linux-kernel - Re: [RFC 16/33] KVM: x86/mmu: Expose R/W/X flags during memory fault exits

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Date:   Tue, 28 Nov 2023 09:36:00 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Nicolas Saenz Julienne <nsaenz@...zon.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
        pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        anelkz@...zon.com, graf@...zon.com, dwmw@...zon.co.uk,
        jgowans@...zon.com, corbert@....net, kys@...rosoft.com,
        haiyangz@...rosoft.com, decui@...rosoft.com, x86@...nel.org,
        linux-doc@...r.kernel.org
Subject: Re: [RFC 16/33] KVM: x86/mmu: Expose R/W/X flags during memory
 fault exits

On Wed, 2023-11-08 at 11:17 +0000, Nicolas Saenz Julienne wrote:
> Include the fault's read, write and execute status when exiting to
> user-space.
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenz@...zon.com>
> ---
>  arch/x86/kvm/mmu/mmu.c   | 4 ++--
>  include/linux/kvm_host.h | 9 +++++++--
>  include/uapi/linux/kvm.h | 6 ++++++
>  3 files changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 4e02d506cc25..feca077c0210 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4300,8 +4300,8 @@ static inline u8 kvm_max_level_for_order(int order)
>  static void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
>  					      struct kvm_page_fault *fault)
>  {
> -	kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
> -				      PAGE_SIZE, fault->write, fault->exec,
> +	kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT, PAGE_SIZE,
> +				      fault->write, fault->exec, fault->user,
>  				      fault->is_private);
>  }
>  
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 71e1e8cf8936..631fd532c97a 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2367,14 +2367,19 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr)
>  static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
>  						 gpa_t gpa, gpa_t size,
>  						 bool is_write, bool is_exec,
> -						 bool is_private)
> +						 bool is_read, bool is_private)

It almost feels like there is a need for a struct to hold all of those parameters.

>  {
>  	vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
>  	vcpu->run->memory_fault.gpa = gpa;
>  	vcpu->run->memory_fault.size = size;
>  
> -	/* RWX flags are not (yet) defined or communicated to userspace. */
>  	vcpu->run->memory_fault.flags = 0;
> +	if (is_read)
> +		vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_READ;
> +	if (is_write)
> +		vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_WRITE;
> +	if (is_exec)
> +		vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_EXECUTE;
>  	if (is_private)
>  		vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE;
>  }
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 03f5c08fd7aa..0ddffb8b0c99 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -533,7 +533,13 @@ struct kvm_run {
>  		} notify;
>  		/* KVM_EXIT_MEMORY_FAULT */
>  		struct {
> +#define KVM_MEMORY_EXIT_FLAG_READ	(1ULL << 0)
> +#define KVM_MEMORY_EXIT_FLAG_WRITE	(1ULL << 1)
> +#define KVM_MEMORY_EXIT_FLAG_EXECUTE	(1ULL << 2)
>  #define KVM_MEMORY_EXIT_FLAG_PRIVATE	(1ULL << 3)
> +#define KVM_MEMORY_EXIT_NO_ACCESS                            \
> +	(KVM_MEMORY_EXIT_FLAG_NR | KVM_MEMORY_EXIT_FLAG_NW | \
> +	 KVM_MEMORY_EXIT_FLAG_NX)
>  			__u64 flags;
>  			__u64 gpa;
>  			__u64 size;


I don't think that KVM_MEMORY_EXIT_FLAG_NR, KVM_MEMORY_EXIT_FLAG_NW, KVM_MEMORY_EXIT_FLAG_NX are defined anywhere.
Also why KVM_MEMORY_EXIT_NO_ACCESS is needed - userspace can infer it from the lack of other access flags.

Best regards,
	Maxim Levitsky