[RFC v2 00/27] Kernel Address Space Isolation

From: Alexandre Chartre
Date: Thu Jul 11 2019 - 10:28:50 EST


This is version 2 of the "KVM Address Space Isolation" RFC. The code
has been completely changed compared to v1 and it now provides a generic
kernel framework which provides Address Space Isolation; and KVM is now
a simple consumer of that framework. That's why the RFC title has been
changed from "KVM Address Space Isolation" to "Kernel Address Space

Kernel Address Space Isolation aims to use address spaces to isolate some
parts of the kernel (for example KVM) to prevent leaking sensitive data
between hyper-threads under speculative execution attacks. You can refer
to the first version of this RFC for more context:


The new code is still a proof of concept. It is much more stable than v1:
I am able to run a VM with a full OS (and also a nested VM) with multiple
vcpus. But it looks like there are still some corner cases which cause the
system to crash/hang.

I am looking for feedback about this new approach where address space
isolation is provided by the kernel, and KVM is a just a consumer of this
new framework.


- Address Space Isolation (ASI) is now provided as a kernel framework:
interfaces for creating and managing an ASI are provided by the kernel,
there are not implemented in KVM.

- An ASI is associated with a page-table, we don't use mm anymore. Entering
isolation is done by just updating CR3 to use the ASI page-table. Exiting
isolation restores CR3 with the CR3 value present before entering isolation.

- Isolation is exited at the beginning of any interrupt/exception handler,
and on context switch.

- Isolation doesn't disable interrupt, but if an interrupt occurs the
interrupt handler will exit isolation.

- The current stack is mapped when entering isolation and unmapped when
exiting isolation.

- The current task is not mapped by default, but there's an option to map it.
In such a case, the current task is mapped when entering isolation and
unmap when exiting isolation.

- Kernel code mapped to the ASI page-table has been reduced to:
. the entire kernel (I still need to test with only the kernel text)
. the cpu entry area (because we need the GDT to be mapped)
. the cpu ASI session (for managing ASI)
. the current stack

- Optionally, an ASI can request the following kernel mapping to be added:
. the stack canary
. the cpu offsets (this_cpu_off)
. the current task
. RCU data (rcu_data)
. CPU HW events (cpu_hw_events).

All these optional mappings are used for KVM isolation.


The proposed patches provides a framework for creating an Address Space
Isolation (ASI) (represented by a struct asi). The ASI has a page-table which
can be populated by copying mappings from the kernel page-table. The ASI can
then be entered/exited by switching between the kernel page-table and the
ASI page-table. In addition, any interrupt, exception or context switch
will automatically abort and exit the isolation. Finally patches use the
ASI framework to implement KVM isolation.

- 01-03: Core of the ASI framework: create/destroy ASI, enter/exit/abort
isolation, ASI page-fault handler.

- 04-14: Functions to manage, populate and clear an ASI page-table.

- 15-20: ASI core mappings and optional mappings.

- 21: Make functions to read cr3/cr4 ASI aware

- 22-26: Use ASI in KVM to provide isolation for VMExit handlers.

API Overview:
Here is a short description of the main ASI functions provided by the framwork.

struct asi *asi_create(int map_flags)

Create an Address Space Isolation (ASI). map_flags can be used to specify
optional kernel mapping to be added to the ASI page-table (for example,
ASI_MAP_STACK_CANARY to map the stack canary).

void asi_destroy(struct asi *asi)

Destroy an ASI.

int asi_enter(struct asi *asi)

Enter isolation for the specified ASI. This switches from the kernel page-table
to the page-table associated with the ASI.

void asi_exit(struct asi *asi)

Exit isolation for the specified ASI. This switches back to the kernel

int asi_map(struct asi *asi, void *ptr, unsigned long size);

Copy kernel mapping to the specified ASI page-table.

void asi_unmap(struct asi *asi, void *ptr);

Clear kernel mapping from the specified ASI page-table.

Alexandre Chartre (23):
mm/x86: Introduce kernel address space isolation
mm/asi: Abort isolation on interrupt, exception and context switch
mm/asi: Handle page fault due to address space isolation
mm/asi: Functions to track buffers allocated for an ASI page-table
mm/asi: Add ASI page-table entry offset functions
mm/asi: Add ASI page-table entry allocation functions
mm/asi: Add ASI page-table entry set functions
mm/asi: Functions to populate an ASI page-table from a VA range
mm/asi: Helper functions to map module into ASI
mm/asi: Keep track of VA ranges mapped in ASI page-table
mm/asi: Functions to clear ASI page-table entries for a VA range
mm/asi: Function to copy page-table entries for percpu buffer
mm/asi: Add asi_remap() function
mm/asi: Handle ASI mapped range leaks and overlaps
mm/asi: Initialize the ASI page-table with core mappings
mm/asi: Option to map current task into ASI
rcu: Move tree.h static forward declarations to tree.c
rcu: Make percpu rcu_data non-static
mm/asi: Add option to map RCU data
mm/asi: Add option to map cpu_hw_events
mm/asi: Make functions to read cr3/cr4 ASI aware
KVM: x86/asi: Populate the KVM ASI page-table
KVM: x86/asi: Map KVM memslots and IO buses into KVM ASI

Liran Alon (3):
KVM: x86/asi: Introduce address_space_isolation module parameter
KVM: x86/asi: Introduce KVM address space isolation
KVM: x86/asi: Switch to KVM address space on entry to guest

arch/x86/entry/entry_64.S | 42 ++-
arch/x86/include/asm/asi.h | 237 ++++++++
arch/x86/include/asm/mmu_context.h | 20 +-
arch/x86/include/asm/tlbflush.h | 10 +
arch/x86/kernel/asm-offsets.c | 4 +
arch/x86/kvm/Makefile | 3 +-
arch/x86/kvm/mmu.c | 2 +-
arch/x86/kvm/vmx/isolation.c | 231 ++++++++
arch/x86/kvm/vmx/vmx.c | 14 +-
arch/x86/kvm/vmx/vmx.h | 24 +
arch/x86/kvm/x86.c | 68 +++-
arch/x86/kvm/x86.h | 1 +
arch/x86/mm/Makefile | 2 +
arch/x86/mm/asi.c | 459 +++++++++++++++
arch/x86/mm/asi_pagetable.c | 1077 ++++++++++++++++++++++++++++++++++++
arch/x86/mm/fault.c | 7 +
include/linux/kvm_host.h | 7 +
kernel/rcu/tree.c | 56 ++-
kernel/rcu/tree.h | 56 +--
kernel/sched/core.c | 4 +
security/Kconfig | 10 +
21 files changed, 2269 insertions(+), 65 deletions(-)
create mode 100644 arch/x86/include/asm/asi.h
create mode 100644 arch/x86/kvm/vmx/isolation.c
create mode 100644 arch/x86/mm/asi.c
create mode 100644 arch/x86/mm/asi_pagetable.c