[PATCH 0/2] Support userspace hypercalls for TDX

[Tim Merrifield <tim.merrifield@broadcom.com>]

VMCALL and VMMCALL instructions are used by x86 guests to request services
from the host VMM. Both VMCALL and VMMCALL are not restricted to CPL 0.
This allows userspace software like open-vm-tools to communicate directly
with the VMM.

In the context of confidential VMs, direct communication with the host may
violate the security model. Existing binaries that make use of hypercalls
and are not hardened against malicious hypervisors can become a possible
attack surface. For this reason, user-level VMCALLs are not currently
forwarded to the host on TDX VMs. This breaks any user-level software that
use these instructions.

But if user-level software is aware of the risks and has been hardened to
address any known violations of the security model, then it seems
reasonable to allow hypercalls from this process to proceed.

This patchset introduces a new x86 process control flag to address this
concern. By setting the TIF_COCO_USER_HCALL thread information flag, the
process opts in to user-level hypercalls. When TDX is enabled, the VMCALL
will #VE and control will be transferred to a hypervisor-specific
hypercall handler (similar to how things work today for SEV with
sev_es_hcall_prepare/sev_es_hcall_finish). The flag has no effect on
non-TDX VMs. Other confidential computing technologies could use this flag
to provide limited access to user-level hypercalls.

Tim Merrifield (2):
  x86/tdx: Add prctl to allow userlevel TDX hypercalls
  x86/vmware: VMware support for TDX userspace hypercalls

 arch/x86/coco/tdx/tdx.c            | 18 +++++++++++
 arch/x86/include/asm/thread_info.h |  2 ++
 arch/x86/include/asm/x86_init.h    |  1 +
 arch/x86/include/uapi/asm/prctl.h  |  3 ++
 arch/x86/kernel/cpu/vmware.c       | 51 +++++++++++++++++++++++-------
 arch/x86/kernel/process.c          | 20 ++++++++++++
 6 files changed, 84 insertions(+), 11 deletions(-)

-- 
2.40.1