Re: [PATCH v5 12/17] KVM: arm64: Save protected-nVHE (pKVM) hyp stacktrace

Oliver Upton
2022-07-22T15:33:52Z

Hi Kalesh,

On Wed, Jul 20, 2022 at 10:57:23PM -0700, Kalesh Singh wrote:
> In protected nVHE mode, the host cannot access private owned hypervisor
> memory. Also the hypervisor aims to remains simple to reduce the attack
> surface and does not provide any printk support.
>
> For the above reasons, the approach taken to provide hypervisor stacktraces
> in protected mode is:
> 1) Unwind and save the hyp stack addresses in EL2 to a shared buffer
> with the host (done in this patch).
> 2) Delegate the dumping and symbolization of the addresses to the
> host in EL1 (later patch in the series).
>
> On hyp_panic(), the hypervisor prepares the stacktrace before returning to
> the host.
>
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> ---

Tried reworking this a bit and here is what I arrived at, WDYT?
Untested, of course :)

--
Thanks,
Oliver

diff --git a/arch/arm64/include/asm/stacktrace/nvhe.h b/arch/arm64/include/asm/stacktrace/nvhe.h
index 3078501f8e22..05d7e03e0a8c 100644
--- a/arch/arm64/include/asm/stacktrace/nvhe.h
+++ b/arch/arm64/include/asm/stacktrace/nvhe.h
@@ -21,6 +21,23 @@

#include <asm/stacktrace/common.h>

+/*
+ * kvm_nvhe_unwind_init - Start an unwind from the given nVHE HYP fp and pc
+ *
+ * @state : unwind_state to initialize
+ * @fp : frame pointer at which to start the unwinding.
+ * @pc : program counter at which to start the unwinding.
+ */
+static inline void kvm_nvhe_unwind_init(struct unwind_state *state,
+ unsigned long fp,
+ unsigned long pc)
+{
+ unwind_init_common(state, NULL);
+
+ state->fp = fp;
+ state->pc = pc;
+}
+
static inline bool on_accessible_stack(const struct task_struct *tsk,
unsigned long sp, unsigned long size,
struct stack_info *info)
diff --git a/arch/arm64/kvm/hyp/nvhe/stacktrace.c b/arch/arm64/kvm/hyp/nvhe/stacktrace.c
index 96c8b93320eb..644276fb02af 100644
--- a/arch/arm64/kvm/hyp/nvhe/stacktrace.c
+++ b/arch/arm64/kvm/hyp/nvhe/stacktrace.c
@@ -11,4 +11,69 @@ DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)], overflow_stack)

#ifdef CONFIG_PROTECTED_NVHE_STACKTRACE
DEFINE_PER_CPU(unsigned long [NVHE_STACKTRACE_SIZE/sizeof(long)], pkvm_stacktrace);
+
+/*
+ * pkvm_save_backtrace_entry - Saves a protected nVHE HYP stacktrace entry
+ *
+ * @arg : the index of the entry in the stacktrace buffer
+ * @where : the program counter corresponding to the stack frame
+ *
+ * Save the return address of a stack frame to the shared stacktrace buffer.
+ * The host can access this shared buffer from EL1 to dump the backtrace.
+ */
+static bool pkvm_save_backtrace_entry(void *arg, unsigned long where)
+{
+ unsigned long *stacktrace = this_cpu_ptr(pkvm_stacktrace);
+ int *idx = (int *)arg;
+
+ /*
+ * Need 2 free slots: 1 for current entry and 1 for the
+ * delimiter.
+ */
+ if (*idx > ARRAY_SIZE(pkvm_stacktrace) - 2)
+ return false;
+
+ stacktrace[*idx] = where;
+ stacktrace[++*idx] = 0UL;
+
+ return true;
+}
+
+/*
+ * pkvm_save_backtrace - Saves the protected nVHE HYP stacktrace
+ *
+ * @fp : frame pointer at which to start the unwinding.
+ * @pc : program counter at which to start the unwinding.
+ *
+ * Save the unwinded stack addresses to the shared stacktrace buffer.
+ * The host can access this shared buffer from EL1 to dump the backtrace.
+ */
+static void pkvm_save_backtrace(unsigned long fp, unsigned long pc)
+{
+ struct unwind_state state;
+ int idx = 0;
+
+ kvm_nvhe_unwind_init(&state, fp, pc);
+
+ unwind(&state, pkvm_save_backtrace_entry, &idx);
+}
+#else /* !CONFIG_PROTECTED_NVHE_STACKTRACE */
+static void pkvm_save_backtrace(unsigned long fp, unsigned long pc)
+{
+}
#endif /* CONFIG_PROTECTED_NVHE_STACKTRACE */
+
+/*
+ * kvm_nvhe_prepare_backtrace - prepare to dump the nVHE backtrace
+ *
+ * @fp : frame pointer at which to start the unwinding.
+ * @pc : program counter at which to start the unwinding.
+ *
+ * Saves the information needed by the host to dump the nVHE hypervisor
+ * backtrace.
+ */
+void kvm_nvhe_prepare_backtrace(unsigned long fp, unsigned long pc)
+{
+ if (is_protected_kvm_enabled())
+ pkvm_save_backtrace(fp, pc);
+}
diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
index 6db801db8f27..64e13445d0d9 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -34,6 +34,8 @@ DEFINE_PER_CPU(struct kvm_host_data, kvm_host_data);
DEFINE_PER_CPU(struct kvm_cpu_context, kvm_hyp_ctxt);
DEFINE_PER_CPU(unsigned long, kvm_hyp_vector);

+extern void kvm_nvhe_prepare_backtrace(unsigned long fp, unsigned long pc);
+
static void __activate_traps(struct kvm_vcpu *vcpu)
{
u64 val;
@@ -375,6 +377,10 @@ asmlinkage void __noreturn hyp_panic(void)
__sysreg_restore_state_nvhe(host_ctxt);
}

+ /* Prepare to dump kvm nvhe hyp stacktrace */
+ kvm_nvhe_prepare_backtrace((unsigned long)__builtin_frame_address(0),
+ _THIS_IP_);
+
__hyp_do_panic(host_ctxt, spsr, elr, par);
unreachable();
}