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

[Alexey Kardashevskiy]

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@xxxxxxx>
---
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