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

[Marc Zyngier <maz@kernel.org>]

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.

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