Re: [V2 09/11] KVM: selftests: Make ucall work with encrypted guests

Peter Gonda
2022-08-02T13:52:00Z

On Tue, Aug 2, 2022 at 3:49 AM Andrew Jones <andrew.jones@linux.dev> wrote:
>
> On Mon, Aug 01, 2022 at 01:11:07PM -0700, Peter Gonda wrote:
> > Add support for encrypted, SEV, guests in the ucall framework. If
> > encryption is enabled set up a pool of ucall structs in the guests'
> > shared memory region. This was suggested in the thread on "[RFC PATCH
> > 00/10] KVM: selftests: Add support for test-selectable ucall
> > implementations". Using a listed as suggested there doesn't work well
> > because the list is setup using HVAs not GVAs so use a bitmap + array
> > solution instead to get the same pool result.
> >
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: Peter Gonda <pgonda@google.com>
> > ---
> > .../selftests/kvm/include/kvm_util_base.h | 3 +
> > .../selftests/kvm/include/ucall_common.h | 14 +--
> > .../testing/selftests/kvm/lib/ucall_common.c | 112 +++++++++++++++++-
> > 3 files changed, 115 insertions(+), 14 deletions(-)
> >
> > diff --git a/tools/testing/selftests/kvm/include/kvm_util_base.h b/tools/testing/selftests/kvm/include/kvm_util_base.h
> > index 8ce9e5be70a3..ad4abc6be1ab 100644
> > --- a/tools/testing/selftests/kvm/include/kvm_util_base.h
> > +++ b/tools/testing/selftests/kvm/include/kvm_util_base.h
> > @@ -102,6 +102,9 @@ struct kvm_vm {
> > int stats_fd;
> > struct kvm_stats_header stats_header;
> > struct kvm_stats_desc *stats_desc;
> > +
> > + bool use_ucall_list;
>
> use_ucall_pool
>
> > + struct list_head ucall_list;
> > };

Will do. I also need to remove this |ucall_list| member.

> >
> >
> > diff --git a/tools/testing/selftests/kvm/include/ucall_common.h b/tools/testing/selftests/kvm/include/ucall_common.h
> > index c1bc8e33ef3f..a96220ac6024 100644
> > --- a/tools/testing/selftests/kvm/include/ucall_common.h
> > +++ b/tools/testing/selftests/kvm/include/ucall_common.h
> > @@ -22,6 +22,10 @@ enum {
> > struct ucall {
> > uint64_t cmd;
> > uint64_t args[UCALL_MAX_ARGS];
> > +
> > + /* For encrypted guests. */
>
> Please, no 'encrypted' words in ucall files. ucalls shouldn't care about
> guest types. Indeed, the summary of this patch could even drop the word
> 'encrypted'. This patch is adding support for ucall pools, which is
> motivated by the need to support encrypted guests (the motivation should
> go in the commit message, but otherwise the patch should be ucall specific
> and guest type agnostic)
>
> > + uint64_t idx;
>
> We don't need 'idx' because 'hva' will always be at the
> idx * sizeof(struct ucall) offset of ucall_hdr->ucalls, which means
> we can always calculate it,
>
> static inline size_t uc_pool_idx(struct ucall *uc)
> {
> return uc->hva - ucall_hdr->ucalls;
> }

Good call, I didn't think of that.

>
> > + struct ucall *hva;
> > };
> >
> > void ucall_arch_init(struct kvm_vm *vm, void *arg);
> > @@ -32,15 +36,9 @@ uint64_t ucall_arch_get_ucall(struct kvm_vcpu *vcpu);
> > void ucall(uint64_t cmd, int nargs, ...);
> > uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc);
> >
> > -static inline void ucall_init(struct kvm_vm *vm, void *arg)
> > -{
> > - ucall_arch_init(vm, arg);
> > -}
> > +void ucall_init(struct kvm_vm *vm, void *arg);
> >
> > -static inline void ucall_uninit(struct kvm_vm *vm)
> > -{
> > - ucall_arch_uninit(vm);
> > -}
> > +void ucall_uninit(struct kvm_vm *vm);
> >
> > #define GUEST_SYNC_ARGS(stage, arg1, arg2, arg3, arg4) \
> > ucall(UCALL_SYNC, 6, "hello", stage, arg1, arg2, arg3, arg4)
> > diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
> > index a060252bab40..feb0173179ec 100644
> > --- a/tools/testing/selftests/kvm/lib/ucall_common.c
> > +++ b/tools/testing/selftests/kvm/lib/ucall_common.c
> > @@ -1,22 +1,122 @@
> > // SPDX-License-Identifier: GPL-2.0-only
> > #include "kvm_util.h"
> > +#include "linux/types.h"
> > +#include "linux/bitmap.h"
> > +#include "linux/atomic.h"
> > +
> > +struct ucall_header {
> > + DECLARE_BITMAP(in_use, KVM_MAX_VCPUS);
> > + struct ucall ucalls[KVM_MAX_VCPUS];
> > +};
> > +
> > +static bool use_ucall_list;
>
> use_ucall_pool
>
> > +static struct ucall_header *ucall_hdr;
>
> ucall_pool

Will update naming here.

>
> > +
> > +void ucall_init(struct kvm_vm *vm, void *arg)
> > +{
> > + struct ucall *uc;
> > + struct ucall_header *hdr;
> > + vm_vaddr_t vaddr;
> > + int i;
> > +
> > + use_ucall_list = vm->use_ucall_list;
> > + sync_global_to_guest(vm, use_ucall_list);
> > + if (!use_ucall_list)
> > + goto out;
> > +
> > + TEST_ASSERT(!ucall_hdr,
> > + "Only a single encrypted guest at a time for ucalls.");
>
> single VM at a time
>
> And I'd leave that on a single line. checkpatch allows up to 100 chars and
> I'm included to use all 110 chars of it.

Sounds good.

>
> > + vaddr = vm_vaddr_alloc_shared(vm, sizeof(*hdr), vm->page_size);
> > + hdr = (struct ucall_header *)addr_gva2hva(vm, vaddr);
> > + memset(hdr, 0, sizeof(*hdr));
> > +
> > + for (i = 0; i < KVM_MAX_VCPUS; ++i) {
> > + uc = &hdr->ucalls[i];
> > + uc->hva = uc;
> > + uc->idx = i;
> > + }
> > +
> > + ucall_hdr = (struct ucall_header *)vaddr;
> > + sync_global_to_guest(vm, ucall_hdr);
> > +
> > +out:
> > + ucall_arch_init(vm, arg);
> > +}
> > +
> > +void ucall_uninit(struct kvm_vm *vm)
> > +{
> > + use_ucall_list = false;
> > + ucall_hdr = NULL;
>
> It's unlikely we'd ever change the ucall setup on a running VM,
> but we should sync these changes to the guest for good measure.

Hmm I'll need to have some notion of encrypted guests here then. Since
the guests page tables also get encrypted we can no longer get the
gva2gpa translations so sync_global_to_guest() cannot be done due to
the addr_ga2hva(). So is it OK if this call references encryption like
below?

use_ucall_list = false;
ucall_hdr = NULL;
if (!vm->memencrypt.encrypted) {
sync_global_to_guest(vm, use_ucall_list);
sync_global_to_guest(vm, ucall_hdr);
}

>
> > +
> > + ucall_arch_uninit(vm);
> > +}
> > +
> > +static struct ucall *ucall_alloc(void)
> > +{
> > + struct ucall *uc = NULL;
> > + int i;
> > +
> > + if (!use_ucall_list)
> > + goto out;
> > +
> > + for (i = 0; i < KVM_MAX_VCPUS; ++i) {
> > + if (atomic_test_and_set_bit(i, ucall_hdr->in_use))
> > + continue;
> > +
> > + uc = &ucall_hdr->ucalls[i];
> > + }
>
> This is not what I suggested in the last revision and it's still wrong.
> Here, you're still looping through all of the pool and returning the last
> one.
>
> What I suggested was
>
> for (i = 0; i < KVM_MAX_VCPUS; ++i) {
> if (!atomic_test_and_set_bit(i, ucall_hdr->in_use)) {
> uc = &ucall_hdr->ucalls[i];
> break;
> }
> }
>
> Notice how we leave the loop early when we find a free uc.

Ah my mistake, I didn't get this fix into this series

>
> We should also zero the contents of the uc buffer before returning.

Ack will do.

>
> > +
> > +out:
> > + return uc;
> > +}
> > +
> > +static void ucall_free(struct ucall *uc)
> > +{
> > + if (!use_ucall_list)
> > + return;
> > +
> > + clear_bit(uc->idx, ucall_hdr->in_use);
>
> This seems to be the one and only use of idx which is another argument for
> dropping the variable and just calculating it instead.

Will do.

>
> > +}
> > +
> > +static vm_vaddr_t get_ucall_addr(struct ucall *uc)
> > +{
> > + if (use_ucall_list)
> > + return (vm_vaddr_t)uc->hva;
>
> A comment explaining that this hva has already been synchronized
> with the guest would be good. Or maybe a different name for the
> pointer than hva, one which conveys that it's a pointer that works
> as both an hva and gva would be better.

I don't think this pointer does work as a gva though since its created
by just taking &uc during host execution. I can add a comment that
mentions this is a hva is already translated for the host and not for
guest use other than communicating with the host. I thought 'hva' was
a good name here since it is literally the hva of this ucall struct,
what name would you prefer?

>
>
> > +
> > + return (vm_vaddr_t)uc;
> > +}
> >
> > void ucall(uint64_t cmd, int nargs, ...)
> > {
> > - struct ucall uc = {
> > - .cmd = cmd,
> > - };
>
> This zeros all members except cmd.
>
> > + struct ucall *uc;
> > + struct ucall tmp;
>
> And this makes tmp full of stack garbage, so in the non uc-pool case we
> no longer have an equivalent uc. Please initialize tmp the same way uc
> was initialized.

Ack, I'll get this fixed.

>
> > va_list va;
> > int i;
> >
> > + uc = ucall_alloc();
> > + if (!uc)
> > + uc = &tmp;
> > +
> > + uc->cmd = cmd;
> > +
> > nargs = min(nargs, UCALL_MAX_ARGS);
> >
> > va_start(va, nargs);
> > for (i = 0; i < nargs; ++i)
> > - uc.args[i] = va_arg(va, uint64_t);
> > + uc->args[i] = va_arg(va, uint64_t);
> > va_end(va);
> >
> > - ucall_arch_do_ucall((vm_vaddr_t)&uc);
> > + ucall_arch_do_ucall(get_ucall_addr(uc));
>
> We don't need get_ucall_addr(). Just do the if-else right here
>
> if (use_ucall_list)
> ucall_arch_do_ucall((vm_vaddr_t)uc->hva);
> else
> ucall_arch_do_ucall((vm_vaddr_t)uc);

Will do.

>
> > +
> > + ucall_free(uc);
> > +}
> > +
> > +static void *get_ucall_hva(struct kvm_vm *vm, uint64_t uc)
> > +{
> > + if (vm->use_ucall_list)
> > + return (void *)uc;
> > +
> > + return addr_gva2hva(vm, uc);
> > }
> >
> > uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc)
> > @@ -27,7 +127,7 @@ uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc)
> > if (!uc)
> > uc = &ucall;
> >
> > - addr = addr_gva2hva(vcpu->vm, ucall_arch_get_ucall(vcpu));
> > + addr = get_ucall_hva(vcpu->vm, ucall_arch_get_ucall(vcpu));
>
> Wait, I thought we wanted ucall_arch_get_ucall() to return hvas?
> It would make more sense if it did, since it's called from the
> host.

As you noted in [V2 07/11] I updated ucall_arch_get_ucall() to return
the gva. I figured this was just more boilerplate which could be
consolidated given all archs right now just use the addr_gva2hva()
helper to do this translation. This change also allowed for the
"use_ucall_pool" to be completely contained in ucall_common.c instead
of spilling into each arch specific file. If there is a preference to
have the arch specific calls translate from gva to hva I can do that,
I think I'll need to have them all check if "use_ucall_pool" is set
though. Thoughts?

We can still update ucall_arch_get_ucall() to return a pointer type so
that we can return NULL instead of 0.

>
> > if (addr) {
> > memcpy(uc, addr, sizeof(*uc));
> > vcpu_run_complete_io(vcpu);
> > --
> > 2.37.1.455.g008518b4e5-goog
> >
>
> Thanks,
> drew