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

Borislav Petkov
2023-01-10T18:03:52Z

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.

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.
>
> 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.

--
Regards/Gruss,
Boris.