[PATCH v2 6/9] KVM: arm64: Detect and handle hypervisor stack overflows

[Kalesh Singh <kaleshsingh@google.com>]

The hypervisor stacks (for both nVHE Hyp mode and nVHE protected mode)
are aligned to twice their size (PAGE_SIZE), meaning that any valid stack
address has PAGE_SHIFT bit as 0. This allows us to conveniently check for
overflow in the exception entry without corrupting any GPRs. We won't
recover from a stack overflow so panic the hypervisor.

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
 arch/arm64/kvm/hyp/nvhe/host.S   | 16 ++++++++++++++++
 arch/arm64/kvm/hyp/nvhe/switch.c |  5 +++++
 2 files changed, 21 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/host.S b/arch/arm64/kvm/hyp/nvhe/host.S
index 3d613e721a75..78e4b612ac06 100644
--- a/arch/arm64/kvm/hyp/nvhe/host.S
+++ b/arch/arm64/kvm/hyp/nvhe/host.S
@@ -153,6 +153,10 @@ SYM_FUNC_END(__host_hvc)
 
 .macro invalid_host_el2_vect
 	.align 7
+
+	/* Test stack overflow without corrupting GPRs */
+	test_sp_overflow PAGE_SHIFT, .L__hyp_sp_overflow\@
+
 	/* If a guest is loaded, panic out of it. */
 	stp	x0, x1, [sp, #-16]!
 	get_loaded_vcpu x0, x1
@@ -165,6 +169,18 @@ SYM_FUNC_END(__host_hvc)
 	 * been partially clobbered by __host_enter.
 	 */
 	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
+
+	bl	hyp_panic_bad_stack
+	ASM_BUG()
 .endm
 
 .macro invalid_host_el1_vect
diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
index 6410d21d8695..5a2e1ab79913 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -369,6 +369,11 @@ void __noreturn hyp_panic(void)
 	unreachable();
 }
 
+void __noreturn hyp_panic_bad_stack(void)
+{
+	hyp_panic();
+}
+
 asmlinkage void kvm_unexpected_el2_exception(void)
 {
 	return __kvm_unexpected_el2_exception();
-- 
2.35.1.473.g83b2b277ed-goog