Re: [RFC 00/10] Process-local memory allocations for hiding KVM secrets

[Sean Christopherson]

On Wed, Jun 12, 2019 at 07:08:24PM +0200, Marius Hillenbrand wrote:
> The Linux kernel has a global address space that is the same for any
> kernel code. This address space becomes a liability in a world with
> processor information leak vulnerabilities, such as L1TF. With the right
> cache load gadget, an attacker-controlled hyperthread pair can leak
> arbitrary data via L1TF. Disabling hyperthreading is one recommended
> mitigation, but it comes with a large performance hit for a wide range
> of workloads.
> 
> An alternative mitigation is to not make certain data in the kernel
> globally visible, but only when the kernel executes in the context of
> the process where this data belongs to.
>
> This patch series proposes to introduce a region for what we call
> process-local memory into the kernel's virtual address space. Page
> tables and mappings in that region will be exclusive to one address
> space, instead of implicitly shared between all kernel address spaces.
> Any data placed in that region will be out of reach of cache load
> gadgets that execute in different address spaces. To implement
> process-local memory, we introduce a new interface kmalloc_proclocal() /
> kfree_proclocal() that allocates and maps pages exclusively into the
> current kernel address space. As a first use case, we move architectural
> state of guest CPUs in KVM out of reach of other kernel address spaces.

Can you briefly describe what types of attacks this is intended to
mitigate?  E.g. guest-guest, userspace-guest, etc...  I don't want to
make comments based on my potentially bad assumptions.
 
> The patch set is a prototype for x86-64 that we have developed on top of
> kernel 4.20.17 (with cherry-picked commit d253ca0c3865 "x86/mm/cpa: Add
> set_direct_map_*() functions"). I am aware that the integration with KVM
> will see some changes while rebasing to 5.x. Patches 7 and 8, in

Ha, "some" :-)

> particular, help make patch 9 more readable, but will be dropped in
> rebasing. We have tested the code on both Intel and AMDs, launching VMs
> in a loop. So far, we have not done in-depth performance evaluation.
> Impact on starting VMs was within measurement noise.