[PATCH 0/6] KVM: Avoid a lurking guest_memfd ABI mess

Sean Christopherson
2025-09-26T16:31:20Z

Add a guest_memfd flag, DEFAULT_SHARED, to let userspace explicitly state
whether the underlying memory should default to private vs. shared. As-is,
the default state is implicitly derived from the MMAP flag: guest_memfd
without MMAP is private, and with MMAP is shared. That implicit behavior
is going to create a mess of an ABI once in-place conversion support comes
along.

If the default state is implicit, then x86 CoCo VMs will end up with default
state that varies based on whether or not a guest_memfd instance is
configured for mmap() support. To avoid breaking guest<=>host ABI for CoCo
VMs when utilizing in-place conversion, i.e. MMAP, userspace would need to
immediately convert all memory from shared=>private.

Ackerley's RFC for in-place conversion fudged around this by adding a flag
to let userspace set the default to _private_, but that will result in a
messy and hard to document ABI. For x86 CoCo VMs, memory would be private
by default, unless MMAP but not INIT_PRIVATE is specified. For everything
else, memory would be shared by default, sort of? Because without MMAP,
the memory would be inaccessible, leading to Schrödinger's cat situation.

Since odds are very good we'll end up with a flag of some kind, add one now
(for 6.18) so that the default state is explicit and simple: without
DEFAULT_SHARED == private, with DEFAULT_SHARED == shared.

As a bonus, this allows for adding test coverage that KVM rejects faults to
private memory.

Ackerley Tng (1):
KVM: selftests: Add test coverage for guest_memfd without
GUEST_MEMFD_FLAG_MMAP

Sean Christopherson (5):
KVM: guest_memfd: Add DEFAULT_SHARED flag, reject user page faults if
not set
KVM: selftests: Stash the host page size in a global in the
guest_memfd test
KVM: selftests: Create a new guest_memfd for each testcase
KVM: selftests: Add wrappers for mmap() and munmap() to assert success
KVM: selftests: Verify that faulting in private guest_memfd memory
fails

Documentation/virt/kvm/api.rst | 10 +-
include/uapi/linux/kvm.h | 3 +-
.../testing/selftests/kvm/guest_memfd_test.c | 162 +++++++++++-------
.../testing/selftests/kvm/include/kvm_util.h | 25 +++
tools/testing/selftests/kvm/lib/kvm_util.c | 44 ++---
tools/testing/selftests/kvm/mmu_stress_test.c | 5 +-
.../selftests/kvm/s390/ucontrol_test.c | 16 +-
.../selftests/kvm/set_memory_region_test.c | 17 +-
virt/kvm/guest_memfd.c | 6 +-
9 files changed, 169 insertions(+), 119 deletions(-)

base-commit: a6ad54137af92535cfe32e19e5f3bc1bb7dbd383
--
2.51.0.536.g15c5d4f767-goog