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SubjectRe: [PATCH kernel v4] KVM: SEV: Enable data breakpoints in SEV-ES
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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@amd.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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