[PATCH v8 5/6] 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 such  that any valid stack address has PAGE_SHIFT bit as 1.
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>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
---

Changes in v8:
  - branch to hyp_panic_bad_stack(), instead of branch and link, per Marc

Changes in v7:
  - Add Fuad's Reviewed-by and Tested-by tags.

Changes in v5:
  - Valid stack addresses now have PAGE_SHIFT bit as 1 instead of 0

Changes in v3:
  - Remove test_sp_overflow macro, per Mark
  - Add asmlinkage attribute for hyp_panic, hyp_panic_bad_stack, per Ard


 arch/arm64/kvm/hyp/nvhe/host.S   | 24 ++++++++++++++++++++++++
 arch/arm64/kvm/hyp/nvhe/switch.c |  7 ++++++-
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/host.S b/arch/arm64/kvm/hyp/nvhe/host.S
index 3d613e721a75..09b5254fb497 100644
--- a/arch/arm64/kvm/hyp/nvhe/host.S
+++ b/arch/arm64/kvm/hyp/nvhe/host.S
@@ -153,6 +153,18 @@ SYM_FUNC_END(__host_hvc)
 
 .macro invalid_host_el2_vect
 	.align 7
+
+	/*
+	 * Test whether the SP has overflowed, without corrupting a GPR.
+	 * nVHE hypervisor stacks are aligned so that the PAGE_SHIFT bit
+	 * of SP should always be 1.
+	 */
+	add	sp, sp, x0			// sp' = sp + x0
+	sub	x0, sp, x0			// x0' = sp' - x0 = (sp + x0) - x0 = sp
+	tbz	x0, #PAGE_SHIFT, .L__hyp_sp_overflow\@
+	sub	x0, sp, x0			// x0'' = sp' - x0' = (sp + x0) - sp = x0
+	sub	sp, sp, x0			// sp'' = sp' - x0 = (sp + x0) - x0 = sp
+
 	/* If a guest is loaded, panic out of it. */
 	stp	x0, x1, [sp, #-16]!
 	get_loaded_vcpu x0, x1
@@ -165,6 +177,18 @@ 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
+
+	b	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..703a5d3f611b 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -347,7 +347,7 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
 	return exit_code;
 }
 
-void __noreturn hyp_panic(void)
+asmlinkage void __noreturn hyp_panic(void)
 {
 	u64 spsr = read_sysreg_el2(SYS_SPSR);
 	u64 elr = read_sysreg_el2(SYS_ELR);
@@ -369,6 +369,11 @@ void __noreturn hyp_panic(void)
 	unreachable();
 }
 
+asmlinkage void __noreturn hyp_panic_bad_stack(void)
+{
+	hyp_panic();
+}
+
 asmlinkage void kvm_unexpected_el2_exception(void)
 {
 	return __kvm_unexpected_el2_exception();
-- 
2.36.0.rc0.470.gd361397f0d-goog