linux-kernel - Re: [PATCH v2] KVM: nVMX: fix the layout of struct kvm_vmx_nested_state

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Message-ID: <CALMp9eR+DYVH0UZvbNKUNArzPdf1mvAoxakzj++szaVCD0Fcpw@mail.gmail.com>
Date:   Mon, 13 Jul 2020 11:23:59 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Peter Shier <pshier@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: nVMX: fix the layout of struct kvm_vmx_nested_state_hdr

On Mon, Jul 13, 2020 at 9:22 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Before commit 850448f35aaf ("KVM: nVMX: Fix VMX preemption timer
> migration") struct kvm_vmx_nested_state_hdr looked like:
>
> struct kvm_vmx_nested_state_hdr {
>         __u64 vmxon_pa;
>         __u64 vmcs12_pa;
>         struct {
>                 __u16 flags;
>         } smm;
> }
>
> The ABI got broken by the above mentioned commit and an attempt
> to fix that was made in commit 83d31e5271ac ("KVM: nVMX: fixes for
> preemption timer migration") which made the structure look like:
>
> struct kvm_vmx_nested_state_hdr {
>         __u64 vmxon_pa;
>         __u64 vmcs12_pa;
>         struct {
>                 __u16 flags;
>         } smm;
>         __u32 flags;
>         __u64 preemption_timer_deadline;
> };
>
> The problem with this layout is that before both changes compilers were
> allocating 24 bytes for this and although smm.flags is padded to 8 bytes,
> it is initialized as a 2 byte value. Chances are that legacy userspaces
> using old layout will be passing uninitialized bytes which will slip into
> what is now known as 'flags'.
>
> Suggested-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Fixes: 850448f35aaf ("KVM: nVMX: Fix VMX preemption timer migration")
> Fixes: 83d31e5271ac ("KVM: nVMX: fixes for preemption timer migration")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>

Oops!

Reviewed-by: Jim Mattson <jmattson@...gle.com>