On Sat, Apr 25, 2020 at 10:43:21AM +0100, Marc Zyngier wrote:
> We currently save/restore sp_el0 in C code. This is a bit unsafe,
> as a lot of the C code expects 'current' to be accessible from
> there (and the opportunity to run kernel code in HYP is specially
> great with VHE).
>
> Instead, let's move the save/restore of sp_el0 to the assembly
> code (in __guest_enter), making sure that sp_el0 is correct
> very early on when we exit the guest, and is preserved as long
> as possible to its host value when we enter the guest.
>
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
Makes sense to me in principle, but I haven't reviewed the code in
detail:
Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
Mark.
> ---
> arch/arm64/kvm/hyp/entry.S | 23 +++++++++++++++++++++++
> arch/arm64/kvm/hyp/sysreg-sr.c | 17 +++--------------
> 2 files changed, 26 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index d22d0534dd600..90186cf6473e0 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -18,6 +18,7 @@
>
> #define CPU_GP_REG_OFFSET(x) (CPU_GP_REGS + x)
> #define CPU_XREG_OFFSET(x) CPU_GP_REG_OFFSET(CPU_USER_PT_REGS + 8*x)
> +#define CPU_SP_EL0_OFFSET (CPU_XREG_OFFSET(30) + 8)
>
> .text
> .pushsection .hyp.text, "ax"
> @@ -47,6 +48,16 @@
> ldp x29, lr, [\ctxt, #CPU_XREG_OFFSET(29)]
> .endm
>
> +.macro save_sp_el0 ctxt, tmp
> + mrs \tmp, sp_el0
> + str \tmp, [\ctxt, #CPU_SP_EL0_OFFSET]
> +.endm
> +
> +.macro restore_sp_el0 ctxt, tmp
> + ldr \tmp, [\ctxt, #CPU_SP_EL0_OFFSET]
> + msr sp_el0, \tmp
> +.endm
> +
> /*
> * u64 __guest_enter(struct kvm_vcpu *vcpu,
> * struct kvm_cpu_context *host_ctxt);
> @@ -60,6 +71,9 @@ SYM_FUNC_START(__guest_enter)
> // Store the host regs
> save_callee_saved_regs x1
>
> + // Save the host's sp_el0
> + save_sp_el0 x1, x2
> +
> // Now the host state is stored if we have a pending RAS SError it must
> // affect the host. If any asynchronous exception is pending we defer
> // the guest entry. The DSB isn't necessary before v8.2 as any SError
> @@ -83,6 +97,9 @@ alternative_else_nop_endif
> // when this feature is enabled for kernel code.
> ptrauth_switch_to_guest x29, x0, x1, x2
>
> + // Restore the guest's sp_el0
> + restore_sp_el0 x29, x0
> +
> // Restore guest regs x0-x17
> ldp x0, x1, [x29, #CPU_XREG_OFFSET(0)]
> ldp x2, x3, [x29, #CPU_XREG_OFFSET(2)]
> @@ -130,6 +147,9 @@ SYM_INNER_LABEL(__guest_exit, SYM_L_GLOBAL)
> // Store the guest regs x18-x29, lr
> save_callee_saved_regs x1
>
> + // Store the guest's sp_el0
> + save_sp_el0 x1, x2
> +
> get_host_ctxt x2, x3
>
> // Macro ptrauth_switch_to_guest format:
> @@ -139,6 +159,9 @@ SYM_INNER_LABEL(__guest_exit, SYM_L_GLOBAL)
> // when this feature is enabled for kernel code.
> ptrauth_switch_to_host x1, x2, x3, x4, x5
>
> + // Restore the hosts's sp_el0
> + restore_sp_el0 x2, x3
> +
> // Now restore the host regs
> restore_callee_saved_regs x2
>
> diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c b/arch/arm64/kvm/hyp/sysreg-sr.c
> index 75b1925763f16..6d2df9fe0b5d2 100644
> --- a/arch/arm64/kvm/hyp/sysreg-sr.c
> +++ b/arch/arm64/kvm/hyp/sysreg-sr.c
> @@ -15,8 +15,9 @@
> /*
> * Non-VHE: Both host and guest must save everything.
> *
> - * VHE: Host and guest must save mdscr_el1 and sp_el0 (and the PC and pstate,
> - * which are handled as part of the el2 return state) on every switch.
> + * VHE: Host and guest must save mdscr_el1 and sp_el0 (and the PC and
> + * pstate, which are handled as part of the el2 return state) on every
> + * switch (sp_el0 is being dealt with in the assembly code).
> * tpidr_el0 and tpidrro_el0 only need to be switched when going
> * to host userspace or a different VCPU. EL1 registers only need to be
> * switched when potentially going to run a different VCPU. The latter two
> @@ -26,12 +27,6 @@
> static void __hyp_text __sysreg_save_common_state(struct kvm_cpu_context *ctxt)
> {
> ctxt->sys_regs[MDSCR_EL1] = read_sysreg(mdscr_el1);
> -
> - /*
> - * The host arm64 Linux uses sp_el0 to point to 'current' and it must
> - * therefore be saved/restored on every entry/exit to/from the guest.
> - */
> - ctxt->gp_regs.regs.sp = read_sysreg(sp_el0);
> }
>
> static void __hyp_text __sysreg_save_user_state(struct kvm_cpu_context *ctxt)
> @@ -99,12 +94,6 @@ NOKPROBE_SYMBOL(sysreg_save_guest_state_vhe);
> static void __hyp_text __sysreg_restore_common_state(struct kvm_cpu_context *ctxt)
> {
> write_sysreg(ctxt->sys_regs[MDSCR_EL1], mdscr_el1);
> -
> - /*
> - * The host arm64 Linux uses sp_el0 to point to 'current' and it must
> - * therefore be saved/restored on every entry/exit to/from the guest.
> - */
> - write_sysreg(ctxt->gp_regs.regs.sp, sp_el0);
> }
>
> static void __hyp_text __sysreg_restore_user_state(struct kvm_cpu_context *ctxt)
> --
> 2.26.2
>
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