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Date: Tue, 14 Mar 2023 20:43:32 +1100
From: Alexey Kardashevskiy <aik@....com>
To: Sean Christopherson <seanjc@...gle.com>,
Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
Thomas Gleixner <tglx@...utronix.de>,
Paolo Bonzini <pbonzini@...hat.com>,
Ingo Molnar <mingo@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org
Subject: Re: [PATCH kernel v4] KVM: SEV: Enable data breakpoints in SEV-ES
Ping? Thanks,
On 21/2/23 16:19, Alexey Kardashevskiy wrote:
> Ping? Thanks,
>
> On 3/2/23 16:14, Alexey Kardashevskiy wrote:
>> Prior to SEV-ES, KVM stored/loaded host debug registers upon switching
>> to/from a VM. Changing those registers inside a running SEV VM
>> triggered #VC exit to KVM.
>>
>> SEV-ES added the encrypted state (ES) which uses an encrypted guest page
>> for the VM state (VMSA). The hardware saves/restores certain registers on
>> VMRUN/VMEXIT according to a swap type (A, B, C), see
>> "Table B-3. Swap Types" in the AMD Architecture Programmer’s Manual
>> volume 2.
>>
>> AMD Milan (Fam 19h) introduces support for the debug registers swapping.
>> DR6 and DR7 are always swapped. DR[0-3] and DR[0-3]_ADDR_MASK are swapped
>> a type B when SEV_FEATURES[5] ("DebugSwap") is set.
>>
>> Enable DebugSwap in VMSA. But only do so if CPUID Fn80000021_EAX[0]
>> ("NoNestedDataBp", "Processor ignores nested data breakpoints") is
>> supported by the SOC as otherwise a malicious SEV-ES guest can set up
>> data breakpoints on the #VC IDT entry/stack and cause an infinite loop.
>>
>> Eliminate DR7 and #DB intercepts as:
>> - they are not needed when DebugSwap is supported;
>> - #VC for these intercepts is most likely not supported anyway and
>> kills the VM.
>> Keep DR7 intercepted unless DebugSwap enabled to prevent the infinite #DB
>> loop DoS.
>>
>> While at this, move set_/clr_dr_intercepts to .c and move #DB intercept
>> next to DR7 intercept.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@....com>
>> ---
>> Changes:
>> v4:
>> * removed sev_es_is_debug_swap_enabled() helper
>> * made sev_es_debug_swap_enabled (module param) static
>> * set sev_feature early in sev_es_init_vmcb() and made intercepts
>> dependend on it vs. module param
>> * move set_/clr_dr_intercepts to .c
>>
>> v3:
>> * rewrote the commit log again
>> * rebased on tip/master to use recently defined
>> X86_FEATURE_NO_NESTED_DATA_BP
>> * s/boot_cpu_has/cpu_feature_enabled/
>>
>> v2:
>> * debug_swap moved from vcpu to module_param
>> * rewrote commit log
>>
>> ---
>> Tested with:
>> ===
>> int x;
>> int main(int argc, char *argv[])
>> {
>> x = 1;
>> return 0;
>> }
>> ===
>> gcc -g a.c
>> rsync a.out ruby-954vm:~/
>> ssh -t ruby-954vm 'gdb -ex "file a.out" -ex "watch x" -ex r'
>>
>> where ruby-954vm is a VM.
>>
>> With "/sys/module/kvm_amd/parameters/debug_swap = 0", gdb does not stop
>> on the watchpoint, with "= 1" - gdb does.
>> ---
>> arch/x86/include/asm/svm.h | 1 +
>> arch/x86/kvm/svm/svm.h | 42 -------------
>> arch/x86/kvm/svm/sev.c | 24 ++++++++
>> arch/x86/kvm/svm/svm.c | 65 +++++++++++++++++++-
>> 4 files changed, 87 insertions(+), 45 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
>> index cb1ee53ad3b1..665515c7edae 100644
>> --- a/arch/x86/include/asm/svm.h
>> +++ b/arch/x86/include/asm/svm.h
>> @@ -278,6 +278,7 @@ enum avic_ipi_failure_cause {
>> #define AVIC_HPA_MASK ~((0xFFFULL << 52) | 0xFFF)
>> #define VMCB_AVIC_APIC_BAR_MASK 0xFFFFFFFFFF000ULL
>> +#define SVM_SEV_FEAT_DEBUG_SWAP BIT(5)
>> struct vmcb_seg {
>> u16 selector;
>> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
>> index 4826e6cc611b..653fd09929df 100644
>> --- a/arch/x86/kvm/svm/svm.h
>> +++ b/arch/x86/kvm/svm/svm.h
>> @@ -389,48 +389,6 @@ static inline bool vmcb12_is_intercept(struct
>> vmcb_ctrl_area_cached *control, u3
>> return test_bit(bit, (unsigned long *)&control->intercepts);
>> }
>> -static inline void set_dr_intercepts(struct vcpu_svm *svm)
>> -{
>> - struct vmcb *vmcb = svm->vmcb01.ptr;
>> -
>> - if (!sev_es_guest(svm->vcpu.kvm)) {
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR0_READ);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR1_READ);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR2_READ);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR3_READ);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR4_READ);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR5_READ);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_READ);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR0_WRITE);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR1_WRITE);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR2_WRITE);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR3_WRITE);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR4_WRITE);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR5_WRITE);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_WRITE);
>> - }
>> -
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
>> -
>> - recalc_intercepts(svm);
>> -}
>> -
>> -static inline void clr_dr_intercepts(struct vcpu_svm *svm)
>> -{
>> - struct vmcb *vmcb = svm->vmcb01.ptr;
>> -
>> - vmcb->control.intercepts[INTERCEPT_DR] = 0;
>> -
>> - /* DR7 access must remain intercepted for an SEV-ES guest */
>> - if (sev_es_guest(svm->vcpu.kvm)) {
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
>> - }
>> -
>> - recalc_intercepts(svm);
>> -}
>> -
>> static inline void set_exception_intercept(struct vcpu_svm *svm, u32
>> bit)
>> {
>> struct vmcb *vmcb = svm->vmcb01.ptr;
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index 86d6897f4806..af775410c5eb 100644
>> --- a/arch/x86/kvm/svm/sev.c
>> +++ b/arch/x86/kvm/svm/sev.c
>> @@ -21,6 +21,7 @@
>> #include <asm/pkru.h>
>> #include <asm/trapnr.h>
>> #include <asm/fpu/xcr.h>
>> +#include <asm/debugreg.h>
>> #include "mmu.h"
>> #include "x86.h"
>> @@ -52,9 +53,14 @@ module_param_named(sev, sev_enabled, bool, 0444);
>> /* enable/disable SEV-ES support */
>> static bool sev_es_enabled = true;
>> module_param_named(sev_es, sev_es_enabled, bool, 0444);
>> +
>> +/* enable/disable SEV-ES DebugSwap support */
>> +static bool sev_es_debug_swap_enabled = true;
>> +module_param_named(debug_swap, sev_es_debug_swap_enabled, bool, 0644);
>> #else
>> #define sev_enabled false
>> #define sev_es_enabled false
>> +#define sev_es_debug_swap false
>> #endif /* CONFIG_KVM_AMD_SEV */
>> static u8 sev_enc_bit;
>> @@ -2249,6 +2255,8 @@ void __init sev_hardware_setup(void)
>> out:
>> sev_enabled = sev_supported;
>> sev_es_enabled = sev_es_supported;
>> + if (!sev_es_enabled ||
>> !cpu_feature_enabled(X86_FEATURE_NO_NESTED_DATA_BP))
>> + sev_es_debug_swap_enabled = false;
>> #endif
>> }
>> @@ -2940,6 +2948,7 @@ int sev_es_string_io(struct vcpu_svm *svm, int
>> size, unsigned int port, int in)
>> static void sev_es_init_vmcb(struct vcpu_svm *svm)
>> {
>> struct kvm_vcpu *vcpu = &svm->vcpu;
>> + struct sev_es_save_area *save = svm->sev_es.vmsa;
>> svm->vmcb->control.nested_ctl |= SVM_NESTED_CTL_SEV_ES_ENABLE;
>> svm->vmcb->control.virt_ext |= LBR_CTL_ENABLE_MASK;
>> @@ -2988,6 +2997,9 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
>> if (guest_cpuid_has(&svm->vcpu, X86_FEATURE_RDTSCP))
>> svm_clr_intercept(svm, INTERCEPT_RDTSCP);
>> }
>> +
>> + if (sev_es_debug_swap_enabled)
>> + save->sev_features |= SVM_SEV_FEAT_DEBUG_SWAP;
>> }
>> void sev_init_vmcb(struct vcpu_svm *svm)
>> @@ -3027,6 +3039,18 @@ void sev_es_prepare_switch_to_guest(struct
>> sev_es_save_area *hostsa)
>> /* MSR_IA32_XSS is restored on VMEXIT, save the currnet host
>> value */
>> hostsa->xss = host_xss;
>> +
>> + /* The DebugSwap SEV feature does Type B swaps of DR[0-3] */
>> + if (sev_es_debug_swap_enabled) {
>> + hostsa->dr0 = native_get_debugreg(0);
>> + hostsa->dr1 = native_get_debugreg(1);
>> + hostsa->dr2 = native_get_debugreg(2);
>> + hostsa->dr3 = native_get_debugreg(3);
>> + hostsa->dr0_addr_mask = amd_get_dr_addr_mask(0);
>> + hostsa->dr1_addr_mask = amd_get_dr_addr_mask(1);
>> + hostsa->dr2_addr_mask = amd_get_dr_addr_mask(2);
>> + hostsa->dr3_addr_mask = amd_get_dr_addr_mask(3);
>> + }
>> }
>> void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index 60c7c880266b..f8e222bee22a 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -671,6 +671,65 @@ static int svm_cpu_init(int cpu)
>> }
>> +static void set_dr_intercepts(struct vcpu_svm *svm)
>> +{
>> + struct vmcb *vmcb = svm->vmcb01.ptr;
>> + bool intercept;
>> +
>> + if (!sev_es_guest(svm->vcpu.kvm)) {
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR0_READ);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR1_READ);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR2_READ);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR3_READ);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR4_READ);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR5_READ);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_READ);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR0_WRITE);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR1_WRITE);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR2_WRITE);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR3_WRITE);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR4_WRITE);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR5_WRITE);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_WRITE);
>> + }
>> +
>> + if (sev_es_guest(svm->vcpu.kvm)) {
>> + struct sev_es_save_area *save = svm->sev_es.vmsa;
>> +
>> + intercept = !(save->sev_features & SVM_SEV_FEAT_DEBUG_SWAP);
>> + } else {
>> + intercept = true;
>> + }
>> +
>> + if (intercept) {
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
>> + set_exception_intercept(svm, DB_VECTOR);
>> + }
>> +
>> + recalc_intercepts(svm);
>> +}
>> +
>> +static void clr_dr_intercepts(struct vcpu_svm *svm)
>> +{
>> + struct vmcb *vmcb = svm->vmcb01.ptr;
>> + struct sev_es_save_area *save = svm->sev_es.vmsa;
>> +
>> + vmcb->control.intercepts[INTERCEPT_DR] = 0;
>> +
>> + /*
>> + * DR7 access must remain intercepted for an SEV-ES guest unless
>> DebugSwap
>> + * (depends on NO_NESTED_DATA_BP) is enabled as otherwise a VM
>> writing to DR7
>> + * from the #DB handler may trigger infinite loop of #DB's.
>> + */
>> + if (sev_es_guest(svm->vcpu.kvm) && (save->sev_features &
>> SVM_SEV_FEAT_DEBUG_SWAP)) {
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
>> + }
>> +
>> + recalc_intercepts(svm);
>> +}
>> +
>> static int direct_access_msr_slot(u32 msr)
>> {
>> u32 i;
>> @@ -1184,13 +1243,11 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
>> if (!kvm_vcpu_apicv_active(vcpu))
>> svm_set_intercept(svm, INTERCEPT_CR8_WRITE);
>> - set_dr_intercepts(svm);
>> -
>> set_exception_intercept(svm, PF_VECTOR);
>> set_exception_intercept(svm, UD_VECTOR);
>> set_exception_intercept(svm, MC_VECTOR);
>> set_exception_intercept(svm, AC_VECTOR);
>> - set_exception_intercept(svm, DB_VECTOR);
>> +
>> /*
>> * Guest access to VMware backdoor ports could legitimately
>> * trigger #GP because of TSS I/O permission bitmap.
>> @@ -1308,6 +1365,8 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
>> if (sev_guest(vcpu->kvm))
>> sev_init_vmcb(svm);
>> + set_dr_intercepts(svm);
>> +
>> svm_hv_init_vmcb(vmcb);
>> init_vmcb_after_set_cpuid(vcpu);
>
--
Alexey
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