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

Marc Zyngier
2021-07-26T11:27:19Z

On Mon, 26 Jul 2021 10:29:04 +0100,
Quentin Perret <qperret@google.com> wrote:
>
> 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);

Why the different treatment here? If we couldn't map the page in EL2
Stage-1, surely that's also a big problem, isn't it?

> +
> +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;

How comes that this doesn't generate a warning on a GICv2 system when
we try and map the CPU interface via create_hyp_io_mappings()?

>
> 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);
>

Thanks,

M.

--
Without deviation from the norm, progress is not possible.