Re: [PATCH kernel v4] KVM: SEV: Enable data breakpoints in SEV-ES

[Alexey Kardashevskiy <aik@amd.com>]

Ping? (I am told that pinging once a week is ok) Thanks,

On 14/3/23 20:43, Alexey Kardashevskiy wrote:
> 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@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