[PATCH 3/3] KVM: SVM: allow to intercept all exceptions for debug

Maxim Levitsky
2021-03-15T22:11:46Z

Add a new debug module param 'debug_intercept_exceptions' which will allow the
KVM to intercept any guest exception, and forward it to the guest.

This can be very useful for guest debugging and/or KVM debugging with kvm trace.
This is not intended to be used on production systems.

This is based on an idea first shown here:
https://patchwork.kernel.org/project/kvm/patch/20160301192822.GD22677@pd.tnic/

CC: Borislav Petkov <bp@suse.de>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/include/asm/kvm_host.h | 2 +
arch/x86/kvm/svm/svm.c | 77 ++++++++++++++++++++++++++++++++-
arch/x86/kvm/svm/svm.h | 5 ++-
arch/x86/kvm/x86.c | 5 ++-
4 files changed, 85 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index a52f973bdff6d..c8f44a88b3153 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1564,6 +1564,8 @@ int kvm_emulate_rdpmc(struct kvm_vcpu *vcpu);
void kvm_queue_exception(struct kvm_vcpu *vcpu, unsigned nr);
void kvm_queue_exception_e(struct kvm_vcpu *vcpu, unsigned nr, u32 error_code);
void kvm_queue_exception_p(struct kvm_vcpu *vcpu, unsigned nr, unsigned long payload);
+void kvm_queue_exception_e_p(struct kvm_vcpu *vcpu, unsigned nr,
+ u32 error_code, unsigned long payload);
void kvm_requeue_exception(struct kvm_vcpu *vcpu, unsigned nr);
void kvm_requeue_exception_e(struct kvm_vcpu *vcpu, unsigned nr, u32 error_code);
void kvm_inject_page_fault(struct kvm_vcpu *vcpu, struct x86_exception *fault);
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 271196400495f..94156a367a663 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -197,6 +197,9 @@ module_param(sev_es, int, 0444);
bool __read_mostly dump_invalid_vmcb;
module_param(dump_invalid_vmcb, bool, 0644);

+uint debug_intercept_exceptions;
+module_param(debug_intercept_exceptions, uint, 0444);
+
static bool svm_gp_erratum_intercept = true;

static u8 rsm_ins_bytes[] = "\x0f\xaa";
@@ -220,6 +223,8 @@ static const u32 msrpm_ranges[] = {0, 0xc0000000, 0xc0010000};
#define MSRS_RANGE_SIZE 2048
#define MSRS_IN_RANGE (MSRS_RANGE_SIZE * 8 / 2)

+static void init_debug_exceptions_intercept(struct vcpu_svm *svm);
+
u32 svm_msrpm_offset(u32 msr)
{
u32 offset;
@@ -1137,6 +1142,8 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
set_exception_intercept(svm, MC_VECTOR);
set_exception_intercept(svm, AC_VECTOR);
set_exception_intercept(svm, DB_VECTOR);
+
+ init_debug_exceptions_intercept(svm);
/*
* Guest access to VMware backdoor ports could legitimately
* trigger #GP because of TSS I/O permission bitmap.
@@ -1913,6 +1920,17 @@ static int pf_interception(struct kvm_vcpu *vcpu)
u64 fault_address = svm->vmcb->control.exit_info_2;
u64 error_code = svm->vmcb->control.exit_info_1;

+ if ((debug_intercept_exceptions & (1 << PF_VECTOR)))
+ if (npt_enabled && !vcpu->arch.apf.host_apf_flags) {
+ /* If #PF was only intercepted for debug, inject
+ * it directly to the guest, since the mmu code
+ * is not ready to deal with such page faults
+ */
+ kvm_queue_exception_e_p(vcpu, PF_VECTOR,
+ error_code, fault_address);
+ return 1;
+ }
+
return kvm_handle_page_fault(vcpu, error_code, fault_address,
static_cpu_has(X86_FEATURE_DECODEASSISTS) ?
svm->vmcb->control.insn_bytes : NULL,
@@ -3025,7 +3043,7 @@ static int invpcid_interception(struct kvm_vcpu *vcpu)
return kvm_handle_invpcid(vcpu, type, gva);
}

-static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
+static int (*svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
[SVM_EXIT_READ_CR0] = cr_interception,
[SVM_EXIT_READ_CR3] = cr_interception,
[SVM_EXIT_READ_CR4] = cr_interception,
@@ -3099,6 +3117,63 @@ static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
[SVM_EXIT_VMGEXIT] = sev_handle_vmgexit,
};

+static int generic_exception_interception(struct kvm_vcpu *vcpu)
+{
+ /*
+ * Generic exception handler which forwards a guest exception
+ * as-is to the guest.
+ * For exceptions that don't have a special intercept handler.
+ *
+ * Used for 'debug_intercept_exceptions' KVM debug feature only.
+ */
+ struct vcpu_svm *svm = to_svm(vcpu);
+ int exc = svm->vmcb->control.exit_code - SVM_EXIT_EXCP_BASE;
+
+ WARN_ON(exc < 0 || exc > 31);
+
+ if (exc == TS_VECTOR) {
+ /*
+ * SVM doesn't provide us with an error code to be able to
+ * re-inject the #TS exception, so just disable its
+ * interception, and let the guest re-execute the instruction.
+ */
+ vmcb_clr_intercept(&svm->vmcb01.ptr->control,
+ INTERCEPT_EXCEPTION_OFFSET + TS_VECTOR);
+ recalc_intercepts(svm);
+ return 1;
+ } else if (exc == DF_VECTOR) {
+ /* SVM doesn't provide us with an error code for the #DF */
+ kvm_queue_exception_e(vcpu, exc, 0);
+ return 1;
+ }
+
+ if (x86_exception_has_error_code(exc))
+ kvm_queue_exception_e(vcpu, exc, svm->vmcb->control.exit_info_1);
+ else
+ kvm_queue_exception(vcpu, exc);
+ return 1;
+}
+
+static void init_debug_exceptions_intercept(struct vcpu_svm *svm)
+{
+ int exc;
+
+ for (exc = 0 ; exc < 32 ; exc++) {
+ if (!(debug_intercept_exceptions & (1 << exc)))
+ continue;
+
+ /* Those are defined to have undefined behavior in the SVM spec */
+ if (exc == 2 || exc == 9)
+ continue;
+
+ set_exception_intercept(svm, exc);
+
+ if (!svm_exit_handlers[SVM_EXIT_EXCP_BASE + exc])
+ svm_exit_handlers[SVM_EXIT_EXCP_BASE + exc] =
+ generic_exception_interception;
+ }
+}
+
static void dump_vmcb(struct kvm_vcpu *vcpu)
{
struct vcpu_svm *svm = to_svm(vcpu);
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 8e276c4fb33df..e0ff9ca996df8 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -32,6 +32,7 @@ static const u32 host_save_user_msrs[] = {
#define MSRPM_OFFSETS 16
extern u32 msrpm_offsets[MSRPM_OFFSETS] __read_mostly;
extern bool npt_enabled;
+extern uint debug_intercept_exceptions;

enum {
VMCB_INTERCEPTS, /* Intercept vectors, TSC offset,
@@ -333,7 +334,9 @@ static inline void clr_exception_intercept(struct vcpu_svm *svm, u32 bit)
struct vmcb *vmcb = svm->vmcb01.ptr;

WARN_ON_ONCE(bit >= 32);
- vmcb_clr_intercept(&vmcb->control, INTERCEPT_EXCEPTION_OFFSET + bit);
+
+ if (!((1 << bit) & debug_intercept_exceptions))
+ vmcb_clr_intercept(&vmcb->control, INTERCEPT_EXCEPTION_OFFSET + bit);

recalc_intercepts(svm);
}
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b75d990fcf12b..be509944622bd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -627,12 +627,13 @@ void kvm_queue_exception_p(struct kvm_vcpu *vcpu, unsigned nr,
}
EXPORT_SYMBOL_GPL(kvm_queue_exception_p);

-static void kvm_queue_exception_e_p(struct kvm_vcpu *vcpu, unsigned nr,
- u32 error_code, unsigned long payload)
+void kvm_queue_exception_e_p(struct kvm_vcpu *vcpu, unsigned nr,
+ u32 error_code, unsigned long payload)
{
kvm_multiple_exception(vcpu, nr, true, error_code,
true, payload, false);
}
+EXPORT_SYMBOL_GPL(kvm_queue_exception_e_p);

int kvm_complete_insn_gp(struct kvm_vcpu *vcpu, int err)
{
--
2.26.2