Re: [PATCH kernel v2 2/3] KVM: SEV: Enable data breakpoints in SEV-ES

Tom Lendacky
2023-01-10T19:08:48Z

On 1/10/23 12:00, Borislav Petkov wrote:
> On Fri, Dec 09, 2022 at 03:38:03PM +1100, Alexey Kardashevskiy wrote:
>>

>> "DR7 access must remain intercepted for an SEV-ES guest" - I could not
>> figure out the exact reasoning why it is there in the first place,
>> IIUC this is to prevent loop of #DBs in the VM.
>
> Let's ask Mr. Lendacky:
>
> 8d4846b9b150 ("KVM: SVM: Prevent debugging under SEV-ES")

The DR7 requirements were to prevent a malicious SEV-ES guest from setting
up data breakpoints on the #VC IDT entry/stack and causing an infinite loop.

Thanks,
Tom

>
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index efaaef2b7ae1..800ea2a778cc 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"
>> @@ -52,11 +53,21 @@ 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 */
>>
>> +bool sev_es_is_debug_swap_enabled(void)
>> +{
>> + return sev_es_debug_swap_enabled;
>> +}
>> +
>> static u8 sev_enc_bit;
>> static DECLARE_RWSEM(sev_deactivate_lock);
>> static DEFINE_MUTEX(sev_bitmap_lock);
>> @@ -604,6 +615,9 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
>> save->xss = svm->vcpu.arch.ia32_xss;
>> save->dr6 = svm->vcpu.arch.dr6;
>>
>> + if (sev_es_is_debug_swap_enabled())
>> + save->sev_features |= SVM_SEV_FEAT_DEBUG_SWAP;
>> +
>> pr_debug("Virtual Machine Save Area (VMSA):\n");
>> print_hex_dump_debug("", DUMP_PREFIX_NONE, 16, 1, save, sizeof(*save), false);
>>
>> @@ -2249,6 +2263,9 @@ void __init sev_hardware_setup(void)
>> out:
>> sev_enabled = sev_supported;
>> sev_es_enabled = sev_es_supported;
>> + if (sev_es_debug_swap_enabled)
>> + sev_es_debug_swap_enabled = sev_es_enabled &&
>> + boot_cpu_has(X86_FEATURE_NO_NESTED_DATA_BP);
>
> check_for_deprecated_apis: WARNING: arch/x86/kvm/svm/sev.c:2268: Do not use boot_cpu_has() - use cpu_feature_enabled() instead.
>