linux-kernel - Re: [PATCH v2 1/2] KVM: SVM: free sev_*asid

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Date:   Tue, 6 Jun 2023 20:51:57 +0200
From:   Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     pbonzini@...hat.com,
        Stéphane Graber <stgraber@...ntu.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 1/2] KVM: SVM: free sev_*asid_bitmap init if SEV init fails

On Tue, Jun 6, 2023 at 8:45 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Mon, May 22, 2023, Alexander Mikhalitsyn wrote:
> > If misc_cg_set_capacity() fails for some reason then we have
> > a memleak for sev_reclaim_asid_bitmap/sev_asid_bitmap. It's
> > not a case right now, because misc_cg_set_capacity() just can't
> > fail and check inside it is always successful.
> >
> > But let's fix that for code consistency.
> >
> > Cc: Sean Christopherson <seanjc@...gle.com>
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Stï¿½phane Graber <stgraber@...ntu.com>
> > Cc: kvm@...r.kernel.org
> > Cc: linux-kernel@...r.kernel.org
> > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
> > ---
> >  arch/x86/kvm/svm/sev.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > index 69ae5e1b3120..cc832a8d1bca 100644
> > --- a/arch/x86/kvm/svm/sev.c
> > +++ b/arch/x86/kvm/svm/sev.c
> > @@ -2216,8 +2216,13 @@ void __init sev_hardware_setup(void)
> >       }
> >
> >       sev_asid_count = max_sev_asid - min_sev_asid + 1;
> > -     if (misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count))
> > +     if (misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count)) {
> > +             bitmap_free(sev_reclaim_asid_bitmap);
> > +             sev_reclaim_asid_bitmap = NULL;
> > +             bitmap_free(sev_asid_bitmap);
> > +             sev_asid_bitmap = NULL;
> >               goto out;
> > +     }
>
> Blech, didn't look close enough at v1.  I think I'd rather yell and continue on.
> If misc_cg_set_capacity() were to fail, debugging would be unnecessarily painful,
> and at least as things stand today, there's nothing userspace can do to remedy
> the problem except by manually disabling SEV and/or SEV-ES.

Hi Sean,

I agree with your point. Let's go this way!

Kind regards,
Alex

>
> ---
> From: Sean Christopherson <seanjc@...gle.com>
> Date: Tue, 6 Jun 2023 11:34:28 -0700
> Subject: [PATCH] KVM: SVM: WARN, but continue, if misc_cg_set_capacity() fails
>
> WARN and continue if misc_cg_set_capacity() fails, as the only scenario
> in which it can fail is if the specified resource is invalid, which should
> never happen when CONFIG_KVM_AMD_SEV=y.  Deliberately not bailing "fixes"
> a theoretical bug where KVM would leak the ASID bitmaps on failure, which
> again can't happen.
>
> If the impossible should happen, the end result is effectively the same
> with respect to SEV and SEV-ES (they are unusable), while continuing on
> has the advantage of letting KVM load, i.e. userspace can still run
> non-SEV guests.
>
> Reported-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/sev.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index d65578d8784d..07756b7348ae 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2216,9 +2216,7 @@ void __init sev_hardware_setup(void)
>         }
>
>         sev_asid_count = max_sev_asid - min_sev_asid + 1;
> -       if (misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count))
> -               goto out;
> -
> +       WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count));
>         sev_supported = true;
>
>         /* SEV-ES support requested? */
> @@ -2243,9 +2241,7 @@ void __init sev_hardware_setup(void)
>                 goto out;
>
>         sev_es_asid_count = min_sev_asid - 1;
> -       if (misc_cg_set_capacity(MISC_CG_RES_SEV_ES, sev_es_asid_count))
> -               goto out;
> -
> +       WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV_ES, sev_es_asid_count));
>         sev_es_supported = true;
>
>  out:
>
> base-commit: 6d1bc9754b04075d938b47cf7f7800814b8911a7
> --
>