[PATCH 4/7] KVM: arm64: Allocate guard pages near hyp stacks

[Kalesh Singh]

From: Quentin Perret <qperret at google.com>

Allocate unbacked VA space underneath each stack page to ensure stack
overflows get trapped and don't corrupt memory silently.

The stack is aligned to twice its size (PAGE_SIZE), meaning that any
valid stack address has PAGE_SHIFT bit as 0. This allows us to easily
check for overflow in the exception entry without corrupting any GPRs.

Signed-off-by: Quentin Perret <qperret at google.com>
[ Kalesh - Update commit text and comments,
           refactor, add overflow handling ]
Signed-off-by: Kalesh Singh <kaleshsingh at google.com>
---
 arch/arm64/kvm/hyp/nvhe/host.S   | 16 ++++++++++++++++
 arch/arm64/kvm/hyp/nvhe/setup.c  | 19 ++++++++++++++++++-
 arch/arm64/kvm/hyp/nvhe/switch.c |  5 +++++
 3 files changed, 39 insertions(+), 1 deletion(-)

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/setup.c b/arch/arm64/kvm/hyp/nvhe/setup.c
index 99e178cf4249..114053dff228 100644
--- a/arch/arm64/kvm/hyp/nvhe/setup.c
+++ b/arch/arm64/kvm/hyp/nvhe/setup.c
@@ -105,7 +105,24 @@ static int recreate_hyp_mappings(phys_addr_t phys, unsigned long size,
 		if (ret)
 			return ret;
 
-		/* Map stack pages in the 'private' VA range */
+		/*
+		 * Allocate 'private' VA range for stack guard pages.
+		 *
+		 * The 'private' VA range grows upward and stacks downwards, so
+		 * allocate the guard page first. But make sure to align the
+		 * stack itself with PAGE_SIZE * 2 granularity to ease overflow
+		 * detection in the entry assembly code.
+		 */
+		do {
+			start = (void *)hyp_alloc_private_va_range(PAGE_SIZE);
+			if (IS_ERR(start))
+				return PTR_ERR(start);
+		} while (IS_ALIGNED((u64) start, PAGE_SIZE * 2));
+
+		/*
+		 * Map stack pages in the 'private' VA range above the allocated
+		 * guard pages.
+		 */
 		end = (void *)__hyp_pa(per_cpu_ptr(&kvm_init_params, i)->stack_hyp_va);
 		start = end - PAGE_SIZE;
 		start = (void *)__pkvm_create_private_mapping((phys_addr_t)start,
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();
-- 
2.35.1.265.g69c8d7142f-goog