Re: [PATCH] KVM: SVM: Do not terminate SEV-ES guests on GHCB validation failure

From: Tom Lendacky
Date: Fri Dec 03 2021 - 13:59:53 EST

On 12/3/21 10:39 AM, Sean Christopherson wrote:
On Thu, Dec 02, 2021, Tom Lendacky wrote:

- return -EINVAL;
+ return false;

I'd really prefer that this helper continue to return 0/-EINVAL, there's no hint
in the function name that this return true/false. And given the usage, there's
no advantage to returning true/false. On the contrary, if there's a future
condition where this needs to exit to userspace, we'll end up switching this all
back to int.

I don't have any objection to that.



}
scratch_va = kvzalloc(len, GFP_KERNEL_ACCOUNT);
if (!scratch_va)
- return -ENOMEM;

...because this is wrong. Failure to allocate memory should exit to userspace,
not report an error to the guest.

+ goto e_scratch;
if (kvm_read_guest(svm->vcpu.kvm, scratch_gpa_beg, scratch_va, len)) {
/* Unable to copy scratch area from guest */
pr_err("vmgexit: kvm_read_guest for scratch area failed\n");
kvfree(scratch_va);
- return -EFAULT;
+ goto e_scratch;

Same here, failure to read guest memory is a userspace issue and needs to be
reported to userspace.

But it could be a guest issue as well... whichever is preferred is ok by me.


}
/*

IMO, this should be the patch (compile tested only).

I can test this, but probably won't be able to get to it until Monday.

Thanks,
Tom


---
arch/x86/include/asm/sev-common.h | 11 +++++
arch/x86/kvm/svm/sev.c | 75 +++++++++++++++++++------------
2 files changed, 58 insertions(+), 28 deletions(-)

diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
index 2cef6c5a52c2..6acaf5af0a3d 100644
--- a/arch/x86/include/asm/sev-common.h
+++ b/arch/x86/include/asm/sev-common.h
@@ -73,4 +73,15 @@

#define GHCB_RESP_CODE(v) ((v) & GHCB_MSR_INFO_MASK)

+/*
+ * Error codes related to GHCB input that can be communicated back to the guest
+ * by setting the lower 32-bits of the GHCB SW_EXITINFO1 field to 2.
+ */
+#define GHCB_ERR_NOT_REGISTERED 1
+#define GHCB_ERR_INVALID_USAGE 2
+#define GHCB_ERR_INVALID_SCRATCH_AREA 3
+#define GHCB_ERR_MISSING_INPUT 4
+#define GHCB_ERR_INVALID_INPUT 5
+#define GHCB_ERR_INVALID_EVENT 6
+
#endif
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 713e3daa9574..60c6d7b216eb 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2357,20 +2357,25 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
{
struct kvm_vcpu *vcpu;
struct ghcb *ghcb;
- u64 exit_code = 0;
+ u64 exit_code;
+ u64 reason;

ghcb = svm->sev_es.ghcb;

- /* Only GHCB Usage code 0 is supported */
- if (ghcb->ghcb_usage)
- goto vmgexit_err;
-
/*
- * Retrieve the exit code now even though is may not be marked valid
+ * Retrieve the exit code now even though it may not be marked valid
* as it could help with debugging.
*/
exit_code = ghcb_get_sw_exit_code(ghcb);

+ /* Only GHCB Usage code 0 is supported */
+ if (ghcb->ghcb_usage) {
+ reason = GHCB_ERR_INVALID_USAGE;
+ goto vmgexit_err;
+ }
+
+ reason = GHCB_ERR_MISSING_INPUT;
+
if (!ghcb_sw_exit_code_is_valid(ghcb) ||
!ghcb_sw_exit_info_1_is_valid(ghcb) ||
!ghcb_sw_exit_info_2_is_valid(ghcb))
@@ -2449,6 +2454,7 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
case SVM_VMGEXIT_UNSUPPORTED_EVENT:
break;
default:
+ reason = GHCB_ERR_INVALID_EVENT;
goto vmgexit_err;
}

@@ -2457,22 +2463,25 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
vmgexit_err:
vcpu = &svm->vcpu;

- if (ghcb->ghcb_usage) {
+ if (reason == GHCB_ERR_INVALID_USAGE) {
vcpu_unimpl(vcpu, "vmgexit: ghcb usage %#x is not valid\n",
ghcb->ghcb_usage);
+ } else if (reason == GHCB_ERR_INVALID_EVENT) {
+ vcpu_unimpl(vcpu, "vmgexit: exit code %#llx is not valid\n",
+ exit_code);
} else {
- vcpu_unimpl(vcpu, "vmgexit: exit reason %#llx is not valid\n",
+ vcpu_unimpl(vcpu, "vmgexit: exit code %#llx input is not valid\n",
exit_code);
dump_ghcb(svm);
}

- vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
- vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON;
- vcpu->run->internal.ndata = 2;
- vcpu->run->internal.data[0] = exit_code;
- vcpu->run->internal.data[1] = vcpu->arch.last_vmentry_cpu;
+ /* Clear the valid entries fields */
+ memset(ghcb->save.valid_bitmap, 0, sizeof(ghcb->save.valid_bitmap));

- return -EINVAL;
+ ghcb_set_sw_exit_info_1(ghcb, 2);
+ ghcb_set_sw_exit_info_2(ghcb, reason);
+
+ return 1;
}

void sev_es_unmap_ghcb(struct vcpu_svm *svm)
@@ -2542,14 +2551,14 @@ static int setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
scratch_gpa_beg = ghcb_get_sw_scratch(ghcb);
if (!scratch_gpa_beg) {
pr_err("vmgexit: scratch gpa not provided\n");
- return -EINVAL;
+ goto e_scratch;
}

scratch_gpa_end = scratch_gpa_beg + len;
if (scratch_gpa_end < scratch_gpa_beg) {
pr_err("vmgexit: scratch length (%#llx) not valid for scratch address (%#llx)\n",
len, scratch_gpa_beg);
- return -EINVAL;
+ goto e_scratch;
}

if ((scratch_gpa_beg & PAGE_MASK) == control->ghcb_gpa) {
@@ -2567,7 +2576,7 @@ static int setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
scratch_gpa_end > ghcb_scratch_end) {
pr_err("vmgexit: scratch area is outside of GHCB shared buffer area (%#llx - %#llx)\n",
scratch_gpa_beg, scratch_gpa_end);
- return -EINVAL;
+ goto e_scratch;
}

scratch_va = (void *)svm->sev_es.ghcb;
@@ -2580,7 +2589,7 @@ static int setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
if (len > GHCB_SCRATCH_AREA_LIMIT) {
pr_err("vmgexit: scratch area exceeds KVM limits (%#llx requested, %#llx limit)\n",
len, GHCB_SCRATCH_AREA_LIMIT);
- return -EINVAL;
+ goto e_scratch;
}
scratch_va = kvzalloc(len, GFP_KERNEL_ACCOUNT);
if (!scratch_va)
@@ -2608,6 +2617,12 @@ static int setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
svm->sev_es.ghcb_sa_len = len;

return 0;
+
+e_scratch:
+ ghcb_set_sw_exit_info_1(ghcb, 2);
+ ghcb_set_sw_exit_info_2(ghcb, GHCB_ERR_INVALID_SCRATCH_AREA);
+
+ return 1;
}

static void set_ghcb_msr_bits(struct vcpu_svm *svm, u64 value, u64 mask,
@@ -2658,7 +2673,7 @@ static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)

ret = svm_invoke_exit_handler(vcpu, SVM_EXIT_CPUID);
if (!ret) {
- ret = -EINVAL;
+ /* Error, keep GHCB MSR value as-is */
break;
}

@@ -2694,10 +2709,13 @@ static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)
GHCB_MSR_TERM_REASON_POS);
pr_info("SEV-ES guest requested termination: %#llx:%#llx\n",
reason_set, reason_code);
- fallthrough;
+
+ ret = -EINVAL;
+ break;
}
default:
- ret = -EINVAL;
+ /* Error, keep GHCB MSR value as-is */
+ break;
}

trace_kvm_vmgexit_msr_protocol_exit(svm->vcpu.vcpu_id,
@@ -2721,14 +2739,18 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)

if (!ghcb_gpa) {
vcpu_unimpl(vcpu, "vmgexit: GHCB gpa is not set\n");
- return -EINVAL;
+
+ /* Without a GHCB, just return right back to the guest */
+ return 1;
}

if (kvm_vcpu_map(vcpu, ghcb_gpa >> PAGE_SHIFT, &svm->sev_es.ghcb_map)) {
/* Unable to map GHCB from guest */
vcpu_unimpl(vcpu, "vmgexit: error mapping GHCB [%#llx] from guest\n",
ghcb_gpa);
- return -EINVAL;
+
+ /* Without a GHCB, just return right back to the guest */
+ return 1;
}

svm->sev_es.ghcb = svm->sev_es.ghcb_map.hva;
@@ -2788,11 +2810,8 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
default:
pr_err("svm: vmgexit: unsupported AP jump table request - exit_info_1=%#llx\n",
control->exit_info_1);
- ghcb_set_sw_exit_info_1(ghcb, 1);
- ghcb_set_sw_exit_info_2(ghcb,
- X86_TRAP_UD |
- SVM_EVTINJ_TYPE_EXEPT |
- SVM_EVTINJ_VALID);
+ ghcb_set_sw_exit_info_1(ghcb, 2);
+ ghcb_set_sw_exit_info_2(ghcb, GHCB_ERR_INVALID_INPUT);
}

ret = 1;

base-commit: 70f433c2193fbfb5541ef98f973e087ddf2f9dfb
--