linux-kernel - Re: [PATCH v2 7/9] x86: Use current USER_CS to setup correct context on vmx entry

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Date:   Thu, 1 Oct 2020 14:52:32 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Robert Richter <rric@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>, kernel@...labora.com
Subject: Re: [PATCH v2 7/9] x86: Use current USER_CS to setup correct context
 on vmx entry

On Thu, Oct 1, 2020 at 1:59 PM Gabriel Krisman Bertazi
<krisman@...labora.com> wrote:
>
> vmx_prepare_switch_to_guest shouldn't use is_64bit_mm, which has a
> very specific use in uprobes.  Use the user_64bit_mode helper instead.
> This reduces the usage of is_64bit_mm, which is awkward, since it relies
> on the personality at load time, which is fine for uprobes, but doesn't
> seem fine here.
>
> I tested this by running VMs with 64 and 32 bits payloads from 64/32
> programs.
>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@...labora.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 7b2a068f08c1..b5aafd9e5f5d 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1172,7 +1172,7 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
>         savesegment(es, host_state->es_sel);
>
>         gs_base = cpu_kernelmode_gs_base(cpu);
> -       if (likely(is_64bit_mm(current->mm))) {
> +       if (likely(user_64bit_mode(current_pt_regs()))) {
>                 current_save_fsgs();
>                 fs_sel = current->thread.fsindex;
>                 gs_sel = current->thread.gsindex;

I disagree with this one.  This whole code path is nonsense.  Can you
just remove the condition entirely and use the 64-bit path
unconditionally?
]