Re: [PATCH v4 3/4] KVM: x86: model canonical checks more precisely

[Sean Christopherson]

On Fri, Sep 06, 2024, Maxim Levitsky wrote:
> As a result of a recent investigation, it was determined that x86 CPUs
> which support 5-level paging, don't always respect CR4.LA57 when doing
> canonical checks.
> 
> In particular:
> 
> 1. MSRs which contain a linear address, allow full 57-bitcanonical address
> regardless of CR4.LA57 state. For example: MSR_KERNEL_GS_BASE.
> 
> 2. All hidden segment bases and GDT/IDT bases also behave like MSRs.
> This means that full 57-bit canonical address can be loaded to them
> regardless of CR4.LA57, both using MSRS (e.g GS_BASE) and instructions
> (e.g LGDT).
> 
> 3. TLB invalidation instructions also allow the user to use full 57-bit
> address regardless of the CR4.LA57.
> 
> Finally, it must be noted that the CPU doesn't prevent the user from
> disabling 5-level paging, even when the full 57-bit canonical address is
> present in one of the registers mentioned above (e.g GDT base).
> 
> In fact, this can happen without any userspace help, when the CPU enters
> SMM mode - some MSRs, for example MSR_KERNEL_GS_BASE are left to contain
> a non-canonical address in regard to the new mode.
> 
> Since most of the affected MSRs and all segment bases can be read and
> written freely by the guest without any KVM intervention, this patch makes
> the emulator closely follow hardware behavior, which means that the
> emulator doesn't take in the account the guest CPUID support for 5-level
> paging, and only takes in the account the host CPU support.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---
>  arch/x86/kvm/emulate.c       |  2 +-
>  arch/x86/kvm/mmu/mmu.c       |  2 +-
>  arch/x86/kvm/vmx/nested.c    | 22 ++++++++--------
>  arch/x86/kvm/vmx/pmu_intel.c |  2 +-
>  arch/x86/kvm/vmx/sgx.c       |  2 +-
>  arch/x86/kvm/vmx/vmx.c       |  4 +--
>  arch/x86/kvm/x86.c           |  8 +++---
>  arch/x86/kvm/x86.h           | 49 ++++++++++++++++++++++++++++++++++--
>  8 files changed, 68 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 8c8061884a019..60986f67c35a8 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -654,7 +654,7 @@ static inline bool emul_is_noncanonical_address(u64 la,
>  						struct x86_emulate_ctxt *ctxt,
>  						unsigned int flags)
>  {
> -	return !ctxt->ops->is_canonical_addr(ctxt, la, 0);
> +	return !ctxt->ops->is_canonical_addr(ctxt, la, flags);

And conversely, passing flags to ->is_canonical_addr() belongs in the patch that
adds the plumbing.  Even though flags isn't truly consumed until this patch,
passing '0' in this helper is confusing and an unnecessary risk.