[PATCH v2 15/16] KVM: arm64: Restrict EL2 stage-1 changes in protected mode

Quentin Perret
2021-07-26T09:30:08Z

The host kernel is currently able to change EL2 stage-1 mappings without
restrictions thanks to the __pkvm_create_mappings() hypercall. But in a
world where the host is no longer part of the TCB, this clearly poses a
problem.

To fix this, introduce a new hypercall to allow the host to share a
physical memory page with the hypervisor, and remove the
__pkvm_create_mappings() variant. The new hypercall implements
ownership and permission checks before allowing the sharing operation,
and it annotates the shared page in the hypervisor stage-1 and host
stage-2 page-tables.

Signed-off-by: Quentin Perret <qperret@google.com>
---
arch/arm64/include/asm/kvm_asm.h | 2 +-
arch/arm64/kvm/hyp/include/nvhe/mem_protect.h | 1 +
arch/arm64/kvm/hyp/nvhe/hyp-main.c | 11 +--
arch/arm64/kvm/hyp/nvhe/mem_protect.c | 90 +++++++++++++++++++
arch/arm64/kvm/mmu.c | 28 +++++-
5 files changed, 120 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index 9f0bf2109be7..78db818ae2c9 100644
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -59,7 +59,7 @@
#define __KVM_HOST_SMCCC_FUNC___vgic_v3_save_aprs 13
#define __KVM_HOST_SMCCC_FUNC___vgic_v3_restore_aprs 14
#define __KVM_HOST_SMCCC_FUNC___pkvm_init 15
-#define __KVM_HOST_SMCCC_FUNC___pkvm_create_mappings 16
+#define __KVM_HOST_SMCCC_FUNC___pkvm_host_share_hyp 16
#define __KVM_HOST_SMCCC_FUNC___pkvm_create_private_mapping 17
#define __KVM_HOST_SMCCC_FUNC___pkvm_cpu_set_vector 18
#define __KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize 19
diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
index 8e5725d032b2..1b8e59a9d065 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
@@ -49,6 +49,7 @@ extern struct host_kvm host_kvm;

int __pkvm_prot_finalize(void);
int __pkvm_mark_hyp(phys_addr_t start, phys_addr_t end);
+int __pkvm_host_share_hyp(u64 pfn);

int host_stage2_idmap_locked(u64 start, u64 end, enum kvm_pgtable_prot prot);
int kvm_host_prepare_stage2(void *pgt_pool_base);
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index 1632f001f4ed..dd155646ec86 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -140,14 +140,11 @@ static void handle___pkvm_cpu_set_vector(struct kvm_cpu_context *host_ctxt)
cpu_reg(host_ctxt, 1) = pkvm_cpu_set_vector(slot);
}

-static void handle___pkvm_create_mappings(struct kvm_cpu_context *host_ctxt)
+static void handle___pkvm_host_share_hyp(struct kvm_cpu_context *host_ctxt)
{
- DECLARE_REG(unsigned long, start, host_ctxt, 1);
- DECLARE_REG(unsigned long, size, host_ctxt, 2);
- DECLARE_REG(unsigned long, phys, host_ctxt, 3);
- DECLARE_REG(enum kvm_pgtable_prot, prot, host_ctxt, 4);
+ DECLARE_REG(u64, pfn, host_ctxt, 1);

- cpu_reg(host_ctxt, 1) = __pkvm_create_mappings(start, size, phys, prot);
+ cpu_reg(host_ctxt, 1) = __pkvm_host_share_hyp(pfn);
}

static void handle___pkvm_create_private_mapping(struct kvm_cpu_context *host_ctxt)
@@ -193,7 +190,7 @@ static const hcall_t host_hcall[] = {
HANDLE_FUNC(__vgic_v3_restore_aprs),
HANDLE_FUNC(__pkvm_init),
HANDLE_FUNC(__pkvm_cpu_set_vector),
- HANDLE_FUNC(__pkvm_create_mappings),
+ HANDLE_FUNC(__pkvm_host_share_hyp),
HANDLE_FUNC(__pkvm_create_private_mapping),
HANDLE_FUNC(__pkvm_prot_finalize),
HANDLE_FUNC(__pkvm_mark_hyp),
diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 223c541a7051..75273166d2c5 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -315,6 +315,96 @@ static int host_stage2_idmap(u64 addr)
return ret;
}

+static inline bool check_prot(enum kvm_pgtable_prot prot,
+ enum kvm_pgtable_prot required,
+ enum kvm_pgtable_prot denied)
+{
+ return (prot & (required | denied)) == required;
+}
+
+int __pkvm_host_share_hyp(u64 pfn)
+{
+ phys_addr_t addr = hyp_pfn_to_phys(pfn);
+ enum kvm_pgtable_prot prot, cur;
+ void * virt = __hyp_va(addr);
+ enum pkvm_page_state state;
+ kvm_pte_t pte;
+ u32 level;
+ int ret;
+
+ if (!range_is_memory(addr, addr + PAGE_SIZE))
+ return -EINVAL;
+
+ hyp_spin_lock(&host_kvm.lock);
+ hyp_spin_lock(&pkvm_pgd_lock);
+
+ ret = kvm_pgtable_get_leaf(&host_kvm.pgt, addr, &pte, &level);
+ if (ret)
+ goto unlock;
+ if (!pte)
+ goto map_shared;
+
+ /*
+ * Check attributes in the host stage-2 PTE. We need the page to be:
+ * - mapped RWX as we're sharing memory;
+ * - not borrowed, as that implies absence of ownership.
+ * Otherwise, we can't let it got through
+ */
+ cur = kvm_pgtable_stage2_pte_prot(pte);
+ prot = pkvm_mkstate(0, PKVM_PAGE_SHARED_BORROWED);
+ if (!check_prot(cur, KVM_PGTABLE_PROT_RWX, prot)) {
+ ret = -EPERM;
+ goto unlock;
+ }
+
+ state = pkvm_getstate(cur);
+ if (state == PKVM_PAGE_OWNED)
+ goto map_shared;
+
+ /*
+ * Tolerate double-sharing the same page, but this requires
+ * cross-checking the hypervisor stage-1.
+ */
+ if (state != PKVM_PAGE_SHARED_OWNED) {
+ ret = -EPERM;
+ goto unlock;
+ }
+
+ ret = kvm_pgtable_get_leaf(&pkvm_pgtable, (u64)virt, &pte, &level);
+ if (ret)
+ goto unlock;
+
+ /*
+ * If the page has been shared with the hypervisor, it must be
+ * SHARED_BORROWED already.
+ */
+ cur = kvm_pgtable_hyp_pte_prot(pte);
+ prot = pkvm_mkstate(PAGE_HYP, PKVM_PAGE_SHARED_BORROWED);
+ if (!check_prot(cur, prot, ~prot))
+ ret = EPERM;
+ goto unlock;
+
+map_shared:
+ /*
+ * If the page is not yet shared, adjust mappings in both page-tables
+ * while both locks are held.
+ */
+ prot = pkvm_mkstate(PAGE_HYP, PKVM_PAGE_SHARED_BORROWED);
+ ret = pkvm_create_mappings_locked(virt, virt + PAGE_SIZE, prot);
+ if (ret)
+ goto unlock;
+
+ prot = pkvm_mkstate(KVM_PGTABLE_PROT_RWX, PKVM_PAGE_SHARED_OWNED);
+ ret = host_stage2_idmap_locked(addr, addr + PAGE_SIZE, prot);
+ BUG_ON(ret);
+
+unlock:
+ hyp_spin_unlock(&pkvm_pgd_lock);
+ hyp_spin_unlock(&host_kvm.lock);
+
+ return ret;
+}
+
int __pkvm_mark_hyp(phys_addr_t start, phys_addr_t end)
{
int ret;
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 0625bf2353c2..cbab146cda6a 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -259,10 +259,8 @@ static int __create_hyp_mappings(unsigned long start, unsigned long size,
{
int err;

- if (!kvm_host_owns_hyp_mappings()) {
- return kvm_call_hyp_nvhe(__pkvm_create_mappings,
- start, size, phys, prot);
- }
+ if (WARN_ON(!kvm_host_owns_hyp_mappings()))
+ return -EINVAL;

mutex_lock(&kvm_hyp_pgd_mutex);
err = kvm_pgtable_hyp_map(hyp_pgtable, start, size, phys, prot);
@@ -282,6 +280,21 @@ static phys_addr_t kvm_kaddr_to_phys(void *kaddr)
}
}

+static int pkvm_share_hyp(phys_addr_t start, phys_addr_t end)
+{
+ phys_addr_t addr;
+ int ret;
+
+ for (addr = ALIGN_DOWN(start, PAGE_SIZE); addr < end; addr += PAGE_SIZE) {
+ ret = kvm_call_hyp_nvhe(__pkvm_host_share_hyp,
+ __phys_to_pfn(addr));
+ if (ret)
+ return ret;
+ }
+
+ return 0;
+}
+
/**
* create_hyp_mappings - duplicate a kernel virtual address range in Hyp mode
* @from: The virtual kernel start address of the range
@@ -302,6 +315,13 @@ int create_hyp_mappings(void *from, void *to, enum kvm_pgtable_prot prot)
if (is_kernel_in_hyp_mode())
return 0;

+ if (!kvm_host_owns_hyp_mappings()) {
+ if (WARN_ON(prot != PAGE_HYP))
+ return -EPERM;
+ return pkvm_share_hyp(kvm_kaddr_to_phys(from),
+ kvm_kaddr_to_phys(to));
+ }
+
start = start & PAGE_MASK;
end = PAGE_ALIGN(end);

--
2.32.0.432.gabb21c7263-goog