Re: [PATCH] KVM: x86/vmx: Do not skip segment attributes if unusable bit is set

[Jim Mattson]

On Mon, Nov 14, 2022 at 8:50 AM Hendrik Borghorst <hborghor@xxxxxxxxx> wrote:
>
> When serializing and deserializing kvm_sregs, attributes of the segment
> descriptors are stored by user space. For unusable segments,
> vmx_segment_access_rights skips all attributes and sets them to 0.
>
> This means we zero out the DPL (Descriptor Privilege Level) for unusable
> entries.
>
> Unusable segments are - contrary to their name - usable in 64bit mode and
> are used by guests to for example create a linear map through the
> NULL selector.
>
> VMENTER checks if SS.DPL is correct depending on the CS segment type.
> For types 9 (Execute Only) and 11 (Execute Read), CS.DPL must be equal to
> SS.DPL [1].
>
> We have seen real world guests setting CS to a usable segment with DPL=3
> and SS to an unusable segment with DPL=3. Once we go through an sregs
> get/set cycle, SS.DPL turns to 0. This causes the virtual machine to crash
> reproducibly.
>
> This commit changes the attribute logic to always preserve attributes for
> unusable segments. According to [2] SS.DPL is always saved on VM exits,
> regardless of the unusable bit so user space applications should have saved
> the information on serialization correctly.
>
> [3] specifies that besides SS.DPL the rest of the attributes of the
> descriptors are undefined after VM entry if unusable bit is set. So, there
> should be no harm in setting them all to the previous state.
>
> [1] Intel SDM Vol 3C 26.3.1.2 Checks on Guest Segment Registers
> [2] Intel SDM Vol 3C 27.3.2 Saving Segment Registers and Descriptor-Table
> Registers
> [3] Intel SDM Vol 3C 26.3.2.2 Loading Guest Segment Registers and
> Descriptor-Table Registers
>
> Cc: Alexander Graf <graf@xxxxxxxxx>
> Signed-off-by: Hendrik Borghorst <hborghor@xxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/vmx.c | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 63247c57c72c..4ae248e87f5e 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3412,18 +3412,15 @@ static u32 vmx_segment_access_rights(struct kvm_segment *var)
>  {
>         u32 ar;
>
> -       if (var->unusable || !var->present)
> -               ar = 1 << 16;
> -       else {
> -               ar = var->type & 15;
> -               ar |= (var->s & 1) << 4;
> -               ar |= (var->dpl & 3) << 5;
> -               ar |= (var->present & 1) << 7;
> -               ar |= (var->avl & 1) << 12;
> -               ar |= (var->l & 1) << 13;
> -               ar |= (var->db & 1) << 14;
> -               ar |= (var->g & 1) << 15;
> -       }
> +       ar = var->type & 15;
> +       ar |= (var->s & 1) << 4;
> +       ar |= (var->dpl & 3) << 5;
> +       ar |= (var->present & 1) << 7;
> +       ar |= (var->avl & 1) << 12;
> +       ar |= (var->l & 1) << 13;
> +       ar |= (var->db & 1) << 14;
> +       ar |= (var->g & 1) << 15;
> +       ar |= (var->unusable || !var->present) << 16;
>
>         return ar;
>  }
Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>