linux-kernel - Re: [PATCH 1/8] KVM: SEV-ES: fix length of string I/O

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Date:   Sun, 24 Oct 2021 18:31:01 -0700
From:   Marc Orr <marcorr@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        fwilhelm@...gle.com, seanjc@...gle.com, oupton@...gle.com,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/8] KVM: SEV-ES: fix length of string I/O

On Wed, Oct 13, 2021 at 9:56 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> The size of the data in the scratch buffer is not divided by the size of
> each port I/O operation, so vcpu->arch.pio.count ends up being larger
> than it should be by a factor of size.
>
> Cc: stable@...r.kernel.org
> Fixes: 7ed9abfe8e9f ("KVM: SVM: Support string IO operations for an SEV-ES guest")
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/svm/sev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index c36b5fe4c27c..e672493b5d8d 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2583,7 +2583,7 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
>                 return -EINVAL;
>
>         return kvm_sev_es_string_io(&svm->vcpu, size, port,
> -                                   svm->ghcb_sa, svm->ghcb_sa_len, in);
> +                                   svm->ghcb_sa, svm->ghcb_sa_len / size, in);
>  }
>
>  void sev_es_init_vmcb(struct vcpu_svm *svm)
> --
> 2.27.0
>
>

I could be missing something, but I'm pretty sure that this is wrong.
The GHCB spec says that `exit_info_2` is the `rep` count. Not the
string length.

For example, given a `rep outsw` instruction, with `ECX` set to `8`,
the rep count written into `SW_EXITINFO2` should be eight x86 words
(i.e., 16 bytes) and the IO size should be one x86 word (i.e., 2
bytes). In other words, the code was correct before this patch. This
patch is incorrectly dividing the rep count by the IO size, causing
the string IO to be truncated.