Re: [PATCH v3 03/17] arm64: kvm: stop treating register x18 as caller save

[Kees Cook]

On Thu, Oct 31, 2019 at 09:46:23AM -0700, samitolvanen@xxxxxxxxxx wrote:
> In preparation of reserving x18, stop treating it as caller save in
> the KVM guest entry/exit code. Currently, the code assumes there is
> no need to preserve it for the host, given that it would have been
> assumed clobbered anyway by the function call to __guest_enter().
> Instead, preserve its value and restore it upon return.
> 
> Link: https://patchwork.kernel.org/patch/9836891/
> Co-developed-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> [ updated commit message, switched from x18 to x29 for the guest context ]
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  arch/arm64/kvm/hyp/entry.S | 41 +++++++++++++++++++-------------------
>  1 file changed, 20 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index e5cc8d66bf53..c3c2d842c609 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -23,6 +23,7 @@
>  	.pushsection	.hyp.text, "ax"
>  
>  .macro save_callee_saved_regs ctxt
> +	str	x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
>  	stp	x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
>  	stp	x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
>  	stp	x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
> @@ -32,6 +33,8 @@
>  .endm
>  
>  .macro restore_callee_saved_regs ctxt
> +	// We assume \ctxt is not x18-x28
> +	ldr	x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
>  	ldp	x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
>  	ldp	x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
>  	ldp	x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
> @@ -48,7 +51,7 @@ ENTRY(__guest_enter)
>  	// x0: vcpu
>  	// x1: host context
>  	// x2-x17: clobbered by macros
> -	// x18: guest context
> +	// x29: guest context
>  
>  	// Store the host regs
>  	save_callee_saved_regs x1
> @@ -67,31 +70,28 @@ alternative_else_nop_endif
>  	ret
>  
>  1:
> -	add	x18, x0, #VCPU_CONTEXT
> +	add	x29, x0, #VCPU_CONTEXT
>  
>  	// Macro ptrauth_switch_to_guest format:
>  	// 	ptrauth_switch_to_guest(guest cxt, tmp1, tmp2, tmp3)
>  	// The below macro to restore guest keys is not implemented in C code
>  	// as it may cause Pointer Authentication key signing mismatch errors
>  	// when this feature is enabled for kernel code.
> -	ptrauth_switch_to_guest x18, x0, x1, x2
> +	ptrauth_switch_to_guest x29, x0, x1, x2
>  
>  	// Restore guest regs x0-x17
> -	ldp	x0, x1,   [x18, #CPU_XREG_OFFSET(0)]
> -	ldp	x2, x3,   [x18, #CPU_XREG_OFFSET(2)]
> -	ldp	x4, x5,   [x18, #CPU_XREG_OFFSET(4)]
> -	ldp	x6, x7,   [x18, #CPU_XREG_OFFSET(6)]
> -	ldp	x8, x9,   [x18, #CPU_XREG_OFFSET(8)]
> -	ldp	x10, x11, [x18, #CPU_XREG_OFFSET(10)]
> -	ldp	x12, x13, [x18, #CPU_XREG_OFFSET(12)]
> -	ldp	x14, x15, [x18, #CPU_XREG_OFFSET(14)]
> -	ldp	x16, x17, [x18, #CPU_XREG_OFFSET(16)]
> -
> -	// Restore guest regs x19-x29, lr
> -	restore_callee_saved_regs x18
> -
> -	// Restore guest reg x18
> -	ldr	x18,      [x18, #CPU_XREG_OFFSET(18)]
> +	ldp	x0, x1,   [x29, #CPU_XREG_OFFSET(0)]
> +	ldp	x2, x3,   [x29, #CPU_XREG_OFFSET(2)]
> +	ldp	x4, x5,   [x29, #CPU_XREG_OFFSET(4)]
> +	ldp	x6, x7,   [x29, #CPU_XREG_OFFSET(6)]
> +	ldp	x8, x9,   [x29, #CPU_XREG_OFFSET(8)]
> +	ldp	x10, x11, [x29, #CPU_XREG_OFFSET(10)]
> +	ldp	x12, x13, [x29, #CPU_XREG_OFFSET(12)]
> +	ldp	x14, x15, [x29, #CPU_XREG_OFFSET(14)]
> +	ldp	x16, x17, [x29, #CPU_XREG_OFFSET(16)]
> +
> +	// Restore guest regs x18-x29, lr
> +	restore_callee_saved_regs x29
>  
>  	// Do not touch any register after this!
>  	eret
> @@ -114,7 +114,7 @@ ENTRY(__guest_exit)
>  	// Retrieve the guest regs x0-x1 from the stack
>  	ldp	x2, x3, [sp], #16	// x0, x1
>  
> -	// Store the guest regs x0-x1 and x4-x18
> +	// Store the guest regs x0-x1 and x4-x17
>  	stp	x2, x3,   [x1, #CPU_XREG_OFFSET(0)]
>  	stp	x4, x5,   [x1, #CPU_XREG_OFFSET(4)]
>  	stp	x6, x7,   [x1, #CPU_XREG_OFFSET(6)]
> @@ -123,9 +123,8 @@ ENTRY(__guest_exit)
>  	stp	x12, x13, [x1, #CPU_XREG_OFFSET(12)]
>  	stp	x14, x15, [x1, #CPU_XREG_OFFSET(14)]
>  	stp	x16, x17, [x1, #CPU_XREG_OFFSET(16)]
> -	str	x18,      [x1, #CPU_XREG_OFFSET(18)]
>  
> -	// Store the guest regs x19-x29, lr
> +	// Store the guest regs x18-x29, lr
>  	save_callee_saved_regs x1
>  
>  	get_host_ctxt	x2, x3
> -- 
> 2.24.0.rc0.303.g954a862665-goog
> 

-- 
Kees Cook