/nvhe/mem_protect.c | 2 +-
 arch/arm64/kvm/hyp/nvhe/pkvm.c        | 8 ++++++--
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index d4c332a918035..5b624b1ab32e8 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -186,7 +186,7 @@ static void *guest_s2_zalloc_pages_exact(size_t size)
 {
 	void *addr = hyp_alloc_pages(&current_vm->pool, get_order(size));
 
-	WARN_ON(size != (PAGE_SIZE << get_order(size)));
+	WARN_ON(!addr || size != (PAGE_SIZE << get_order(size)));
 	hyp_split_page(hyp_virt_to_page(addr));
 
 	return addr;
diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
index fc0475f0877d1..03e530e15b4a2 100644
--- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
+++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
@@ -892,8 +892,8 @@ void teardown_selftest_vm(void)
  *
  * host_kvm: A pointer to the host's struct kvm.
  * pgd_hva: The host va of the area being donated for the stage-2 PGD for
- *	    the VM. Must be page aligned. Its size is implied by the VM's
- *	    VTCR.
+ *	    the VM. Must be aligned to its size, which is implied by the
+ *	    VM's VTCR.
  *
  * Return 0 success, negative error code on failure.
  */
@@ -924,6 +924,10 @@ int __pkvm_init_vm(struct kvm *host_kvm, void __kern *pgd_hva)
 
 	vm_size = pkvm_get_hyp_vm_size(nr_vcpus);
 	pgd_size = kvm_pgtable_stage2_pgd_size(host_mmu.arch.mmu.vtcr);
+	if (!IS_ALIGNED((unsigned long)pgd_hva, pgd_size)) {
+		ret = -EINVAL;
+		goto err_unpin_kvm;
+	}
 
 	hyp_vm = hyp_alloc(vm_size);
 	if (!hyp_vm) {
-- 
2.47.3