[PATCH v7 00/12] Direct Map Removal Support for guest_memfd

Patrick Roy
2025-09-24T15:17:48Z

From: Patrick Roy <roypat@amazon.co.uk>

[ based on kvm/next ]

Unmapping virtual machine guest memory from the host kernel's direct map is a
successful mitigation against Spectre-style transient execution issues: If the
kernel page tables do not contain entries pointing to guest memory, then any
attempted speculative read through the direct map will necessarily be blocked
by the MMU before any observable microarchitectural side-effects happen. This
means that Spectre-gadgets and similar cannot be used to target virtual machine
memory. Roughly 60% of speculative execution issues fall into this category [1,
Table 1].

This patch series extends guest_memfd with the ability to remove its memory
from the host kernel's direct map, to be able to attain the above protection
for KVM guests running inside guest_memfd.

Additionally, a Firecracker branch with support for these VMs can be found on
GitHub [2].

For more details, please refer to the v5 cover letter [v5]. No
substantial changes in design have taken place since.

=== Changes Since v6 ===

- Drop patch for passing struct address_space to ->free_folio(), due to
possible races with freeing of the address_space. (Hugh)
- Stop using PG_uptodate / gmem preparedness tracking to keep track of
direct map state. Instead, use the lowest bit of folio->private. (Mike, David)
- Do direct map removal when establishing mapping of gmem folio instead
of at allocation time, due to impossibility of handling direct map
removal errors in kvm_gmem_populate(). (Patrick)
- Do TLB flushes after direct map removal, and provide a module
parameter to opt out from them, and a new patch to export
flush_tlb_kernel_range() to KVM. (Will)

[1]: https://download.vusec.net/papers/quarantine_raid23.pdf
[2]: https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding
[RFCv1]: https://lore.kernel.org/kvm/20240709132041.3625501-1-roypat@amazon.co.uk/
[RFCv2]: https://lore.kernel.org/kvm/20240910163038.1298452-1-roypat@amazon.co.uk/
[RFCv3]: https://lore.kernel.org/kvm/20241030134912.515725-1-roypat@amazon.co.uk/
[v4]: https://lore.kernel.org/kvm/20250221160728.1584559-1-roypat@amazon.co.uk/
[v5]: https://lore.kernel.org/kvm/20250828093902.2719-1-roypat@amazon.co.uk/
[v6]: https://lore.kernel.org/kvm/20250912091708.17502-1-roypat@amazon.co.uk/

Patrick Roy (12):
arch: export set_direct_map_valid_noflush to KVM module
x86/tlb: export flush_tlb_kernel_range to KVM module
mm: introduce AS_NO_DIRECT_MAP
KVM: guest_memfd: Add stub for kvm_arch_gmem_invalidate
KVM: guest_memfd: Add flag to remove from direct map
KVM: guest_memfd: add module param for disabling TLB flushing
KVM: selftests: load elf via bounce buffer
KVM: selftests: set KVM_MEM_GUEST_MEMFD in vm_mem_add() if guest_memfd
!= -1
KVM: selftests: Add guest_memfd based vm_mem_backing_src_types
KVM: selftests: cover GUEST_MEMFD_FLAG_NO_DIRECT_MAP in existing
selftests
KVM: selftests: stuff vm_mem_backing_src_type into vm_shape
KVM: selftests: Test guest execution from direct map removed gmem

Documentation/virt/kvm/api.rst | 5 ++
arch/arm64/include/asm/kvm_host.h | 12 ++++
arch/arm64/mm/pageattr.c | 1 +
arch/loongarch/mm/pageattr.c | 1 +
arch/riscv/mm/pageattr.c | 1 +
arch/s390/mm/pageattr.c | 1 +
arch/x86/include/asm/tlbflush.h | 3 +-
arch/x86/mm/pat/set_memory.c | 1 +
arch/x86/mm/tlb.c | 1 +
include/linux/kvm_host.h | 9 +++
include/linux/pagemap.h | 16 +++++
include/linux/secretmem.h | 18 -----
include/uapi/linux/kvm.h | 2 +
lib/buildid.c | 4 +-
mm/gup.c | 19 ++----
mm/mlock.c | 2 +-
mm/secretmem.c | 8 +--
.../testing/selftests/kvm/guest_memfd_test.c | 2 +
.../testing/selftests/kvm/include/kvm_util.h | 37 ++++++++---
.../testing/selftests/kvm/include/test_util.h | 8 +++
tools/testing/selftests/kvm/lib/elf.c | 8 +--
tools/testing/selftests/kvm/lib/io.c | 23 +++++++
tools/testing/selftests/kvm/lib/kvm_util.c | 61 +++++++++--------
tools/testing/selftests/kvm/lib/test_util.c | 8 +++
tools/testing/selftests/kvm/lib/x86/sev.c | 1 +
.../selftests/kvm/pre_fault_memory_test.c | 1 +
.../selftests/kvm/set_memory_region_test.c | 50 ++++++++++++--
.../kvm/x86/private_mem_conversions_test.c | 7 +-
virt/kvm/guest_memfd.c | 66 +++++++++++++++++--
virt/kvm/kvm_main.c | 8 +++
30 files changed, 290 insertions(+), 94 deletions(-)

base-commit: a6ad54137af92535cfe32e19e5f3bc1bb7dbd383
--
2.51.0