Re: [PATCH v7 5/6] KVM: arm64: Detect and handle hypervisor stack overflows

[Marc Zyngier <maz@kernel.org>]

On Fri, 08 Apr 2022 21:03:28 +0100,
Kalesh Singh <kaleshsingh@google.com> wrote:
> 
> 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 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..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

Why bl? You clearly don't expect to return here, given that you have
an ASM_BUG() right below, and that you are calling a __no_return
function. I think we should be consistent with the rest of the code
and just do a simple branch.

It also gives us a chance to preserve an extra register from the
context.

> +	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();
> -- 
> 2.35.1.1178.g4f1659d476-goog
> 
> 

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.
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