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

Alexey Kardashevskiy
2023-01-12T05:46:02Z

On 11/1/23 05:00, Borislav Petkov wrote:
> On Fri, Dec 09, 2022 at 03:38:03PM +1100, Alexey Kardashevskiy wrote:
>> AMD Milan (Fam 19h) introduces support for the swapping, as type 'B',
>
> "type B" means nothing to people who don't have an intimate APM knowledge.
>
> Let's try again, this time with a more accessible formulation:
>
> "The debug registers are handled a bit differently when doing a world switch of a
> SEV-ES guest: the guest debug registers values are saved and restored as usual
> and as one would expect.

Well, SEV-ES KVM (ES == Encrypted State) does not save/restore them for
the guest (well, as I would expect) as the guest registers are not
visible to host to save, they are intercepted and the VM does this GHCB
dance with VMGEXIT(SVM_EXIT_WRITE_DR7).

> The *host* debug registers are not saved to the host save area so if the
> host is doing any debug activity, that host should take care to stash its debug
> registers values into the host save area before running guests.
>
> See Table B-3. Swap Types and the AMD APM volume 2."
>
> And now you can go into detail explaining which regs exactly and so on.
>
>> of DR[0-3] and DR[0-3]_ADDR_MASK registers. Software enables this by
>> setting SEV_FEATURES[5] (called "DebugSwap") in the VMSA which makes
>> data breakpoints work in SEV-ES VMs.
>>
>> For type 'B' swaps the hardware saves/restores the VM state on
>> VMEXIT/VMRUN in VMSA, and restores the host state on VMEXIT.
>
> Yeah, close but I'd prefer a more detailed explanation and a reference to the
> APM so that people can follow and read more info if needed.

Well, the only place in APM is that "Table B-3. Swap Types and the AMD
APM volume 2", and it is pretty brief, do I miss something? Thanks,

> >> Enable DebugSwap in VMSA but only if CPUID Fn80000021_EAX[0]
>> ("NoNestedDataBp", "Processor ignores nested data breakpoints") is
>> supported by the SOC as otherwise a malicious guest can cause
>> the infinite #DB loop DoS.
>>
>> Save DR[0-3] / DR[0-3]_ADDR_MASK in the host save area before VMRUN
>> as type 'B' swap does not do this part.
>>
>> 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.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
>> ---
>> Changes:
>> v2:
>> * debug_swap moved from vcpu to module_param
>> * rewrote commit log
>>
>> ---
>>
>> "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")
>
>> 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.
>

--
Alexey