[PATCH v2 5/5] KVM: arm64: Unwind and dump nVHE hypervisor stacktrace

[Kalesh Singh <kaleshsingh@google.com>]

On hyp_panic(), the hypervisor dumps the addresses for its stacktrace
entries to a page shared with the host. The host then symbolizes and
prints the hyp stacktrace before panicking itself.

Example stacktrace:

[  122.051187] kvm [380]: Invalid host exception to nVHE hyp!
[  122.052467] kvm [380]: nVHE HYP call trace:
[  122.052814] kvm [380]: [<ffff800008f5b550>] __kvm_nvhe___pkvm_vcpu_init_traps+0x1f0/0x1f0
[  122.053865] kvm [380]: [<ffff800008f560f0>] __kvm_nvhe_hyp_panic+0x130/0x1c0
[  122.054367] kvm [380]: [<ffff800008f56190>] __kvm_nvhe___kvm_vcpu_run+0x10/0x10
[  122.054878] kvm [380]: [<ffff800008f57a40>] __kvm_nvhe_handle___kvm_vcpu_run+0x30/0x50
[  122.055412] kvm [380]: [<ffff800008f57d2c>] __kvm_nvhe_handle_trap+0xbc/0x160
[  122.055911] kvm [380]: [<ffff800008f56864>] __kvm_nvhe___host_exit+0x64/0x64
[  122.056417] kvm [380]: ---- end of nVHE HYP call trace ----

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
---

Changes in v2:
  - Add Mark's Reviewed-by tag

 arch/arm64/include/asm/stacktrace.h | 42 ++++++++++++++--
 arch/arm64/kernel/stacktrace.c      | 75 +++++++++++++++++++++++++++++
 arch/arm64/kvm/handle_exit.c        |  4 ++
 arch/arm64/kvm/hyp/nvhe/switch.c    |  4 ++
 4 files changed, 121 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index f5af9a94c5a6..3063912107b0 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -5,6 +5,7 @@
 #ifndef __ASM_STACKTRACE_H
 #define __ASM_STACKTRACE_H
 
+#include <linux/kvm_host.h>
 #include <linux/percpu.h>
 #include <linux/sched.h>
 #include <linux/sched/task_stack.h>
@@ -19,10 +20,12 @@ enum stack_type {
 #ifndef __KVM_NVHE_HYPERVISOR__
 	STACK_TYPE_TASK,
 	STACK_TYPE_IRQ,
-	STACK_TYPE_OVERFLOW,
 	STACK_TYPE_SDEI_NORMAL,
 	STACK_TYPE_SDEI_CRITICAL,
+#else /* __KVM_NVHE_HYPERVISOR__ */
+	STACK_TYPE_HYP,
 #endif /* !__KVM_NVHE_HYPERVISOR__ */
+	STACK_TYPE_OVERFLOW,
 	STACK_TYPE_UNKNOWN,
 	__NR_STACK_TYPES
 };
@@ -55,6 +58,9 @@ static inline bool on_stack(unsigned long sp, unsigned long size,
 extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
 			   const char *loglvl);
 
+extern void hyp_dump_backtrace(unsigned long hyp_offset);
+
+DECLARE_PER_CPU(unsigned long, kvm_arm_hyp_stacktrace_page);
 DECLARE_PER_CPU(unsigned long *, irq_stack_ptr);
 
 static inline bool on_irq_stack(unsigned long sp, unsigned long size,
@@ -91,8 +97,32 @@ static inline bool on_overflow_stack(unsigned long sp, unsigned long size,
 static inline bool on_overflow_stack(unsigned long sp, unsigned long size,
 			struct stack_info *info) { return false; }
 #endif
-#endif /* !__KVM_NVHE_HYPERVISOR__ */
+#else /* __KVM_NVHE_HYPERVISOR__ */
+
+extern void hyp_save_backtrace(void);
+
+DECLARE_PER_CPU(unsigned long [PAGE_SIZE/sizeof(long)], overflow_stack);
+DECLARE_PER_CPU(struct kvm_nvhe_init_params, kvm_init_params);
+
+static inline bool on_overflow_stack(unsigned long sp, unsigned long size,
+				 struct stack_info *info)
+{
+	unsigned long low = (unsigned long)this_cpu_ptr(overflow_stack);
+	unsigned long high = low + PAGE_SIZE;
+
+	return on_stack(sp, size, low, high, STACK_TYPE_OVERFLOW, info);
+}
+
+static inline bool on_hyp_stack(unsigned long sp, unsigned long size,
+				 struct stack_info *info)
+{
+	struct kvm_nvhe_init_params *params = this_cpu_ptr(&kvm_init_params);
+	unsigned long high = params->stack_hyp_va;
+	unsigned long low = high - PAGE_SIZE;
 
+	return on_stack(sp, size, low, high, STACK_TYPE_HYP, info);
+}
+#endif /* !__KVM_NVHE_HYPERVISOR__ */
 
 /*
  * We can only safely access per-cpu stacks from current in a non-preemptible
@@ -105,6 +135,9 @@ static inline bool on_accessible_stack(const struct task_struct *tsk,
 	if (info)
 		info->type = STACK_TYPE_UNKNOWN;
 
+	if (on_overflow_stack(sp, size, info))
+		return true;
+
 #ifndef __KVM_NVHE_HYPERVISOR__
 	if (on_task_stack(tsk, sp, size, info))
 		return true;
@@ -112,10 +145,11 @@ static inline bool on_accessible_stack(const struct task_struct *tsk,
 		return false;
 	if (on_irq_stack(sp, size, info))
 		return true;
-	if (on_overflow_stack(sp, size, info))
-		return true;
 	if (on_sdei_stack(sp, size, info))
 		return true;
+#else /* __KVM_NVHE_HYPERVISOR__ */
+	if (on_hyp_stack(sp, size, info))
+		return true;
 #endif /* !__KVM_NVHE_HYPERVISOR__ */
 
 	return false;
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index f346b4c66f1c..c81dea9760ac 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -104,6 +104,7 @@ static int notrace __unwind_next(struct task_struct *tsk,
 	 *
 	 * TASK -> IRQ -> OVERFLOW -> SDEI_NORMAL
 	 * TASK -> SDEI_NORMAL -> SDEI_CRITICAL -> OVERFLOW
+	 * HYP -> OVERFLOW
 	 *
 	 * ... but the nesting itself is strict. Once we transition from one
 	 * stack to another, it's never valid to unwind back to that first
@@ -242,7 +243,81 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
 
 	unwind(task, &state, consume_entry, cookie);
 }
+
+/**
+ * Symbolizes and dumps the hypervisor backtrace from the shared
+ * stacktrace page.
+ */
+noinline notrace void hyp_dump_backtrace(unsigned long hyp_offset)
+{
+	unsigned long *stacktrace_pos =
+		(unsigned long *)*this_cpu_ptr(&kvm_arm_hyp_stacktrace_page);
+	unsigned long va_mask = GENMASK_ULL(vabits_actual - 1, 0);
+	unsigned long pc = *stacktrace_pos++;
+
+	kvm_err("nVHE HYP call trace:\n");
+
+	while (pc) {
+		pc &= va_mask;		/* Mask tags */
+		pc += hyp_offset;	/* Convert to kern addr */
+		kvm_err("[<%016lx>] %pB\n", pc, (void *)pc);
+		pc = *stacktrace_pos++;
+	}
+
+	kvm_err("---- end of nVHE HYP call trace ----\n");
+}
 #else /* __KVM_NVHE_HYPERVISOR__ */
 DEFINE_PER_CPU(unsigned long [PAGE_SIZE/sizeof(long)], overflow_stack)
 	__aligned(16);
+
+static int notrace unwind_next(struct task_struct *tsk,
+			       struct unwind_state *state)
+{
+	struct stack_info info;
+
+	return __unwind_next(tsk, state, &info);
+}
+
+/**
+ * Saves a hypervisor stacktrace entry (address) to the shared stacktrace page.
+ */
+static bool hyp_save_backtrace_entry(void *arg, unsigned long where)
+{
+	struct kvm_nvhe_init_params *params = this_cpu_ptr(&kvm_init_params);
+	unsigned long **stacktrace_pos = (unsigned long **)arg;
+	unsigned long stacktrace_start, stacktrace_end;
+
+	stacktrace_start = (unsigned long)params->stacktrace_hyp_va;
+	stacktrace_end = stacktrace_start + PAGE_SIZE - (2 * sizeof(long));
+
+	if ((unsigned long) *stacktrace_pos > stacktrace_end)
+		return false;
+
+	/* Save the entry to the current pos in stacktrace page */
+	**stacktrace_pos = where;
+
+	/* A zero entry delimits the end of the stacktrace. */
+	*(*stacktrace_pos + 1) = 0UL;
+
+	/* Increment the current pos */
+	++*stacktrace_pos;
+
+	return true;
+}
+
+/**
+ * Saves hypervisor stacktrace to the shared stacktrace page.
+ */
+noinline notrace void hyp_save_backtrace(void)
+{
+	struct kvm_nvhe_init_params *params = this_cpu_ptr(&kvm_init_params);
+	void *stacktrace_start = (void *)params->stacktrace_hyp_va;
+	struct unwind_state state;
+
+	unwind_init(&state, (unsigned long)__builtin_frame_address(0),
+			_THIS_IP_);
+
+	unwind(NULL, &state, hyp_save_backtrace_entry, &stacktrace_start);
+}
+
 #endif /* !__KVM_NVHE_HYPERVISOR__ */
diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 2e61a987b0d5..f1a6b556ec32 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -17,6 +17,7 @@
 #include <asm/kvm_emulate.h>
 #include <asm/kvm_mmu.h>
 #include <asm/debug-monitors.h>
+#include <asm/stacktrace.h>
 #include <asm/traps.h>
 
 #include <kvm/arm_hypercalls.h>
@@ -325,6 +326,9 @@ void __noreturn __cold nvhe_hyp_panic_handler(u64 esr, u64 spsr,
 				(void *)panic_addr);
 	}
 
+	/* Dump the hypervisor stacktrace */
+	hyp_dump_backtrace(hyp_offset);
+
 	/*
 	 * Hyp has panicked and we're going to handle that by panicking the
 	 * kernel. The kernel offset will be revealed in the panic so we're
diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
index 978f1b94fb25..95d810e86c7d 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -25,6 +25,7 @@
 #include <asm/fpsimd.h>
 #include <asm/debug-monitors.h>
 #include <asm/processor.h>
+#include <asm/stacktrace.h>
 
 #include <nvhe/fixed_config.h>
 #include <nvhe/mem_protect.h>
@@ -395,6 +396,9 @@ asmlinkage void __noreturn hyp_panic(void)
 		__sysreg_restore_state_nvhe(host_ctxt);
 	}
 
+	/* Save the hypervisor stacktrace */
+	hyp_save_backtrace();
+
 	__hyp_do_panic(host_ctxt, spsr, elr, par);
 	unreachable();
 }
-- 
2.36.0.464.gb9c8b46e94-goog

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