Re: [PATCH 09/14] KVM: arm64: Mark host bss and rodata section as shared

[Marc Zyngier <maz@kernel.org>]

On Mon, 19 Jul 2021 11:47:30 +0100,
Quentin Perret <qperret@google.com> wrote:
> 
> 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 page back with the host.
> 
> Signed-off-by: Quentin Perret <qperret@google.com>
> ---
>  arch/arm64/kvm/hyp/include/nvhe/mem_protect.h |  1 +
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c         |  7 ++-
>  arch/arm64/kvm/hyp/nvhe/setup.c               | 52 ++++++++++++++++---
>  3 files changed, 51 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> index 9c227d87c36d..b39047463075 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> @@ -23,6 +23,7 @@ extern struct host_kvm host_kvm;
>  int __pkvm_prot_finalize(void);
>  int __pkvm_mark_hyp(phys_addr_t start, phys_addr_t end);
>  
> +int host_stage2_idmap_locked(u64 start, u64 end, enum kvm_pgtable_prot prot);
>  int kvm_host_prepare_stage2(void *pgt_pool_base);
>  void handle_host_mem_abort(struct kvm_cpu_context *host_ctxt);
>  
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index cdace80d3e28..6f28edf58407 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -235,6 +235,11 @@ static bool host_stage2_want_pte_cb(u64 addr, u64 end, enum kvm_pgtable_prot pro
>  		return prot != KVM_PGTABLE_PROT_RW;
>  }
>  
> +int host_stage2_idmap_locked(u64 start, u64 end, enum kvm_pgtable_prot prot)
> +{
> +	return host_stage2_try(__host_stage2_idmap, start, end, prot);
> +}
> +
>  static int host_stage2_idmap(u64 addr)
>  {
>  	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_RW;
> @@ -250,7 +255,7 @@ static int host_stage2_idmap(u64 addr)
>  	if (ret)
>  		goto unlock;
>  
> -	ret = host_stage2_try(__host_stage2_idmap, range.start, range.end, prot);
> +	ret = host_stage2_idmap_locked(range.start, range.end, prot);
>  unlock:
>  	hyp_spin_unlock(&host_kvm.lock);
>  
> diff --git a/arch/arm64/kvm/hyp/nvhe/setup.c b/arch/arm64/kvm/hyp/nvhe/setup.c
> index 0b574d106519..74dce83a6fad 100644
> --- a/arch/arm64/kvm/hyp/nvhe/setup.c
> +++ b/arch/arm64/kvm/hyp/nvhe/setup.c
> @@ -83,10 +83,6 @@ 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 +91,6 @@ 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 +109,25 @@ 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 ownerhsip from the host to the hypervisor itself to
> +	 * make sure it can't be donated or shared with another entity.
> +	 *
> +	 * The ownership transtion requires matching changes in the host
> +	 * stage-2. This will done later (see finalize_mappings()) once the
> +	 * hyp_vmemmap is addressable.
> +	 */
> +	ret = pkvm_create_mappings(__start_rodata, __end_rodata,
> +				   PAGE_HYP_RO | KVM_PGTABLE_STATE_SHARED);
> +	if (ret)
> +		return ret;
> +
> +	ret = pkvm_create_mappings(__hyp_bss_end, __bss_stop,
> +				   PAGE_HYP_RO | KVM_PGTABLE_STATE_SHARED);
> +	if (ret)
> +		return ret;
> +
>  	return 0;
>  }
>  
> @@ -148,6 +159,27 @@ static void hpool_put_page(void *addr)
>  	hyp_put_page(&hpool, addr);
>  }
>  
> +static int finalize_mappings(void)
> +{
> +	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_RWX;
> +	int ret;
> +
> +	/*
> +	 * The host's .bss and .rodata sections are now conceptually owned by
> +	 * the hypervisor, so mark them as 'borrowed' in the host stage-2. We
> +	 * can safely use host_stage2_idmap_locked() at this point since the
> +	 * host stage-2 has not been enabled yet.
> +	 */
> +	prot |= KVM_PGTABLE_STATE_SHARED | KVM_PGTABLE_STATE_BORROWED;
> +	ret = host_stage2_idmap_locked(__hyp_pa(__start_rodata),
> +				       __hyp_pa(__end_rodata), prot);

Do we really want to map the rodata section as RWX?

> +	if (ret)
> +		return ret;
> +
> +	return host_stage2_idmap_locked(__hyp_pa(__hyp_bss_end),
> +					__hyp_pa(__bss_stop), prot);

If the 'locked' state implies SHARED+BORROWED, maybe consider moving
the ORRing of the prot into host_stage2_idmap_locked()?

> +}
> +
>  void __noreturn __pkvm_init_finalise(void)
>  {
>  	struct kvm_host_data *host_data = this_cpu_ptr(&kvm_host_data);
> @@ -167,6 +199,10 @@ void __noreturn __pkvm_init_finalise(void)
>  	if (ret)
>  		goto out;
>  
> +	ret = finalize_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,

Thanks,

	M.

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

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