Date:   Thu, 2 Apr 2020 11:37:17 -0500
From:   Venu Busireddy <venu.busireddy@...cle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     Ashish Kalra <Ashish.Kalra@....com>, pbonzini@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        joro@...tes.org, bp@...e.de, thomas.lendacky@....com,
        x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        rientjes@...gle.com, srutherford@...gle.com, luto@...nel.org
Subject: Re: [PATCH v6 01/14] KVM: SVM: Add KVM_SEV SEND_START command

On 2020-04-02 07:59:54 -0500, Brijesh Singh wrote:
> Hi Venu,
> 
> Thanks for the feedback.
> 
> On 4/2/20 1:27 AM, Venu Busireddy wrote:
> > On 2020-03-30 06:19:59 +0000, Ashish Kalra wrote:
> >> From: Brijesh Singh <Brijesh.Singh@....com>
> >>
> >> The command is used to create an outgoing SEV guest encryption context.
> >>
> >> Cc: Thomas Gleixner <tglx@...utronix.de>
> >> Cc: Ingo Molnar <mingo@...hat.com>
> >> Cc: "H. Peter Anvin" <hpa@...or.com>
> >> Cc: Paolo Bonzini <pbonzini@...hat.com>
> >> Cc: "Radim Krčmář" <rkrcmar@...hat.com>
> >> Cc: Joerg Roedel <joro@...tes.org>
> >> Cc: Borislav Petkov <bp@...e.de>
> >> Cc: Tom Lendacky <thomas.lendacky@....com>
> >> Cc: x86@...nel.org
> >> Cc: kvm@...r.kernel.org
> >> Cc: linux-kernel@...r.kernel.org
> >> Reviewed-by: Steve Rutherford <srutherford@...gle.com>
> >> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> >> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> >> ---
> >>  .../virt/kvm/amd-memory-encryption.rst        |  27 ++++
> >>  arch/x86/kvm/svm.c                            | 128 ++++++++++++++++++
> >>  include/linux/psp-sev.h                       |   8 +-
> >>  include/uapi/linux/kvm.h                      |  12 ++
> >>  4 files changed, 171 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/Documentation/virt/kvm/amd-memory-encryption.rst b/Documentation/virt/kvm/amd-memory-encryption.rst
> >> index c3129b9ba5cb..4fd34fc5c7a7 100644
> >> --- a/Documentation/virt/kvm/amd-memory-encryption.rst
> >> +++ b/Documentation/virt/kvm/amd-memory-encryption.rst
> >> @@ -263,6 +263,33 @@ Returns: 0 on success, -negative on error
> >>                  __u32 trans_len;
> >>          };
> >>  
> >> +10. KVM_SEV_SEND_START
> >> +----------------------
> >> +
> >> +The KVM_SEV_SEND_START command can be used by the hypervisor to create an
> >> +outgoing guest encryption context.
> >> +
> >> +Parameters (in): struct kvm_sev_send_start
> >> +
> >> +Returns: 0 on success, -negative on error
> >> +
> >> +::
> >> +        struct kvm_sev_send_start {
> >> +                __u32 policy;                 /* guest policy */
> >> +
> >> +                __u64 pdh_cert_uaddr;         /* platform Diffie-Hellman certificate */
> >> +                __u32 pdh_cert_len;
> >> +
> >> +                __u64 plat_certs_uadr;        /* platform certificate chain */
> > Could this please be changed to plat_certs_uaddr, as it is referred to
> > in the rest of the code?
> >
> >> +                __u32 plat_certs_len;
> >> +
> >> +                __u64 amd_certs_uaddr;        /* AMD certificate */
> >> +                __u32 amd_cert_len;
> > Could this please be changed to amd_certs_len, as it is referred to in
> > the rest of the code?
> >
> >> +
> >> +                __u64 session_uaddr;          /* Guest session information */
> >> +                __u32 session_len;
> >> +        };
> >> +
> >>  References
> >>  ==========
> >>  
> >> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> >> index 50d1ebafe0b3..63d172e974ad 100644
> >> --- a/arch/x86/kvm/svm.c
> >> +++ b/arch/x86/kvm/svm.c
> >> @@ -7149,6 +7149,131 @@ static int sev_launch_secret(struct kvm *kvm, struct kvm_sev_cmd *argp)
> >>  	return ret;
> >>  }
> >>  
> >> +/* Userspace wants to query session length. */
> >> +static int
> >> +__sev_send_start_query_session_length(struct kvm *kvm, struct kvm_sev_cmd *argp,
> >> +				      struct kvm_sev_send_start *params)
> >> +{
> >> +	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> >> +	struct sev_data_send_start *data;
> >> +	int ret;
> >> +
> >> +	data = kzalloc(sizeof(*data), GFP_KERNEL_ACCOUNT);
> >> +	if (data == NULL)
> >> +		return -ENOMEM;
> >> +
> >> +	data->handle = sev->handle;
> >> +	ret = sev_issue_cmd(kvm, SEV_CMD_SEND_START, data, &argp->error);
> >> +
> >> +	params->session_len = data->session_len;
> >> +	if (copy_to_user((void __user *)(uintptr_t)argp->data, params,
> >> +				sizeof(struct kvm_sev_send_start)))
> >> +		ret = -EFAULT;
> >> +
> >> +	kfree(data);
> >> +	return ret;
> >> +}
> >> +
> >> +static int sev_send_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
> >> +{
> >> +	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> >> +	struct sev_data_send_start *data;
> >> +	struct kvm_sev_send_start params;
> >> +	void *amd_certs, *session_data;
> >> +	void *pdh_cert, *plat_certs;
> >> +	int ret;
> >> +
> >> +	if (!sev_guest(kvm))
> >> +		return -ENOTTY;
> >> +
> >> +	if (copy_from_user(&params, (void __user *)(uintptr_t)argp->data,
> >> +				sizeof(struct kvm_sev_send_start)))
> >> +		return -EFAULT;
> >> +
> >> +	/* if session_len is zero, userspace wants to query the session length */
> >> +	if (!params.session_len)
> >> +		return __sev_send_start_query_session_length(kvm, argp,
> >> +				&params);
> >> +
> >> +	/* some sanity checks */
> >> +	if (!params.pdh_cert_uaddr || !params.pdh_cert_len ||
> >> +	    !params.session_uaddr || params.session_len > SEV_FW_BLOB_MAX_SIZE)
> >> +		return -EINVAL;
> >> +
> >> +	/* allocate the memory to hold the session data blob */
> >> +	session_data = kmalloc(params.session_len, GFP_KERNEL_ACCOUNT);
> >> +	if (!session_data)
> >> +		return -ENOMEM;
> >> +
> >> +	/* copy the certificate blobs from userspace */
> >> +	pdh_cert = psp_copy_user_blob(params.pdh_cert_uaddr,
> >> +				params.pdh_cert_len);
> >> +	if (IS_ERR(pdh_cert)) {
> >> +		ret = PTR_ERR(pdh_cert);
> >> +		goto e_free_session;
> >> +	}
> >> +
> >> +	plat_certs = psp_copy_user_blob(params.plat_certs_uaddr,
> >> +				params.plat_certs_len);
> >> +	if (IS_ERR(plat_certs)) {
> >> +		ret = PTR_ERR(plat_certs);
> >> +		goto e_free_pdh;
> >> +	}
> >> +
> >> +	amd_certs = psp_copy_user_blob(params.amd_certs_uaddr,
> >> +				params.amd_certs_len);
> >> +	if (IS_ERR(amd_certs)) {
> >> +		ret = PTR_ERR(amd_certs);
> >> +		goto e_free_plat_cert;
> >> +	}
> >> +
> >> +	data = kzalloc(sizeof(*data), GFP_KERNEL_ACCOUNT);
> >> +	if (data == NULL) {
> >> +		ret = -ENOMEM;
> >> +		goto e_free_amd_cert;
> >> +	}
> >> +
> >> +	/* populate the FW SEND_START field with system physical address */
> >> +	data->pdh_cert_address = __psp_pa(pdh_cert);
> >> +	data->pdh_cert_len = params.pdh_cert_len;
> >> +	data->plat_certs_address = __psp_pa(plat_certs);
> >> +	data->plat_certs_len = params.plat_certs_len;
> >> +	data->amd_certs_address = __psp_pa(amd_certs);
> >> +	data->amd_certs_len = params.amd_certs_len;
> >> +	data->session_address = __psp_pa(session_data);
> >> +	data->session_len = params.session_len;
> >> +	data->handle = sev->handle;
> >> +
> >> +	ret = sev_issue_cmd(kvm, SEV_CMD_SEND_START, data, &argp->error);
> >> +
> >> +	if (ret)
> >> +		goto e_free;
> >> +
> >> +	if (copy_to_user((void __user *)(uintptr_t) params.session_uaddr,
> >> +			session_data, params.session_len)) {
> >> +		ret = -EFAULT;
> >> +		goto e_free;
> >> +	}
> > To optimize the amount of data being copied to user space, could the
> > above section of code changed as follows?
> >
> > 	params.session_len = data->session_len;
> > 	if (copy_to_user((void __user *)(uintptr_t) params.session_uaddr,
> > 			session_data, params.session_len)) {
> > 		ret = -EFAULT;
> > 		goto e_free;
> > 	}
> 
> 
> We should not be using the data->session_len, it will cause -EFAULT when
> user has not allocated enough space in the session_uaddr. Lets consider
> the case where user passes session_len=10 but firmware thinks the
> session length should be 64. In that case the data->session_len will
> contains a value of 64 but userspace has allocated space for 10 bytes
> and copy_to_user() will fail. If we are really concern about the amount
> of data getting copied to userspace then use min_t(size_t,
> params.session_len, data->session_len).

We are allocating a buffer of params.session_len size and passing that
buffer, and that length via data->session_len, to the firmware. Why would
the firmware set data->session_len to a larger value, in spite of telling
it that the buffer is only params.session_len long? I thought that only
the reverse is possible, that is, the user sets the params.session_len
to the MAX, but the session data is actually smaller than that size.

Also, if for whatever reason the firmware sets data->session_len to
a larger value than what is passed, what is the user space expected
to do when the call returns? If the user space tries to access
params.session_len amount of data, it will possibly get a memory access
violation, because it did not originally allocate that large a buffer.

If we do go with using min_t(size_t, params.session_len,
data->session_len), then params.session_len should also be set to the
smaller of the two, right?

> >> +
> >> +	params.policy = data->policy;
> >> +	params.session_len = data->session_len;
> >> +	if (copy_to_user((void __user *)(uintptr_t)argp->data, &params,
> >> +				sizeof(struct kvm_sev_send_start)))
> >> +		ret = -EFAULT;
> > Since the only fields that are changed in the kvm_sev_send_start structure
> > are session_len and policy, why do we need to copy the entire structure
> > back to the user? Why not just those two values? Please see the changes
> > proposed to kvm_sev_send_start structure further below to accomplish this.
> 
> I think we also need to consider the code readability while saving the
> CPU cycles. This is very small structure. By duplicating into two calls
> #1 copy params.policy and #2 copy params.session_len we will add more
> CPU cycle. And, If we get creative and rearrange the structure then code
> readability is lost because now the copy will depend on how the
> structure is layout in the memory.

I was not recommending splitting that call into two. That would certainly
be more expensive, than copying the entire structure. That is the reason
why I suggested reordering the members of kvm_sev_send_start. Isn't
there plenty of code where structures are defined in a way to keep the
data movement efficient? :-)

Please see my other comment below.

> 
> >
> > 	params.policy = data->policy;
> > 	if (copy_to_user((void __user *)(uintptr_t)argp->data, &params,
> > 			sizeof(params.policy) + sizeof(params.session_len))
> > 		ret = -EFAULT;
> >> +
> >> +e_free:
> >> +	kfree(data);
> >> +e_free_amd_cert:
> >> +	kfree(amd_certs);
> >> +e_free_plat_cert:
> >> +	kfree(plat_certs);
> >> +e_free_pdh:
> >> +	kfree(pdh_cert);
> >> +e_free_session:
> >> +	kfree(session_data);
> >> +	return ret;
> >> +}
> >> +
> >>  static int svm_mem_enc_op(struct kvm *kvm, void __user *argp)
> >>  {
> >>  	struct kvm_sev_cmd sev_cmd;
> >> @@ -7193,6 +7318,9 @@ static int svm_mem_enc_op(struct kvm *kvm, void __user *argp)
> >>  	case KVM_SEV_LAUNCH_SECRET:
> >>  		r = sev_launch_secret(kvm, &sev_cmd);
> >>  		break;
> >> +	case KVM_SEV_SEND_START:
> >> +		r = sev_send_start(kvm, &sev_cmd);
> >> +		break;
> >>  	default:
> >>  		r = -EINVAL;
> >>  		goto out;
> >> diff --git a/include/linux/psp-sev.h b/include/linux/psp-sev.h
> >> index 5167bf2bfc75..9f63b9d48b63 100644
> >> --- a/include/linux/psp-sev.h
> >> +++ b/include/linux/psp-sev.h
> >> @@ -323,11 +323,11 @@ struct sev_data_send_start {
> >>  	u64 pdh_cert_address;			/* In */
> >>  	u32 pdh_cert_len;			/* In */
> >>  	u32 reserved1;
> >> -	u64 plat_cert_address;			/* In */
> >> -	u32 plat_cert_len;			/* In */
> >> +	u64 plat_certs_address;			/* In */
> >> +	u32 plat_certs_len;			/* In */
> >>  	u32 reserved2;
> >> -	u64 amd_cert_address;			/* In */
> >> -	u32 amd_cert_len;			/* In */
> >> +	u64 amd_certs_address;			/* In */
> >> +	u32 amd_certs_len;			/* In */
> >>  	u32 reserved3;
> >>  	u64 session_address;			/* In */
> >>  	u32 session_len;			/* In/Out */
> >> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> >> index 4b95f9a31a2f..17bef4c245e1 100644
> >> --- a/include/uapi/linux/kvm.h
> >> +++ b/include/uapi/linux/kvm.h
> >> @@ -1558,6 +1558,18 @@ struct kvm_sev_dbg {
> >>  	__u32 len;
> >>  };
> >>  
> >> +struct kvm_sev_send_start {
> >> +	__u32 policy;
> >> +	__u64 pdh_cert_uaddr;
> >> +	__u32 pdh_cert_len;
> >> +	__u64 plat_certs_uaddr;
> >> +	__u32 plat_certs_len;
> >> +	__u64 amd_certs_uaddr;
> >> +	__u32 amd_certs_len;
> >> +	__u64 session_uaddr;
> >> +	__u32 session_len;
> >> +};
> > Redo this structure as below:
> >
> > struct kvm_sev_send_start {
> > 	__u32 policy;
> > 	__u32 session_len;
> > 	__u64 session_uaddr;
> > 	__u64 pdh_cert_uaddr;
> > 	__u32 pdh_cert_len;
> > 	__u64 plat_certs_uaddr;
> > 	__u32 plat_certs_len;
> > 	__u64 amd_certs_uaddr;
> > 	__u32 amd_certs_len;
> > };
> >
> > Or as below, just to make it look better.
> >
> > struct kvm_sev_send_start {
> > 	__u32 policy;
> > 	__u32 session_len;
> > 	__u64 session_uaddr;
> > 	__u32 pdh_cert_len;
> > 	__u64 pdh_cert_uaddr;
> > 	__u32 plat_certs_len;
> > 	__u64 plat_certs_uaddr;
> > 	__u32 amd_certs_len;
> > 	__u64 amd_certs_uaddr;
> > };
> >
> 
> Wherever applicable, I tried  best to not divert from the SEV spec
> structure layout. Anyone who is reading the SEV FW spec  will see a
> similar structure layout in the KVM/PSP header files. I would prefer to
> stick to that approach.

This structure is in uapi, and is anyway different from the
sev_data_send_start, right? Does it really need to stay close to the
firmware structure layout? Just because the firmware folks thought of
a structure layout, that should not prevent our code to be efficient.

> 
> 
> >> +
> >>  #define KVM_DEV_ASSIGN_ENABLE_IOMMU	(1 << 0)
> >>  #define KVM_DEV_ASSIGN_PCI_2_3		(1 << 1)
> >>  #define KVM_DEV_ASSIGN_MASK_INTX	(1 << 2)
> >> -- 
> >> 2.17.1
> >>

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