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

Alexey Kardashevskiy
2022-12-09T04:40:23Z

AMD Milan (Fam 19h) introduces support for the swapping, as type 'B',
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.

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.

---
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 | 16 ++++++++---
arch/x86/kvm/svm/sev.c | 29 ++++++++++++++++++++
arch/x86/kvm/svm/svm.c | 3 +-
4 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index 0361626841bc..373a0edda588 100644
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -273,6 +273,7 @@ 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 199a2ecef1ce..0fae611abe4a 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -386,6 +386,8 @@ static inline bool vmcb12_is_intercept(struct vmcb_ctrl_area_cached *control, u3
return test_bit(bit, (unsigned long *)&control->intercepts);
}

+extern bool sev_es_is_debug_swap_enabled(void);
+
static inline void set_dr_intercepts(struct vcpu_svm *svm)
{
struct vmcb *vmcb = svm->vmcb01.ptr;
@@ -407,8 +409,10 @@ static inline void set_dr_intercepts(struct vcpu_svm *svm)
vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_WRITE);
}

- vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
- vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
+ if (!sev_es_guest(svm->vcpu.kvm) || !sev_es_is_debug_swap_enabled()) {
+ vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
+ vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
+ }

recalc_intercepts(svm);
}
@@ -419,8 +423,12 @@ static inline void clr_dr_intercepts(struct vcpu_svm *svm)

vmcb->control.intercepts[INTERCEPT_DR] = 0;

- /* DR7 access must remain intercepted for an SEV-ES guest */
- if (sev_es_guest(svm->vcpu.kvm)) {
+ /*
+ * 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) && !sev_es_is_debug_swap_enabled()) {
vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
}
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);
#endif
}

@@ -3027,6 +3044,18 @@ 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_is_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 ce362e88a567..697804d46545 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1189,7 +1189,8 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
set_exception_intercept(svm, UD_VECTOR);
set_exception_intercept(svm, MC_VECTOR);
set_exception_intercept(svm, AC_VECTOR);
- set_exception_intercept(svm, DB_VECTOR);
+ if (!sev_es_is_debug_swap_enabled())
+ set_exception_intercept(svm, DB_VECTOR);
/*
* Guest access to VMware backdoor ports could legitimately
* trigger #GP because of TSS I/O permission bitmap.
--
2.38.1