[RFC PATCH v3 0/7] Kernel Address Space Isolation

[Alexandre Chartre]

Hi,

This is version 3 of the "Kernel Address Space Isolation" (ASI) RFC
[1][2]. This version focus on providing the ASI infrastructure and
implementing PTI with ASI.

Compared to version 2, this RFC is much smaller with only 7 patches
(RFCv2 has 26 patches). This is because this RFC contains patches only
for the ASI infrastructure and for implementing PTI with ASI, but it
doesn't contain patches for using ASI with KVM and all related changes
regarding page-table creation and filling.

This patchset is based on the Linux 5.5 source code.

Background
==========
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 CPU hyper-threads under speculative execution attacks.

Over the past years, various speculative attacks (like L1TF or MDS) have
highlighted that data can leak between CPU threads through the CPU (micro)
architecture. In particular, a malicious virtual machine running on a CPU
thread can target data used by a sibling CPU thread from the same CPU core.
Thus, a malicious VM can potentially access data from another VM or from
the host system if they are running on sibling CPU threads.

Core Scheduling [3] can prevent a malicious VM from attacking another VM
by running the same VM on all CPU threads of a CPU core. However a
malicious VM can still target the host system when the sibling CPU thread
exits the VM and returns to the host.

Address Space Isolation can be applied to KVM to mitigate this VM-to-host
attack by removing secrets from the kernel address space used when running
KVM, thus preventing a malicious VM from collecting any sensitive data
from host.

Address Space Isolation can also be used to implement Page Table Isolation
(PTI [4]) which reduces kernel mappings present in user address spaces to
prevent the Meltdown attack.

Details
=======

ASI
---
An ASI is created by calling asi_create() with a specified ASI type. The
ASI type manages data common to all ASI of the same type. It is used, in
particular, to manage per-ASI type TLB/PCID information.

Then the ASI can be entered with asi_enter() and exited with asi_exit().
When an ASI is in used, any interrupt/exception/NMI will cause the ASI to
be interrupted (ASI_INTERRUPT) and the ASI will be resumed (ASI_RESUME)
when the interrupt/exception/NMI returns.

asi_enter()/asi_exit() and ASI_INTERRUPT/ASI_RESUME switch between the
ASI and the full kernel page-table by updating the CR3 register.

If a task using ASI is scheduled out then its ASI state is saved and it
will be restored when the task is scheduled back.

Page fault occurring while ASI is used will either cause the ASI to be
aborted (switch back to the full kernel pagetable) or to be preserved.
The behavior depends on the ASI type. For example, for PTI the ASI is
preserved and the kernel page fault handler handles the fault on behalf
of the ASI. But for KVM ASI, the ASI will be aborted and the fault will
be retried with the full kernel page-table.

PTI
---
PTI is now implemented with ASI (user ASI) if both CONFIG_ADDRESS_SPACE_ISOLATION
and CONFIG_PAGE_TABLE_ISOLATION are set. The behavior of PTI is unchanged
but it is now using the ASI infrastructure. 

For each user process, a user ASI is defined with the PTI pagetable. The
user ASI is used when running userland code, and it is exited when entering
a syscall. The user ASI is re-entered when the syscall returns to userland.

KVM
---
As already mentioned, KVM ASI is not present in this patchset. KVM ASI
will be implemented ontop of this infrastructure. Basically, the KVM ASI
patchset will:
  - define a KVM ASI type (DEFINE_ASI_TYPE)
  - create and fill a page-table to be used by the KVM ASI
  - create a KVM ASI (asi_create_kvm())
  - enter the KVM ASI (asi_enter()) on KVM_RUN ioctl
  - exit the KVM ASI (asi_exit())

Fault occuring when KVM ASI is in used will cause the ASI to be aborted,
and the code will continue running with the full kernel page-table,
until KVM ASI is explicitly reentered.

Status
======
The code looks stable and it supports running a full kernel build and
also ltp tests. Performance impact is expected to be limited as the new
code only adds a small number of assembly instructions on syscall and
interrupts. There's probably also room for reducing this number of
instructions.

Changes 
=======
Summary of changes compared to RFCv2:

- Add ASI Type

- Add generic TLB flushing mechanism for ASI. This mechanism is similar
  to the context tracking done when switching mm.

- When ASI is in used, it is interrupted on interrupt/exception/NMI and
  resumed when the interrupt/exception/NMI returns.

- If a task using ASI is scheduled in/out then save/restore the corresponding
  ASI and update the cpu ASI session.

- Implement PTI with ASI.

- Remove KVM ASI from the patchset. KVM ASI will be provided in a separated
  patchset ontop of the ASI infrastructure.

- Remove functions to manage, populate and clear page-tables. These functions
  were only used to build to the KVM ASI page-table. Also such functions should
  be generic page-table functions and not specific to ASI. Mike Rapoport is also
  looking at making these functions generic.


References
==========
[1] ASI RFCv1 - https://lkml.org/lkml/2019/5/13/515
[2] ASI RFCv2 - https://lore.kernel.org/lkml/1562855138-19507-1-git-send-email-alexandre.chartre@xxxxxxxxxx
[3] Core Scheduling - https://lwn.net/Articles/803652
[4] Page Table Isolation (PTI) - https://www.kernel.org/doc/html/latest/x86/pti.html

---


Alexandre Chartre (7):
  mm/x86: Introduce kernel Address Space Isolation (ASI)
  mm/asi: ASI entry/exit interface
  mm/asi: Improve TLB flushing when switching to an ASI pagetable
  mm/asi: Interrupt ASI on interrupt/exception/NMI
  mm/asi: Switch ASI on task switch
  mm/asi: ASI fault handler
  mm/asi: Implement PTI with ASI

 arch/x86/entry/calling.h           |   31 +++-
 arch/x86/entry/common.c            |   29 +++-
 arch/x86/entry/entry_64.S          |   28 +++
 arch/x86/include/asm/asi.h         |  289 ++++++++++++++++++++++++++
 arch/x86/include/asm/asi_session.h |   24 +++
 arch/x86/include/asm/mmu_context.h |   20 ++-
 arch/x86/include/asm/tlbflush.h    |   23 ++-
 arch/x86/kernel/asm-offsets.c      |    5 +
 arch/x86/mm/Makefile               |    1 +
 arch/x86/mm/asi.c                  |  402 ++++++++++++++++++++++++++++++++++++
 arch/x86/mm/fault.c                |   20 ++
 arch/x86/mm/pti.c                  |   28 ++-
 include/linux/mm_types.h           |    5 +
 include/linux/sched.h              |    9 +
 kernel/fork.c                      |   17 ++
 kernel/sched/core.c                |    8 +
 security/Kconfig                   |   10 +
 17 files changed, 931 insertions(+), 18 deletions(-)
 create mode 100644 arch/x86/include/asm/asi.h
 create mode 100644 arch/x86/include/asm/asi_session.h
 create mode 100644 arch/x86/mm/asi.c