Re: [PATCH v3 3/8] KVM: arm64: Add guard pages for KVM nVHE hypervisor stack

[Kalesh Singh <kaleshsingh@google.com>]

On Thu, Feb 24, 2022 at 4:26 AM Fuad Tabba <tabba@google.com> wrote:
>
> Hi Kalesh,
>
>
>
> On Thu, Feb 24, 2022 at 5:18 AM Kalesh Singh <kaleshsingh@google.com> wrote:
> >
> > Maps the stack pages in the flexible private VA range and allocates
> > guard pages below the stack as unbacked VA space. The stack is aligned
> > to twice its size to aid overflow detection (implemented in a subsequent
> > patch in the series).
> >
> > Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> > ---
> >
> > Changes in v3:
> >   - Handle null ptr in IS_ERR_OR_NULL checks, per Mark
> >
> >  arch/arm64/include/asm/kvm_asm.h |  1 +
> >  arch/arm64/kvm/arm.c             | 32 +++++++++++++++++++++++++++++---
> >  2 files changed, 30 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> > index d5b0386ef765..2e277f2ed671 100644
> > --- a/arch/arm64/include/asm/kvm_asm.h
> > +++ b/arch/arm64/include/asm/kvm_asm.h
> > @@ -169,6 +169,7 @@ struct kvm_nvhe_init_params {
> >         unsigned long tcr_el2;
> >         unsigned long tpidr_el2;
> >         unsigned long stack_hyp_va;
> > +       unsigned long stack_pa;
> >         phys_addr_t pgd_pa;
> >         unsigned long hcr_el2;
> >         unsigned long vttbr;
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index ecc5958e27fe..7a23630c4a7f 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -1541,7 +1541,6 @@ static void cpu_prepare_hyp_mode(int cpu)
> >         tcr |= (idmap_t0sz & GENMASK(TCR_TxSZ_WIDTH - 1, 0)) << TCR_T0SZ_OFFSET;
> >         params->tcr_el2 = tcr;
> >
> > -       params->stack_hyp_va = kern_hyp_va(per_cpu(kvm_arm_hyp_stack_page, cpu) + PAGE_SIZE);
> >         params->pgd_pa = kvm_mmu_get_httbr();
> >         if (is_protected_kvm_enabled())
> >                 params->hcr_el2 = HCR_HOST_NVHE_PROTECTED_FLAGS;
> > @@ -1990,14 +1989,41 @@ static int init_hyp_mode(void)
> >          * Map the Hyp stack pages
> >          */
> >         for_each_possible_cpu(cpu) {
> > +               struct kvm_nvhe_init_params *params = per_cpu_ptr_nvhe_sym(kvm_init_params, cpu);
> >                 char *stack_page = (char *)per_cpu(kvm_arm_hyp_stack_page, cpu);
> > -               err = create_hyp_mappings(stack_page, stack_page + PAGE_SIZE,
> > -                                         PAGE_HYP);
> > +               unsigned long stack_hyp_va, guard_hyp_va;
> >
> > +               /*
> > +                * Private mappings are allocated downwards from io_map_base
> > +                * so allocate the stack first then the guard page.
> > +                *
> > +                * The stack is aligned to twice its size to facilitate overflow
> > +                * detection.
> > +                */
> > +               err = __create_hyp_private_mapping(__pa(stack_page), PAGE_SIZE,
> > +                                               PAGE_SIZE * 2, &stack_hyp_va, PAGE_HYP);
> >                 if (err) {
> >                         kvm_err("Cannot map hyp stack\n");
> >                         goto out_err;
> >                 }
> > +
> > +               /* Allocate unbacked private VA range for stack guard page */
> > +               guard_hyp_va = hyp_alloc_private_va_range(PAGE_SIZE, PAGE_SIZE);
> > +               if (IS_ERR_OR_NULL((void *)guard_hyp_va)) {
> > +                       err = guard_hyp_va ? PTR_ERR((void *)guard_hyp_va) : -ENOMEM;
>
> I am a bit confused by this check. hyp_alloc_private_va_range() always
> returns ERR_PTR(-ENOMEM) if there's an error. Mark's comment (if I
> understood it correctly) was about how you were handling it *in*
> hyp_alloc_private_va_range(), rather than calls *to*
> hyp_alloc_private_va_range().

Mark's comments were for the callers. I think the address can still be
null without returning -ENOMEM (judging from what the check was before
hyp_alloc_private_va_range). You make a good point - I think we can
handle any potential null in *_alloc_private_va_range() and drop the
use of PTR_ERR with IS_ERR_OR_NULL (which seems not a good idea in
general).

>
> > +                       kvm_err("Cannot allocate hyp stack guard page\n");
> > +                       goto out_err;
> > +               }
> > +
> > +               /*
> > +                * Save the stack PA in nvhe_init_params. This will be needed to recreate
> > +                * the stack mapping in protected nVHE mode. __hyp_pa() won't do the right
> > +                * thing there, since the stack has been mapped in the flexible private
> > +                * VA space.
> > +                */
>
> Nit: These comments go over 80 columns, unlike other comments that
> you've added in this file.

Ack. I'll update in the next version.

Thanks,
Kalesh

>
> Thanks,
> /fuad
>
> > +               params->stack_pa = __pa(stack_page) + PAGE_SIZE;
> > +
> > +               params->stack_hyp_va = stack_hyp_va + PAGE_SIZE;
> >         }
> >
> >         for_each_possible_cpu(cpu) {
> > --
> > 2.35.1.473.g83b2b277ed-goog
> >