[PATCH v2 0/5] KVM nVHE Hypervisor stack unwinder

[PATCH v2 0/5] KVM nVHE Hypervisor stack unwinder

[Kalesh Singh]

Hi all,

This v2 of the nVHE hypervisor stack unwinder. The previous version was
posted at:
v1: https://lore.kernel.org/lkml/20220427184716.1949239-1-kaleshsingh@google.com/

The series is based on arm64 for-next/core and is also dependent on
kvm-arm64/hyp-stack-guard [1].

The main changes in this version is splitting some patches into more
incremental changes, per Mark Brown

The relevant parts of the previous cover letter are copied below for
convenience.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/hyp-stack-guard

Thanks,
Kalesh

-------

This new version of the unwinder splits the unwinding and dumping
of the stack between the hypervisor and host:
  - The hypervisor unwinds its stack and dumps the address entries
    into a page shared with the host.
  - The host then symnolizes and prints the hyp stacktrace from
    the shared page.

The new approach doesn't depend on CONFIG_NVHE_EL2_DEBUG,
and allows dumping hyp stacktraces in prodcution environments
(!CONFIG_NVHE_EL2_DEBUG).

arm64/kernel/stacktrace.c is compiled twice: stacktrace.o for the
host kernel and stacktrace.nvhe.o for the hypervisor. This allows
reusing most of the host unwinding logic in the nVHE hypervisor.


Kalesh Singh (5):
  KVM: arm64: Factor out common stack unwinding logic
  KVM: arm64: Compile stacktrace.nvhe.o
  KVM: arm64: Add hypervisor overflow stack
  KVM: arm64: Allocate shared stacktrace pages
  KVM: arm64: Unwind and dump nVHE hypervisor stacktrace

 arch/arm64/include/asm/kvm_asm.h    |   1 +
 arch/arm64/include/asm/stacktrace.h |  58 +++++++++--
 arch/arm64/kernel/stacktrace.c      | 151 +++++++++++++++++++++++-----
 arch/arm64/kvm/arm.c                |  34 +++++++
 arch/arm64/kvm/handle_exit.c        |   4 +
 arch/arm64/kvm/hyp/nvhe/Makefile    |   3 +-
 arch/arm64/kvm/hyp/nvhe/host.S      |   9 +-
 arch/arm64/kvm/hyp/nvhe/setup.c     |  11 ++
 arch/arm64/kvm/hyp/nvhe/switch.c    |   4 +
 9 files changed, 231 insertions(+), 44 deletions(-)

-- 
2.36.0.464.gb9c8b46e94-goog

[PATCH v2 1/5] KVM: arm64: Factor out common stack unwinding logic

[Kalesh Singh]

Factor out the stack unwinding logic common to both the host kernel and
the nVHE hypersivor into __unwind_next(). This allows for reuse in the
nVHE hypervisor stack unwinding (later in this series).

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
 arch/arm64/kernel/stacktrace.c | 36 +++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 0467cb79f080..ee60c279511c 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -81,23 +81,19 @@ NOKPROBE_SYMBOL(unwind_init);
  * records (e.g. a cycle), determined based on the location and fp value of A
  * and the location (but not the fp value) of B.
  */
-static int notrace unwind_next(struct task_struct *tsk,
-			       struct unwind_state *state)
+static int notrace __unwind_next(struct task_struct *tsk,
+				 struct unwind_state *state,
+				 struct stack_info *info)
 {
 	unsigned long fp = state->fp;
-	struct stack_info info;
-
-	/* Final frame; nothing to unwind */
-	if (fp == (unsigned long)task_pt_regs(tsk)->stackframe)
-		return -ENOENT;
 
 	if (fp & 0x7)
 		return -EINVAL;
 
-	if (!on_accessible_stack(tsk, fp, 16, &info))
+	if (!on_accessible_stack(tsk, fp, 16, info))
 		return -EINVAL;
 
-	if (test_bit(info.type, state->stacks_done))
+	if (test_bit(info->type, state->stacks_done))
 		return -EINVAL;
 
 	/*
@@ -113,7 +109,7 @@ static int notrace unwind_next(struct task_struct *tsk,
 	 * stack to another, it's never valid to unwind back to that first
 	 * stack.
 	 */
-	if (info.type == state->prev_type) {
+	if (info->type == state->prev_type) {
 		if (fp <= state->prev_fp)
 			return -EINVAL;
 	} else {
@@ -127,7 +123,25 @@ static int notrace unwind_next(struct task_struct *tsk,
 	state->fp = READ_ONCE_NOCHECK(*(unsigned long *)(fp));
 	state->pc = READ_ONCE_NOCHECK(*(unsigned long *)(fp + 8));
 	state->prev_fp = fp;
-	state->prev_type = info.type;
+	state->prev_type = info->type;
+
+	return 0;
+}
+NOKPROBE_SYMBOL(__unwind_next);
+
+static int notrace unwind_next(struct task_struct *tsk,
+			       struct unwind_state *state)
+{
+	struct stack_info info;
+	int err;
+
+	/* Final frame; nothing to unwind */
+	if (state->fp == (unsigned long)task_pt_regs(tsk)->stackframe)
+		return -ENOENT;
+
+	err = __unwind_next(tsk, state, &info);
+	if (err)
+		return err;
 
 	state->pc = ptrauth_strip_insn_pac(state->pc);
 
-- 
2.36.0.464.gb9c8b46e94-goog

Re: [PATCH v2 1/5] KVM: arm64: Factor out common stack unwinding logic

[Mark Brown]

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On Mon, May 02, 2022 at 12:12:01PM -0700, Kalesh Singh wrote:
> Factor out the stack unwinding logic common to both the host kernel and
> the nVHE hypersivor into __unwind_next(). This allows for reuse in the
> nVHE hypervisor stack unwinding (later in this series).

Reviewed-by: Mark Brown <broonie@kernel.org>

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[PATCH v2 2/5] KVM: arm64: Compile stacktrace.nvhe.o

[Kalesh Singh]

Recompile stack unwinding code for use with the nVHE hypervisor. This is
a preparatory patch that will allow reusing most of the kernel unwinding
logic in the nVHE hypervisor.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---

Changes in v2:
  - Split out refactoring of common unwinding logic into a separate patch,
    per Mark Brown

 arch/arm64/include/asm/stacktrace.h | 18 +++++++++-----
 arch/arm64/kernel/stacktrace.c      | 37 ++++++++++++++++-------------
 arch/arm64/kvm/hyp/nvhe/Makefile    |  3 ++-
 3 files changed, 35 insertions(+), 23 deletions(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index aec9315bf156..f5af9a94c5a6 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -16,12 +16,14 @@
 #include <asm/sdei.h>
 
 enum stack_type {
-	STACK_TYPE_UNKNOWN,
+#ifndef __KVM_NVHE_HYPERVISOR__
 	STACK_TYPE_TASK,
 	STACK_TYPE_IRQ,
 	STACK_TYPE_OVERFLOW,
 	STACK_TYPE_SDEI_NORMAL,
 	STACK_TYPE_SDEI_CRITICAL,
+#endif /* !__KVM_NVHE_HYPERVISOR__ */
+	STACK_TYPE_UNKNOWN,
 	__NR_STACK_TYPES
 };
 
@@ -31,11 +33,6 @@ struct stack_info {
 	enum stack_type type;
 };
 
-extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
-			   const char *loglvl);
-
-DECLARE_PER_CPU(unsigned long *, irq_stack_ptr);
-
 static inline bool on_stack(unsigned long sp, unsigned long size,
 			    unsigned long low, unsigned long high,
 			    enum stack_type type, struct stack_info *info)
@@ -54,6 +51,12 @@ static inline bool on_stack(unsigned long sp, unsigned long size,
 	return true;
 }
 
+#ifndef __KVM_NVHE_HYPERVISOR__
+extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
+			   const char *loglvl);
+
+DECLARE_PER_CPU(unsigned long *, irq_stack_ptr);
+
 static inline bool on_irq_stack(unsigned long sp, unsigned long size,
 				struct stack_info *info)
 {
@@ -88,6 +91,7 @@ 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__ */
 
 
 /*
@@ -101,6 +105,7 @@ static inline bool on_accessible_stack(const struct task_struct *tsk,
 	if (info)
 		info->type = STACK_TYPE_UNKNOWN;
 
+#ifndef __KVM_NVHE_HYPERVISOR__
 	if (on_task_stack(tsk, sp, size, info))
 		return true;
 	if (tsk != current || preemptible())
@@ -111,6 +116,7 @@ static inline bool on_accessible_stack(const struct task_struct *tsk,
 		return true;
 	if (on_sdei_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 ee60c279511c..a84e38d41d38 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -129,6 +129,26 @@ static int notrace __unwind_next(struct task_struct *tsk,
 }
 NOKPROBE_SYMBOL(__unwind_next);
 
+static int notrace unwind_next(struct task_struct *tsk,
+			       struct unwind_state *state);
+
+static void notrace unwind(struct task_struct *tsk,
+			   struct unwind_state *state,
+			   stack_trace_consume_fn consume_entry, void *cookie)
+{
+	while (1) {
+		int ret;
+
+		if (!consume_entry(cookie, state->pc))
+			break;
+		ret = unwind_next(tsk, state);
+		if (ret < 0)
+			break;
+	}
+}
+NOKPROBE_SYMBOL(unwind);
+
+#ifndef __KVM_NVHE_HYPERVISOR__
 static int notrace unwind_next(struct task_struct *tsk,
 			       struct unwind_state *state)
 {
@@ -171,22 +191,6 @@ static int notrace unwind_next(struct task_struct *tsk,
 }
 NOKPROBE_SYMBOL(unwind_next);
 
-static void notrace unwind(struct task_struct *tsk,
-			   struct unwind_state *state,
-			   stack_trace_consume_fn consume_entry, void *cookie)
-{
-	while (1) {
-		int ret;
-
-		if (!consume_entry(cookie, state->pc))
-			break;
-		ret = unwind_next(tsk, state);
-		if (ret < 0)
-			break;
-	}
-}
-NOKPROBE_SYMBOL(unwind);
-
 static bool dump_backtrace_entry(void *arg, unsigned long where)
 {
 	char *loglvl = arg;
@@ -238,3 +242,4 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
 
 	unwind(task, &state, consume_entry, cookie);
 }
+#endif /* !__KVM_NVHE_HYPERVISOR__ */
diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
index f9fe4dc21b1f..c0ff0d6fc403 100644
--- a/arch/arm64/kvm/hyp/nvhe/Makefile
+++ b/arch/arm64/kvm/hyp/nvhe/Makefile
@@ -14,7 +14,8 @@ lib-objs := $(addprefix ../../../lib/, $(lib-objs))
 
 obj-y := timer-sr.o sysreg-sr.o debug-sr.o switch.o tlb.o hyp-init.o host.o \
 	 hyp-main.o hyp-smp.o psci-relay.o early_alloc.o page_alloc.o \
-	 cache.o setup.o mm.o mem_protect.o sys_regs.o pkvm.o
+	 cache.o setup.o mm.o mem_protect.o sys_regs.o pkvm.o \
+	 ../../../kernel/stacktrace.o
 obj-y += ../vgic-v3-sr.o ../aarch32.o ../vgic-v2-cpuif-proxy.o ../entry.o \
 	 ../fpsimd.o ../hyp-entry.o ../exception.o ../pgtable.o
 obj-$(CONFIG_DEBUG_LIST) += list_debug.o
-- 
2.36.0.464.gb9c8b46e94-goog

Re: [PATCH v2 2/5] KVM: arm64: Compile stacktrace.nvhe.o

[Mark Brown]

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On Mon, May 02, 2022 at 12:12:02PM -0700, Kalesh Singh wrote:

> Recompile stack unwinding code for use with the nVHE hypervisor. This is
> a preparatory patch that will allow reusing most of the kernel unwinding
> logic in the nVHE hypervisor.

>  enum stack_type {
> -	STACK_TYPE_UNKNOWN,
> +#ifndef __KVM_NVHE_HYPERVISOR__
>  	STACK_TYPE_TASK,
>  	STACK_TYPE_IRQ,
>  	STACK_TYPE_OVERFLOW,
>  	STACK_TYPE_SDEI_NORMAL,
>  	STACK_TYPE_SDEI_CRITICAL,
> +#endif /* !__KVM_NVHE_HYPERVISOR__ */
> +	STACK_TYPE_UNKNOWN,
>  	__NR_STACK_TYPES
>  };

I don't immediately see a problem with it but I'm curious as to why
STACK_TYPE_UNKNOWN got moved to the end of the list here?  It does mean
that zeroed memory will default to STACK_TYPE_TASK but we're not
actually relying on that.  Otherwise

Reviwed-by: Mark Brown <broonie@kernel.org>

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Re: [PATCH v2 2/5] KVM: arm64: Compile stacktrace.nvhe.o

[Kalesh Singh]

On Wed, May 4, 2022 at 7:17 AM Mark Brown <broonie@kernel.org> wrote:
>
> On Mon, May 02, 2022 at 12:12:02PM -0700, Kalesh Singh wrote:
>
> > Recompile stack unwinding code for use with the nVHE hypervisor. This is
> > a preparatory patch that will allow reusing most of the kernel unwinding
> > logic in the nVHE hypervisor.
>
> >  enum stack_type {
> > -     STACK_TYPE_UNKNOWN,
> > +#ifndef __KVM_NVHE_HYPERVISOR__
> >       STACK_TYPE_TASK,
> >       STACK_TYPE_IRQ,
> >       STACK_TYPE_OVERFLOW,
> >       STACK_TYPE_SDEI_NORMAL,
> >       STACK_TYPE_SDEI_CRITICAL,
> > +#endif /* !__KVM_NVHE_HYPERVISOR__ */
> > +     STACK_TYPE_UNKNOWN,
> >       __NR_STACK_TYPES
> >  };
>
> I don't immediately see a problem with it but I'm curious as to why
> STACK_TYPE_UNKNOWN got moved to the end of the list here?  It does mean
> that zeroed memory will default to STACK_TYPE_TASK but we're not
> actually relying on that.  Otherwise

Hi Mark. Thanks for the reviews.

STACK_TYPE_UNKNOWN was moved to the end purely to group the common
enum values. Not a functionality change as we don't reply on an
uninitialized stack_type

Thanks,
Kalesh

>
> Reviwed-by: Mark Brown <broonie@kernel.org>
>
> --
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.

[PATCH v2 3/5] KVM: arm64: Add hypervisor overflow stack

[Kalesh Singh]

Allocate and switch to 16-byte aligned secondary stack on overflow. This
provides us stack space to better handle overflows; and is used in
a subsequent patch to dump the hypervisor stacktrace.

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
 arch/arm64/kernel/stacktrace.c | 3 +++
 arch/arm64/kvm/hyp/nvhe/host.S | 9 ++-------
 2 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index a84e38d41d38..f346b4c66f1c 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -242,4 +242,7 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
 
 	unwind(task, &state, consume_entry, cookie);
 }
+#else /* __KVM_NVHE_HYPERVISOR__ */
+DEFINE_PER_CPU(unsigned long [PAGE_SIZE/sizeof(long)], overflow_stack)
+	__aligned(16);
 #endif /* !__KVM_NVHE_HYPERVISOR__ */
diff --git a/arch/arm64/kvm/hyp/nvhe/host.S b/arch/arm64/kvm/hyp/nvhe/host.S
index 09b5254fb497..1cd2de4f039e 100644
--- a/arch/arm64/kvm/hyp/nvhe/host.S
+++ b/arch/arm64/kvm/hyp/nvhe/host.S
@@ -179,13 +179,8 @@ SYM_FUNC_END(__host_hvc)
 	b	hyp_panic
 
 .L__hyp_sp_overflow\@:
-	/*
-	 * Reset SP to the top of the stack, to allow handling the hyp_panic.
-	 * This corrupts the stack but is ok, since we won't be attempting
-	 * any unwinding here.
-	 */
-	ldr_this_cpu	x0, kvm_init_params + NVHE_INIT_STACK_HYP_VA, x1
-	mov	sp, x0
+	/* Switch to the overflow stack */
+	adr_this_cpu sp, overflow_stack + PAGE_SIZE, x0
 
 	b	hyp_panic_bad_stack
 	ASM_BUG()
-- 
2.36.0.464.gb9c8b46e94-goog

[PATCH v2 4/5] KVM: arm64: Allocate shared stacktrace pages

[Kalesh Singh]

The nVHE hypervisor can use this shared area to dump its stacktrace
addresses on hyp_panic(). Symbolization and printing the stacktrace can
then be handled by the host in EL1 (done in a later patch in this series).

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
 arch/arm64/include/asm/kvm_asm.h |  1 +
 arch/arm64/kvm/arm.c             | 34 ++++++++++++++++++++++++++++++++
 arch/arm64/kvm/hyp/nvhe/setup.c  | 11 +++++++++++
 3 files changed, 46 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index 2e277f2ed671..ad31ac68264f 100644
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -174,6 +174,7 @@ struct kvm_nvhe_init_params {
 	unsigned long hcr_el2;
 	unsigned long vttbr;
 	unsigned long vtcr;
+	unsigned long stacktrace_hyp_va;
 };
 
 /* Translate a kernel address @ptr into its equivalent linear mapping */
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index d300def44f5c..26005182da20 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -50,6 +50,7 @@ DEFINE_STATIC_KEY_FALSE(kvm_protected_mode_initialized);
 DECLARE_KVM_HYP_PER_CPU(unsigned long, kvm_hyp_vector);
 
 static DEFINE_PER_CPU(unsigned long, kvm_arm_hyp_stack_page);
+DEFINE_PER_CPU(unsigned long, kvm_arm_hyp_stacktrace_page);
 unsigned long kvm_arm_hyp_percpu_base[NR_CPUS];
 DECLARE_KVM_NVHE_PER_CPU(struct kvm_nvhe_init_params, kvm_init_params);
 
@@ -1484,6 +1485,7 @@ static void cpu_prepare_hyp_mode(int cpu)
 	tcr |= (idmap_t0sz & GENMASK(TCR_TxSZ_WIDTH - 1, 0)) << TCR_T0SZ_OFFSET;
 	params->tcr_el2 = tcr;
 
+	params->stacktrace_hyp_va = kern_hyp_va(per_cpu(kvm_arm_hyp_stacktrace_page, cpu));
 	params->pgd_pa = kvm_mmu_get_httbr();
 	if (is_protected_kvm_enabled())
 		params->hcr_el2 = HCR_HOST_NVHE_PROTECTED_FLAGS;
@@ -1777,6 +1779,7 @@ static void teardown_hyp_mode(void)
 	free_hyp_pgds();
 	for_each_possible_cpu(cpu) {
 		free_page(per_cpu(kvm_arm_hyp_stack_page, cpu));
+		free_page(per_cpu(kvm_arm_hyp_stacktrace_page, cpu));
 		free_pages(kvm_arm_hyp_percpu_base[cpu], nvhe_percpu_order());
 	}
 }
@@ -1868,6 +1871,23 @@ static int init_hyp_mode(void)
 		per_cpu(kvm_arm_hyp_stack_page, cpu) = stack_page;
 	}
 
+	/*
+	 * Allocate stacktrace pages for Hypervisor-mode.
+	 * This is used by the hypervisor to share its stacktrace
+	 * with the host on a hyp_panic().
+	 */
+	for_each_possible_cpu(cpu) {
+		unsigned long stacktrace_page;
+
+		stacktrace_page = __get_free_page(GFP_KERNEL);
+		if (!stacktrace_page) {
+			err = -ENOMEM;
+			goto out_err;
+		}
+
+		per_cpu(kvm_arm_hyp_stacktrace_page, cpu) = stacktrace_page;
+	}
+
 	/*
 	 * Allocate and initialize pages for Hypervisor-mode percpu regions.
 	 */
@@ -1975,6 +1995,20 @@ static int init_hyp_mode(void)
 		params->stack_hyp_va = hyp_addr + (2 * PAGE_SIZE);
 	}
 
+	/*
+	 * Map the hyp stacktrace pages.
+	 */
+	for_each_possible_cpu(cpu) {
+		char *stacktrace_page = (char *)per_cpu(kvm_arm_hyp_stacktrace_page, cpu);
+
+		err = create_hyp_mappings(stacktrace_page, stacktrace_page + PAGE_SIZE,
+					  PAGE_HYP);
+		if (err) {
+			kvm_err("Cannot map hyp stacktrace page\n");
+			goto out_err;
+		}
+	}
+
 	for_each_possible_cpu(cpu) {
 		char *percpu_begin = (char *)kvm_arm_hyp_percpu_base[cpu];
 		char *percpu_end = percpu_begin + nvhe_percpu_size();
diff --git a/arch/arm64/kvm/hyp/nvhe/setup.c b/arch/arm64/kvm/hyp/nvhe/setup.c
index e8d4ea2fcfa0..9b81bf2d40d7 100644
--- a/arch/arm64/kvm/hyp/nvhe/setup.c
+++ b/arch/arm64/kvm/hyp/nvhe/setup.c
@@ -135,6 +135,17 @@ static int recreate_hyp_mappings(phys_addr_t phys, unsigned long size,
 
 		/* Update stack_hyp_va to end of the stack's private VA range */
 		params->stack_hyp_va = hyp_addr + (2 * PAGE_SIZE);
+
+		/*
+		 * Map the stacktrace pages as shared and transfer ownership to
+		 * the hypervisor.
+		 */
+		prot = pkvm_mkstate(PAGE_HYP, PKVM_PAGE_SHARED_OWNED);
+		start = (void *)params->stacktrace_hyp_va;
+		end = start + PAGE_SIZE;
+		ret = pkvm_create_mappings(start, end, prot);
+		if (ret)
+			return ret;
 	}
 
 	/*
-- 
2.36.0.464.gb9c8b46e94-goog

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

[Kalesh Singh]

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