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

[Xiaoyao Li]

On 12/12/2024 1:18 PM, Binbin Wu wrote:
> 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 was thinking if it is better to let each specific handling function to 
  update run->hypercall.ret with its own logic. E.g., for this case, 
let kvm_handle_hc_map_gpa_range() to update the run->hypercall.ret.

Reusing the return value of the handling function to update
run->hypercall.ret seems not logically correct to me.

> > 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