Re: [PATCH] KVM: arm64: Don't hypercall before EL2 init

[Marc Zyngier]

On Fri, 13 May 2022 10:26:07 +0100,
Quentin Perret <qperret@xxxxxxxxxx> wrote:
> 
> Will reported the following splat when running with Protected KVM
> enabled:
> 
> [    2.427181] ------------[ cut here ]------------
> [    2.427668] WARNING: CPU: 3 PID: 1 at arch/arm64/kvm/mmu.c:489 __create_hyp_private_mapping+0x118/0x1ac
> [    2.428424] Modules linked in:
> [    2.429040] CPU: 3 PID: 1 Comm: swapper/0 Not tainted 5.18.0-rc2-00084-g8635adc4efc7 #1
> [    2.429589] Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
> [    2.430286] pstate: 80000005 (Nzcv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [    2.430734] pc : __create_hyp_private_mapping+0x118/0x1ac
> [    2.431091] lr : create_hyp_exec_mappings+0x40/0x80
> [    2.431377] sp : ffff80000803baf0
> [    2.431597] x29: ffff80000803bb00 x28: 0000000000000000 x27: 0000000000000000
> [    2.432156] x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
> [    2.432561] x23: ffffcd96c343b000 x22: 0000000000000000 x21: ffff80000803bb40
> [    2.433004] x20: 0000000000000004 x19: 0000000000001800 x18: 0000000000000000
> [    2.433343] x17: 0003e68cf7efdd70 x16: 0000000000000004 x15: fffffc81f602a2c8
> [    2.434053] x14: ffffdf8380000000 x13: ffffcd9573200000 x12: ffffcd96c343b000
> [    2.434401] x11: 0000000000000004 x10: ffffcd96c1738000 x9 : 0000000000000004
> [    2.434812] x8 : ffff80000803bb40 x7 : 7f7f7f7f7f7f7f7f x6 : 544f422effff306b
> [    2.435136] x5 : 000000008020001e x4 : ffff207d80a88c00 x3 : 0000000000000005
> [    2.435480] x2 : 0000000000001800 x1 : 000000014f4ab800 x0 : 000000000badca11
> [    2.436149] Call trace:
> [    2.436600]  __create_hyp_private_mapping+0x118/0x1ac
> [    2.437576]  create_hyp_exec_mappings+0x40/0x80
> [    2.438180]  kvm_init_vector_slots+0x180/0x194
> [    2.458941]  kvm_arch_init+0x80/0x274
> [    2.459220]  kvm_init+0x48/0x354
> [    2.459416]  arm_init+0x20/0x2c
> [    2.459601]  do_one_initcall+0xbc/0x238
> [    2.459809]  do_initcall_level+0x94/0xb4
> [    2.460043]  do_initcalls+0x54/0x94
> [    2.460228]  do_basic_setup+0x1c/0x28
> [    2.460407]  kernel_init_freeable+0x110/0x178
> [    2.460610]  kernel_init+0x20/0x1a0
> [    2.460817]  ret_from_fork+0x10/0x20
> [    2.461274] ---[ end trace 0000000000000000 ]---
> 
> Indeed, the Protected KVM mode promotes __create_hyp_private_mapping()
> to a hypercall as EL1 no longer has access to the hypervisor's stage-1
> page-table. However, the call from kvm_init_vector_slots() happens after
> pKVM has been initialized on the primary CPU, but before it has been
> initialized on secondaries. As such, if the KVM initcall procedure is
> migrated from one CPU to another in this window, the hypercall may end up
> running on a CPU for which EL2 has not been initialized.
> 
> Fortunately, the pKVM hypervisor doesn't rely on the host to re-map the
> vectors in the private range, so the hypercall in question is in fact
> superfluous. Skip it when pKVM is enabled.
> 
> Reported-by: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
> ---
>  arch/arm64/kvm/arm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 523bc934fe2f..7347c133efc4 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -1436,7 +1436,7 @@ static int kvm_init_vector_slots(void)
>  	base = kern_hyp_va(kvm_ksym_ref(__bp_harden_hyp_vecs));
>  	kvm_init_vector_slot(base, HYP_VECTOR_SPECTRE_DIRECT);
>  
> -	if (kvm_system_needs_idmapped_vectors() && !has_vhe()) {
> +	if (kvm_system_needs_idmapped_vectors() && !has_vhe() && !is_protected_kvm_enabled()) {
>  		err = create_hyp_exec_mappings(__pa_symbol(__bp_harden_hyp_vecs),
>  					       __BP_HARDEN_HYP_VECS_SZ, &base);
>  		if (err)

Can we simplify the condition? ARM64_SPECTRE_V3A is only set when
!VHE, and we already bail in kvm_patch_vector_branch() if we see
VHE+V3A, because the combination makes no sense at all. I think this
can be rewritten as:

	if (kvm_system_needs_idmapped_vectors() &&
	    !is_protected_lvm_enabled())

Thoughts?

	M.

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