Errata#1096:
On a nested data page fault when CR.SMAP=1 and the guest data read
generates a SMAP violation, GuestInstrBytes field of the VMCB on a
VMEXIT will incorrectly return 0h instead the correct guest
instruction bytes .
Recommend Workaround:
To determine what instruction the guest was executing the hypervisor
will have to decode the instruction at the instruction pointer.
The recommended workaround can not be implemented for the SEV
guest because guest memory is encrypted with the guest specific key,
and instruction decoder will not be able to decode the instruction
bytes. If we hit this errata in the SEV guest then log the message
and request a guest shutdown.
Reported-by: Venkatesh Srinivas <venkateshs@xxxxxxxxxx>
Cc: Jim Mattson <jmattson@xxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Joerg Roedel <joro@xxxxxxxxxx>
Cc: "Radim Krčmář" <rkrcmar@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
---
Change since v2:
* rename the callback emulate_instruction_possible->need_emulation_on_page_fault
Changes since v1:
* request to shutdown the guest instead of injecting #GP
* use the pr_err_ratelimited to supress the host dmesg
arch/x86/include/asm/kvm_host.h | 2 ++
arch/x86/kvm/mmu.c | 13 +++++------
arch/x86/kvm/svm.c | 40 +++++++++++++++++++++++++++++++++
arch/x86/kvm/vmx/vmx.c | 6 +++++
4 files changed, 54 insertions(+), 7 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 180373360e34..f913d21dafce 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1196,6 +1196,8 @@ struct kvm_x86_ops {
int (*nested_enable_evmcs)(struct kvm_vcpu *vcpu,
uint16_t *vmcs_version);
uint16_t (*nested_get_evmcs_version)(struct kvm_vcpu *vcpu);
+
+ bool (*need_emulation_on_page_fault)(struct kvm_vcpu *vcpu);
};
struct kvm_arch_async_pf {
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index f2d1d230d5b8..68672bd756b5 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -5386,14 +5386,13 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
emulation_type = EMULTYPE_ALLOW_RETRY;
emulate:
/*
- * On AMD platforms, under certain conditions insn_len may be zero on #NPF.
- * This can happen if a guest gets a page-fault on data access but the HW
- * table walker is not able to read the instruction page (e.g instruction
- * page is not present in memory). In those cases we simply restart the
- * guest.
+ * On AMD platform, under certain conditions insn_len may be zero on
+ * #NPF. Lets check if its safe to emulate the instruction.
*/
- if (unlikely(insn && !insn_len))
- return 1;
+ if (unlikely(insn && !insn_len)) {
+ if (!kvm_x86_ops->need_emulation_on_page_fault(vcpu))
+ return 1;
+ }
er = x86_emulate_instruction(vcpu, cr2, emulation_type, insn, insn_len);
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index f13a3a24d360..4cc4d556e852 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -7098,6 +7098,44 @@ static int nested_enable_evmcs(struct kvm_vcpu *vcpu,
return -ENODEV;
}
+static bool need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
+{
+ bool is_user, smap;
+
+ is_user = svm_get_cpl(vcpu) == 3;
+ smap = !kvm_read_cr4_bits(vcpu, X86_CR4_SMAP);
+
+ /*
+ * Detect and workaround Errata 1096 Fam_17h_00_0Fh
+ *
+ * In non SEV guest, hypervisor will be able to read the guest
+ * memory to decode the instruction pointer when insn_len is zero
+ * so we return true to indicate that decoding is possible.
+ *
+ * But in the SEV guest, the guest memory is encrypted with the
+ * guest specific key and hypervisor will not be able to decode the
+ * instruction pointer so we will not able to workaround it. Lets
+ * print the error and request to kill the guest.
+ */
+ if (is_user && smap) {
+ if (!sev_guest(vcpu->kvm))
+ return true;
+
+ pr_err_ratelimited("KVM: Guest triggered AMD Erratum 1096\n");
+ kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
+ }
+
+ /*
+ * Under certain conditions insn_len may be zero on #NPF. This can
+ * happen if a guest gets a page-fault on data access but the HW table
+ * walker is not able to read the instruction page (e.g instruction
+ * page is not present in memory). In those cases we return false
+ * indicating the emulation is not possible and request to retry the
+ * execution.
+ */
+ return false;
+}
+
static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
.cpu_has_kvm_support = has_svm,
.disabled_by_bios = is_disabled,
@@ -7231,6 +7269,8 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
.nested_enable_evmcs = nested_enable_evmcs,
.nested_get_evmcs_version = nested_get_evmcs_version,
+
+ .need_emulation_on_page_fault = need_emulation_on_page_fault,
};
static int __init svm_init(void)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 30a6bcd735ec..25940feddec5 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7527,6 +7527,11 @@ static int enable_smi_window(struct kvm_vcpu *vcpu)
return 0;
}
+static bool need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
+{
+ return false;
+}
+
static __init int hardware_setup(void)
{
unsigned long host_bndcfgs;
@@ -7829,6 +7834,7 @@ static struct kvm_x86_ops vmx_x86_ops __ro_after_init = {
.set_nested_state = NULL,
.get_vmcs12_pages = NULL,
.nested_enable_evmcs = NULL,
+ .need_emulation_on_page_fault = need_emulation_on_page_fault,
};
static void vmx_cleanup_l1d_flush(void)
--
2.17.1
