2023-01-19 22:12:11

by Dionna Amalie Glaze

Subject: [PATCH v3 2/2] kvm: sev: If ccp is busy, report throttled to guest

The ccp driver can be overloaded even with 1 HZ throttling. The return
value of -EBUSY means that there is no firmware error to report back to
user space, so the guest VM would see this as exitinfo2 = 0. The false
success can trick the guest to update its the message sequence number
when it shouldn't have.

Instead, when ccp returns -EBUSY, that is reported to userspace as the
throttling return value.

Cc: Thomas Lendacky <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Peter Gonda <[email protected]>
Cc: Borislav Petkov <[email protected]>

Signed-off-by: Dionna Glaze <[email protected]>
---
arch/x86/kvm/svm/sev.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index cd9372ce6fc2..7da1cc300d7b 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3642,7 +3642,14 @@ static void snp_handle_guest_request(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t
goto unlock;

rc = sev_issue_cmd(kvm, SEV_CMD_SNP_GUEST_REQUEST, &data, &err);
- if (rc)
+
+ /*
+ * The ccp driver can return -EBUSY if the PSP is overloaded, so signal
+ * the request has been throttled.
+ */
+ if (rc == -EBUSY)
+ rc = SNP_GUEST_REQ_THROTTLED;
+ else if (rc)
/* use the firmware error code */
rc = err;

@@ -3713,7 +3720,14 @@ static void snp_handle_ext_guest_request(struct vcpu_svm *svm, gpa_t req_gpa, gp
if (sev->snp_certs_len)
data_npages = sev->snp_certs_len >> PAGE_SHIFT;

- if (rc) {
+ /*
+ * The ccp driver can return -EBUSY if the PSP is overloaded, so signal
+ * the request has been throttled.
+ */
+ if (rc == -EBUSY) {
+ rc = SNP_GUEST_REQ_THROTTLED;
+ goto cleanup;
+ } else if (rc) {
/*
* If buffer length is small then return the expected
* length in rbx.
--
2.39.0.246.g2a6d74b583-goog

2023-01-20 17:47:50

by Tom Lendacky

Subject: Re: [PATCH v3 2/2] kvm: sev: If ccp is busy, report throttled to guest

On 1/19/23 15:34, Dionna Glaze wrote:

Since you're building on SNP hypervisor patches, please keep @Ashish on
direct copy.

> The ccp driver can be overloaded even with 1 HZ throttling. The return
> value of -EBUSY means that there is no firmware error to report back to
> user space, so the guest VM would see this as exitinfo2 = 0. The false
> success can trick the guest to update its the message sequence number
> when it shouldn't have.
>
> Instead, when ccp returns -EBUSY, that is reported to userspace as the
> throttling return value.

Except the CCP driver doesn't return -EBUSY because it is overloaded. It
will simply try to acquire the mutex and continue once it has it.

There are a couple of places that return -EBUSY in the driver for other
reasons, as well as other -E* values. It looks like these need to be
handled properly by the SNP hypervisor patches so that a "success" isn't
reported back.

So this patch isn't necessary, but any -E* return value without having
actually called the firmware needs to be handled properly. @Ashish, please
work with Dionna on this.

Thanks,
Tom

>
> Cc: Thomas Lendacky <[email protected]>
> Cc: Paolo Bonzini <[email protected]>
> Cc: Joerg Roedel <[email protected]>
> Cc: Peter Gonda <[email protected]>
> Cc: Borislav Petkov <[email protected]>
>
> Signed-off-by: Dionna Glaze <[email protected]>
> ---
> arch/x86/kvm/svm/sev.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index cd9372ce6fc2..7da1cc300d7b 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3642,7 +3642,14 @@ static void snp_handle_guest_request(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t
> goto unlock;
>
> rc = sev_issue_cmd(kvm, SEV_CMD_SNP_GUEST_REQUEST, &data, &err);
> - if (rc)
> +
> + /*
> + * The ccp driver can return -EBUSY if the PSP is overloaded, so signal
> + * the request has been throttled.
> + */
> + if (rc == -EBUSY)
> + rc = SNP_GUEST_REQ_THROTTLED;
> + else if (rc)
> /* use the firmware error code */
> rc = err;
>
> @@ -3713,7 +3720,14 @@ static void snp_handle_ext_guest_request(struct vcpu_svm *svm, gpa_t req_gpa, gp
> if (sev->snp_certs_len)
> data_npages = sev->snp_certs_len >> PAGE_SHIFT;
>
> - if (rc) {
> + /*
> + * The ccp driver can return -EBUSY if the PSP is overloaded, so signal
> + * the request has been throttled.
> + */
> + if (rc == -EBUSY) {
> + rc = SNP_GUEST_REQ_THROTTLED;
> + goto cleanup;
> + } else if (rc) {
> /*
> * If buffer length is small then return the expected
> * length in rbx.