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Subject[PATCH v5 5/8] KVM: arm64: Detect and handle hypervisor stack overflows
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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>
---

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..be6d844279b1 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
+
+ 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..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();
--
2.35.1.616.g0bdcbb4464-goog
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