Re: [PATCH v6 01/14] KVM: SVM: Add KVM_SEV SEND_START command

From: Venu Busireddy
Date: Thu Apr 02 2020 - 12:37:56 EST

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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@xxxxxxx>
> >>
> >> The command is used to create an outgoing SEV guest encryption context.
> >>
> >> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> >> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> >> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> >> Cc: "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>
> >> Cc: Joerg Roedel <joro@xxxxxxxxxx>
> >> Cc: Borislav Petkov <bp@xxxxxxx>
> >> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> >> Cc: x86@xxxxxxxxxx
> >> Cc: kvm@xxxxxxxxxxxxxxx
> >> Cc: linux-kernel@xxxxxxxxxxxxxxx
> >> Reviewed-by: Steve Rutherford <srutherford@xxxxxxxxxx>
> >> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> >> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> >> ---
> >> .../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 triedÂ best to not divert from the SEV spec
> structure layout. Anyone who is reading the SEV FW specÂ 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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In reply to: Brijesh Singh: "Re: [PATCH v6 01/14] KVM: SVM: Add KVM_SEV SEND_START command"
Next in thread: Brijesh Singh: "Re: [PATCH v6 01/14] KVM: SVM: Add KVM_SEV SEND_START command"
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