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

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Message-ID: <87o8mclei1.fsf@vitty.brq.redhat.com>
Date:   Fri, 11 Sep 2020 13:49:42 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Colin King <colin.king@...onical.com>
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.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

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?

> 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):

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