Re: [PATCH v2 7/7] KVM: selftests: Avoid infinite loop if ucall_alloc() fails

Oliver Upton
2022-12-09T21:35:35Z

On Fri, Dec 09, 2022 at 09:03:45PM +0000, Sean Christopherson wrote:

[...]

> > - GUEST_ASSERT(0);
> > +out:
> > + /*
> > + * If the guest cannot grab a ucall structure from the pool then the
> > + * only option to get out to userspace is a bare ucall. This is probably
> > + * a good time to mention that guest assertions depend on ucalls with
> > + * arguments too.
> > + */
> > + GUEST_UCALL_NONE();
>
> UCALL_NONE isn't much better than infinite stack recursion, e.g. a test might end
> up passing by dumb luck, or go in the wrong direction because it sometimes handles
> UCALL_NONE.

Oh, I was just seeking an end to my misery. Yeah, we can use a magic
value to signal this instead.

> How about this?

LGTM.

--
Thanks,
Oliver

> From: Sean Christopherson <seanjc@google.com>
> Date: Fri, 9 Dec 2022 12:55:44 -0800
> Subject: [PATCH] KVM: selftests: Use magic value to signal ucall_alloc()
> failure
>
> Use a magic value to signal a ucall_alloc() failure instead of simply
> doing GUEST_ASSERT(). GUEST_ASSERT() relies on ucall_alloc() and so a
> failure puts the guest into an infinite loop.
>
> Use -1 as the magic value, as a real ucall struct should never wrap.
>
> Reported-by: Oliver Upton <oliver.upton@linux.dev>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> tools/testing/selftests/kvm/lib/ucall_common.c | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
> index 0cc0971ce60e..2f0e2ea941cc 100644
> --- a/tools/testing/selftests/kvm/lib/ucall_common.c
> +++ b/tools/testing/selftests/kvm/lib/ucall_common.c
> @@ -4,6 +4,8 @@
> #include "linux/bitmap.h"
> #include "linux/atomic.h"
>
> +#define GUEST_UCALL_FAILED -1
> +
> struct ucall_header {
> DECLARE_BITMAP(in_use, KVM_MAX_VCPUS);
> struct ucall ucalls[KVM_MAX_VCPUS];
> @@ -41,7 +43,8 @@ static struct ucall *ucall_alloc(void)
> struct ucall *uc;
> int i;
>
> - GUEST_ASSERT(ucall_pool);
> + if (!ucall_pool)
> + goto ucall_failed;
>
> for (i = 0; i < KVM_MAX_VCPUS; ++i) {
> if (!test_and_set_bit(i, ucall_pool->in_use)) {
> @@ -51,7 +54,13 @@ static struct ucall *ucall_alloc(void)
> }
> }
>
> - GUEST_ASSERT(0);
> +ucall_failed:
> + /*
> + * If the vCPU cannot grab a ucall structure, make a bare ucall with a
> + * magic value to signal to get_ucall() that things went sideways.
> + * GUEST_ASSERT() depends on ucall_alloc() and so cannot be used here.
> + */
> + ucall_arch_do_ucall(GUEST_UCALL_FAILED);
> return NULL;
> }
>
> @@ -93,6 +102,9 @@ uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc)
>
> addr = ucall_arch_get_ucall(vcpu);
> if (addr) {
> + TEST_ASSERT(addr != (void *)GUEST_UCALL_FAILED,
> + "Guest failed to allocate ucall struct");
> +
> memcpy(uc, addr, sizeof(*uc));
> vcpu_run_complete_io(vcpu);
> } else {
>
> base-commit: dc2efbe4813e0dc4368779bc36c5f0e636cb8eb2
> --
>