Re: [RFC KVM 00/27] KVM Address Space Isolation

[Liran Alon]

> On 13 May 2019, at 17:38, Alexandre Chartre <alexandre.chartre@xxxxxxxxxx> wrote:
> 
> Hi,
> 
> This series aims to introduce the concept of KVM address space isolation.
> This is done as part of the upstream community effort to have exploit
> mitigations for CPU info-leaks vulnerabilities such as L1TF. 
> 
> These patches are based on an original patches from Liran Alon, completed
> with additional patches to effectively create KVM address space different
> from the full kernel address space.

Great job for pushing this forward! Thank you!

> 
> The current code is just an early POC, and it is not fully stable at the
> moment (unfortunately you can expect crashes/hangs, see the "Issues"
> section below). However I would like to start a discussion get feedback
> and opinions about this approach.
> 
> Context
> =======
> 
> The most naive approach to handle L1TF SMT-variant exploit is to just disable
> hyper-threading. But that is not practical for public cloud providers. As a
> second next best alternative, there is an approach to combine coscheduling
> together with flushing L1D cache on every VMEntry. By coscheduling I refer
> to some mechanism which on every VMExit from guest, kicks all sibling
> hyperthreads from guest aswell.
> 
> However, this approach have some open issues:
> 
> 1. Kicking all sibling hyperthreads for every VMExit have significant
>   performance hit for some compute shapes (e.g. Emulated and PV).
> 
> 2. It assumes only CPU core resource which could be leaked by some
>   vulnerability is L1D cache. But future vulnerabilities may also be able
>   to leak other CPU core resources. Therefore, we would prefer to have a
>   mechanism which prevents these resources to be able to be loaded with
>   sensitive data to begin with.
> 
> To better address (2), upstream community has discussed some mechanisms
> related to reducing data that is mapped on kernel virtual address space.
> Specifically:
> 
> a. XPFO: Removes from physmap pages that currently should only be accessed
>   by userspace.
> 
> b. Process-local memory allocations: Allows having a memory area in kernel
>   virtual address space that maps different content per-process. Then,
>   allocations made on this memory area can be hidden from other tasks in
>   the system running in kernel space. Most obvious use it to allocate
>   there per-vCPU and per-VM KVM structures.
> 
> However, both (a)+(b) work in a black-list approach (where we decide which
> data is considered dangerous and remove it from kernel virtual address
> space) and don't address performance hit described at (1).

+Cc Stefan from AWS and Kaya from Google.
(I have sent them my original patch series for review and discuss with them about this subject)
Stefan: Do you know what is Julian's current email address to Cc him as-well?

> 
> 
> Proposal
> ========
> 
> To handle both these points, this series introduce the mechanism of KVM
> address space isolation. Note that this mechanism completes (a)+(b) and
> don't contradict. In case this mechanism is also applied, (a)+(b) should
> still be applied to the full virtual address space as a defence-in-depth).
> 
> The idea is that most of KVM #VMExit handlers code will run in a special
> KVM isolated address space which maps only KVM required code and per-VM
> information. Only once KVM needs to architectually access other (sensitive)
> data, it will switch from KVM isolated address space to full standard
> host address space. At this point, KVM will also need to kick all sibling
> hyperthreads to get out of guest (note that kicking all sibling hyperthreads
> is not implemented in this serie).
> 
> Basically, we will have the following flow:
> 
>  - qemu issues KVM_RUN ioctl
>  - KVM handles the ioctl and calls vcpu_run():
>    . KVM switches from the kernel address to the KVM address space
>    . KVM transfers control to VM (VMLAUNCH/VMRESUME)
>    . VM returns to KVM
>    . KVM handles VM-Exit:
>      . if handling need full kernel then switch to kernel address space

*AND* kick sibling hyperthreads before switching to that address space.
I think itâs important to emphasise that one of the main points of this KVM address space isolation mechanism is to minimise number of times we require to kick sibling hyperthreads outside of guest. By hopefully having the vast majority of VMExits handled on KVM isolated address space.

>      . else continues with KVM address space
>    . KVM loops in vcpu_run() or return
>  - KVM_RUN ioctl returns
> 
> So, the KVM_RUN core function will mainly be executed using the KVM address
> space. The handling of a VM-Exit can require access to the kernel space
> and, in that case, we will switch back to the kernel address space.
> 
> The high-level idea of how this is implemented is to create a separate
> struct_mm for KVM such that a vCPU thread will switch active_mm between
> it's original active_mm and kvm_mm when needed as described above. The
> idea is very similar to how kernel switches between task active_mm and
> efi_mm when calling EFI Runtime Services.
> 
> Note that because we use the kernel TLB Manager to switch between kvm_mm
> and host_mm, we will effectively use TLB with PCID if enabled to make
> these switches fast. As all of this is managed internally in TLB Manager's
> switch_mm().
> 
> 
> Patches
> =======
> 
> The proposed patches implement the necessary framework for creating kvm_mm
> and switching between host_mm and kvm_mm at appropriate times. They also
> provide functions for populating the KVM address space, and implement an
> actual KVM address space much smaller than the full kernel address space.
> 
> - 01-08: add framework for switching between the kernel address space and
>  the KVM address space at appropriate times. Note that these patches do
>  not create or switch the address space yet. Address space switching is
>  implemented in patch 25.
> 
> - 09-18: add a framework for populating and managing the KVM page table;
>  this also include mechanisms to ensure changes are effectively limited
>  to the KVM page table and no change is mistakenly propagated to the
>  kernel page table.
> 
> - 19-23: populate the KVM page table.
> 
> - 24: add page fault handler to handle and report missing mappings when
>  running with the KVM address space. This is based on an original idea
>  from Paul Turner.
> 
> - 25: implement the actual switch between the kernel address space and
>  the KVM address space.
> 
> - 26-27: populate the KVM page table with more data.
> 
> 
> If a fault occurs while running with the KVM address space, it will be
> reported on the console like this:
> 
> [ 4840.727476] KVM isolation: page fault #0 (0) at fast_page_fault+0x13e/0x3e0 [kvm] on ffffea00005331f0 (0xffffea00005331f0)
> 
> If the KVM page_fault_stack module parameter is set to non-zero (that's
> the default) then the stack of the fault will also be reported:
> 
> [ 5025.630374] KVM isolation: page fault #0 (0) at fast_page_fault+0x100/0x3e0 [kvm] on ffff88003c718000 (0xffff88003c718000)
> [ 5025.631918] Call Trace:
> [ 5025.632782]  tdp_page_fault+0xec/0x260 [kvm]
> [ 5025.633395]  kvm_mmu_page_fault+0x74/0x5f0 [kvm]
> [ 5025.644467]  handle_ept_violation+0xc3/0x1a0 [kvm_intel]
> [ 5025.645218]  vmx_handle_exit+0xb9/0x600 [kvm_intel]
> [ 5025.645917]  vcpu_enter_guest+0xb88/0x1580 [kvm]
> [ 5025.646577]  kvm_arch_vcpu_ioctl_run+0x403/0x610 [kvm]
> [ 5025.647313]  kvm_vcpu_ioctl+0x3d5/0x650 [kvm]
> [ 5025.648538]  do_vfs_ioctl+0xaa/0x602
> [ 5025.650502]  SyS_ioctl+0x79/0x84
> [ 5025.650966]  do_syscall_64+0x79/0x1ae
> [ 5025.651487]  entry_SYSCALL_64_after_hwframe+0x151/0x0
> [ 5025.652200] RIP: 0033:0x7f74a2f1d997
> [ 5025.652710] RSP: 002b:00007f749f3ec998 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> [ 5025.653769] RAX: ffffffffffffffda RBX: 0000562caa83e110 RCX: 00007f74a2f1d997
> [ 5025.654769] RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 000000000000000c
> [ 5025.655769] RBP: 0000562caa83e1b3 R08: 0000562ca9b6fa50 R09: 0000000000000006
> [ 5025.656766] R10: 0000000000000000 R11: 0000000000000246 R12: 0000562ca9b552c0
> [ 5025.657764] R13: 0000000000801000 R14: 00007f74a59d4000 R15: 0000562caa83e110
> 
> This allows to find out what is missing in the KVM address space.
> 
> 
> Issues
> ======
> 
> Limited tests have been done so far, and mostly with an empty single-vcpu
> VM (i.e. qemu-system-i386 -enable-kvm -smp 1). Single-vcpu VM is able to
> start and run a full OS but the system will eventually crash/hang at some
> point. Multiple vcpus will crash/hang much faster.
> 
> 
> Performance Impact
> ==================
> 
> As this is a RFC, the effective performance impact hasn't been measured
> yet.

> Current patches introduce two additional context switches (kernel to
> KVM, and KVM to kernel) on each KVM_RUN ioctl.

I have never considered this to be an issue.
By design of this patch series, I treated exits to userspace VMM as slow-path that should not be important to optimise.

> Also additional context
> switches are added if a VM-Exit has to be handled using the full kernel
> address space.

This is by design as well.
The KVM address space should contain enough data that is not-sensitive to be leaked by guest from one hand while still be able to handle the vast majority of exits without exiting the address space on the other hand. If we cannot achieve such a KVM isolated address space, the PoC of the series failed.

> 
> I expect that the KVM address space can eventually be expanded to include
> the ioctl syscall entries.

As mentioned above, I do not see a strong reason to do so. The ioctl syscalls are considered slow-path that shouldnât be important to optimise.

> By doing so, and also adding the KVM page table
> to the process userland page table (which should be safe to do because the
> KVM address space doesn't have any secret), we could potentially handle the
> KVM ioctl without having to switch to the kernel pagetable (thus effectively
> eliminating KPTI for KVM).

From above reasons, I donât think this is important.

> Then the only overhead would be if a VM-Exit has
> to be handled using the full kernel address space.

This was always by design the only overhead. And a major one. Because not only it switches address space but also it will require in the future to kick all sibling hyperthreads outside of guest. The purpose of the series is to created an address space such that most VMExits wonât require to do such kick to the sibling hyperthreads.

-Liran

> 
> 
> Thanks,
> 
> alex.
> 
> ---
> 
> Alexandre Chartre (18):
>  kvm/isolation: function to track buffers allocated for the KVM page
>    table
>  kvm/isolation: add KVM page table entry free functions
>  kvm/isolation: add KVM page table entry offset functions
>  kvm/isolation: add KVM page table entry allocation functions
>  kvm/isolation: add KVM page table entry set functions
>  kvm/isolation: functions to copy page table entries for a VA range
>  kvm/isolation: keep track of VA range mapped in KVM address space
>  kvm/isolation: functions to clear page table entries for a VA range
>  kvm/isolation: improve mapping copy when mapping is already present
>  kvm/isolation: function to copy page table entries for percpu buffer
>  kvm/isolation: initialize the KVM page table with core mappings
>  kvm/isolation: initialize the KVM page table with vmx specific data
>  kvm/isolation: initialize the KVM page table with vmx VM data
>  kvm/isolation: initialize the KVM page table with vmx cpu data
>  kvm/isolation: initialize the KVM page table with the vcpu tasks
>  kvm/isolation: KVM page fault handler
>  kvm/isolation: initialize the KVM page table with KVM memslots
>  kvm/isolation: initialize the KVM page table with KVM buses
> 
> Liran Alon (9):
>  kernel: Export memory-management symbols required for KVM address
>    space isolation
>  KVM: x86: Introduce address_space_isolation module parameter
>  KVM: x86: Introduce KVM separate virtual address space
>  KVM: x86: Switch to KVM address space on entry to guest
>  KVM: x86: Add handler to exit kvm isolation
>  KVM: x86: Exit KVM isolation on IRQ entry
>  KVM: x86: Switch to host address space when may access sensitive data
>  KVM: x86: Optimize branches which checks if address space isolation
>    enabled
>  kvm/isolation: implement actual KVM isolation enter/exit
> 
> arch/x86/include/asm/apic.h    |    4 +-
> arch/x86/include/asm/hardirq.h |   10 +
> arch/x86/include/asm/irq.h     |    1 +
> arch/x86/kernel/cpu/common.c   |    2 +
> arch/x86/kernel/dumpstack.c    |    1 +
> arch/x86/kernel/irq.c          |   11 +
> arch/x86/kernel/ldt.c          |    1 +
> arch/x86/kernel/smp.c          |    2 +-
> arch/x86/kvm/Makefile          |    2 +-
> arch/x86/kvm/isolation.c       | 1773 ++++++++++++++++++++++++++++++++++++++++
> arch/x86/kvm/isolation.h       |   40 +
> arch/x86/kvm/mmu.c             |    3 +-
> arch/x86/kvm/vmx/vmx.c         |  123 +++-
> arch/x86/kvm/x86.c             |   44 +-
> arch/x86/mm/fault.c            |   12 +
> arch/x86/mm/tlb.c              |    4 +-
> arch/x86/platform/uv/tlb_uv.c  |    2 +-
> include/linux/kvm_host.h       |    2 +
> include/linux/percpu.h         |    2 +
> include/linux/sched.h          |    6 +
> mm/memory.c                    |    5 +
> mm/percpu.c                    |    6 +-
> virt/kvm/arm/arm.c             |    4 +
> virt/kvm/kvm_main.c            |    4 +-
> 24 files changed, 2051 insertions(+), 13 deletions(-)
> create mode 100644 arch/x86/kvm/isolation.c
> create mode 100644 arch/x86/kvm/isolation.h
>