Re: [PATCH] i386/kvm: Set return value after handling KVM_EXIT_HYPERCALL

[Binbin Wu]

On 12/12/2024 11:44 AM, Xiaoyao Li wrote:
> On 12/12/2024 11:26 AM, Binbin Wu wrote:
> > Userspace should set the ret field of hypercall after handling
> > KVM_EXIT_HYPERCALL.  Otherwise, a stale value could be returned to KVM.
> >
> > Fixes: 47e76d03b15 ("i386/kvm: Add KVM_EXIT_HYPERCALL handling for 
> > KVM_HC_MAP_GPA_RANGE")
> > Reported-by: Farrah Chen <farrah.chen@intel.com>
> > Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
> > Tested-by: Farrah Chen <farrah.chen@intel.com>
> > ---
> > To test the TDX code in kvm-coco-queue, please apply the patch to the QEMU,
> > otherwise, TDX guest boot could fail.
> > A matching QEMU tree including this patch is here:
> > https://github.com/intel-staging/qemu-tdx/releases/tag/tdx-qemu-upstream-v6.1-fix_kvm_hypercall_return_value
> >
> > Previously, the issue was not triggered because no one would modify the ret
> > value. But with the refactor patch for __kvm_emulate_hypercall() in KVM,
> > https://lore.kernel.org/kvm/20241128004344.4072099-7-seanjc@google.com/, the
> > value could be modified.
> > ---
> >   target/i386/kvm/kvm.c | 8 ++++++--
> >   1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> > index 8e17942c3b..4bcccb48d1 100644
> > --- a/target/i386/kvm/kvm.c
> > +++ b/target/i386/kvm/kvm.c
> > @@ -6005,10 +6005,14 @@ static int kvm_handle_hc_map_gpa_range(struct kvm_run 
> > *run)
> >     static int kvm_handle_hypercall(struct kvm_run *run)
> >   {
> > +    int ret = -EINVAL;
> > +
> >       if (run->hypercall.nr == KVM_HC_MAP_GPA_RANGE)
> > -        return kvm_handle_hc_map_gpa_range(run);
> > +        ret = kvm_handle_hc_map_gpa_range(run);
> > +
> > +    run->hypercall.ret = ret;
>
> Updating run->hypercall.ret is useful only when QEMU needs to re-enter the guest. 
> For the case of ret < 0, QEMU will stop the vcpu.

IMHO, assign run->hypercall.ret anyway should be OK, no need to add a
per-condition on ret, although the value is not used when ret < 0.

Currently, since QEMU will stop the vcpu when ret < 0, this patch doesn't
convert ret to -Exxx that the ABI expects.

> I think we might need re-think on the handling of KVM_EXIT_HYPERCALL. E.g., in 
> what error case should QEMU stop the vcpu, and in what case can QEMU return the 
> error back to the guest via run->hypercall.ret.

Actually, I had the similar question before.
https://lore.kernel.org/kvm/d25cc62c-0f56-4be2-968a-63c8b1d63b5a@linux.intel.com/

It might depends on the hypercall number?
Another option is QEMU always sets run->hypercall.ret appropriately and 
continues the vcpu thread.


> > -    return -EINVAL;
> > +    return ret;
> >   }
> >     #define VMX_INVALID_GUEST_STATE 0x80000021
> >
> > base-commit: ae35f033b874c627d81d51070187fbf55f0bf1a7