Re: [RFC v2 16/32] x86/tdx: Handle MWAIT, MONITOR and WBINVD

[Dan Williams <dan.j.williams@intel.com>]

On Mon, Apr 26, 2021 at 11:02 AM Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
>
> When running as a TDX guest, there are a number of existing,
> privileged instructions that do not work. If the guest kernel
> uses these instructions, the hardware generates a #VE.
>
> You can find the list of unsupported instructions in Intel
> Trust Domain Extensions (Intel® TDX) Module specification,
> sec 9.2.2 and in Guest-Host Communication Interface (GHCI)
> Specification for Intel TDX, sec 2.4.1.
>

Ah, better than the "handle port io" patch, these details at least
give the reader a chance.

> To prevent TD guest from using MWAIT/MONITOR instructions,
> support for these instructions are already disabled by TDX
> module (SEAM). So CPUID flags for these instructions should
> be in disabled state.

Why does this not result in a #UD if the instruction is disabled by
SEAM? How is it possible to execute a disabled instruction (one
precluded by CPUID) to the point where it triggers #VE instead of #UD?

> After the above mentioned preventive measures, if TD guests still
> execute these instructions, add appropriate warning messages in #VE
> handler. For WBIND instruction, since it's related to memory writeback
> and cache flushes, it's mainly used in context of IO devices. Since
> TDX 1.0 does not support non-virtual I/O devices, skipping it should
> not cause any fatal issues.

WBINVD is in a different class than MWAIT/MONITOR since it is not
identified by CPUID, it can't possibly have the same #UD behaviour.
It's not clear why WBINVD is included in the same patch as
MWAIT/MONITOR?

I disagree with the assertion that WBINVD is mainly used in the
context of I/O devices, it's also used for ACPI power management
paths. WBINVD dependent functionality should be dynamically disabled
rather than warned about.

Does a TDX guest support out-of-tree modules?  The kernel is already
tainted when out-of-tree modules are loaded. In other words in-tree
modules preclude forbidden instructions because they can just be
audited, and out-of-tree modules are ok to trigger abrupt failure if
they attempt to use forbidden instructions.

> But to let users know about its usage, use
> WARN() to report about it.. For MWAIT/MONITOR instruction, since its
> unsupported use WARN() to report unsupported usage.

I'm not sure how useful warning is outside of a kernel developer's
debug environment. The kernel should know what instructions are
disabled and which are available. WBINVD in particular has potential
data integrity implications. Code that might lead to a WBINVD usage
should be disabled, not run all the way up to where WBINVD is
attempted and then trigger an after-the-fact WARN_ONCE().

The WBINVD change deserves to be split off from MWAIT/MONITOR, and
more thought needs to be put into where these spurious instruction
usages are arising.

>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> Reviewed-by: Andi Kleen <ak@linux.intel.com>
> ---
>  arch/x86/kernel/tdx.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
> index 3fe617978fc4..294dda5bf3f6 100644
> --- a/arch/x86/kernel/tdx.c
> +++ b/arch/x86/kernel/tdx.c
> @@ -371,6 +371,21 @@ int tdg_handle_virtualization_exception(struct pt_regs *regs,
>         case EXIT_REASON_EPT_VIOLATION:
>                 ve->instr_len = tdg_handle_mmio(regs, ve);
>                 break;
> +       case EXIT_REASON_WBINVD:
> +               /*
> +                * WBINVD is not supported inside TDX guests. All in-
> +                * kernel uses should have been disabled.
> +                */
> +               WARN_ONCE(1, "TD Guest used unsupported WBINVD instruction\n");
> +               break;
> +       case EXIT_REASON_MONITOR_INSTRUCTION:
> +       case EXIT_REASON_MWAIT_INSTRUCTION:
> +               /*
> +                * Something in the kernel used MONITOR or MWAIT despite
> +                * X86_FEATURE_MWAIT being cleared for TDX guests.
> +                */
> +               WARN_ONCE(1, "TD Guest used unsupported MWAIT/MONITOR instruction\n");
> +               break;
>         default:
>                 pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
>                 return -EFAULT;
> --
> 2.25.1
>