linux-kernel - Re: [PATCH][next] KVM: SVM: nested: fix free of uninitialized pointers save and ctl

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Date:   Fri, 11 Sep 2020 09:28:20 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     Colin King <colin.king@...onical.com>,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org
Subject: Re: [PATCH][next] KVM: SVM: nested: fix free of uninitialized
 pointers save and ctl

+Joerg

On Fri, Sep 11, 2020 at 01:49:42PM +0200, Vitaly Kuznetsov wrote:
> Colin King <colin.king@...onical.com> writes:
> 
> > From: Colin Ian King <colin.king@...onical.com>
> >
> > Currently the error exit path to outt_set_gif will kfree on
> > uninitialized
> 
> typo: out_set_gif
> 
> > pointers save and ctl.  Fix this by ensuring these pointers are
> > inintialized to NULL to avoid garbage pointer freeing.
> >
> > Addresses-Coverity: ("Uninitialized pointer read")
> > Fixes: 6ccbd29ade0d ("KVM: SVM: nested: Don't allocate VMCB structures
> > on stack")
> 
> Where is this commit id from? I don't see it in Paolo's kvm tree, if
> it's not yet merged, maybe we should fix it and avoid introducing the
> issue in the first place?

Ya, AFAIK the series as not been applied.

> > Signed-off-by: Colin Ian King <colin.king@...onical.com>
> > ---
> >  arch/x86/kvm/svm/nested.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index 28036629abf8..2b15f49f9e5a 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -1060,8 +1060,8 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
> >  	struct vmcb *hsave = svm->nested.hsave;
> >  	struct vmcb __user *user_vmcb = (struct vmcb __user *)
> >  		&user_kvm_nested_state->data.svm[0];
> > -	struct vmcb_control_area *ctl;
> > -	struct vmcb_save_area *save;
> > +	struct vmcb_control_area *ctl = NULL;
> > +	struct vmcb_save_area *save = NULL;
> >  	int ret;
> >  	u32 cr0;
> 
> I think it would be better if we eliminate 'out_set_gif; completely as
> the 'error path' we have looks a bit weird anyway. Something like
> (untested):

Ya, I agree that duplicating the single line for this one-off case is
preferable to creating a convoluted set of labels.

Joerg, can you fold this change into a prep patch for v4 of your "KVM: SVM:
SEV-ES groundwork" series?

> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 28036629abf8..d1ae94f40907 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1092,7 +1092,8 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
>  
>         if (!(kvm_state->flags & KVM_STATE_NESTED_GUEST_MODE)) {
>                 svm_leave_nested(svm);
> -               goto out_set_gif;
> +               svm_set_gif(svm, !!(kvm_state->flags & KVM_STATE_NESTED_GIF_SET));
> +               return 0;
>         }
>  
>         if (!page_address_valid(vcpu, kvm_state->hdr.svm.vmcb_pa))
> @@ -1145,7 +1146,6 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
>         load_nested_vmcb_control(svm, ctl);
>         nested_prepare_vmcb_control(svm);
>  
> -out_set_gif:
>         svm_set_gif(svm, !!(kvm_state->flags & KVM_STATE_NESTED_GIF_SET));
>  
>         ret = 0;
> 
> -- 
> Vitaly
>