Re: [RFC v2 00/27] Kernel Address Space Isolation

From: Alexandre Chartre
Date: Fri Jul 12 2019 - 09:46:52 EST

On 7/12/19 2:50 PM, Peter Zijlstra wrote:
On Fri, Jul 12, 2019 at 01:56:44PM +0200, Alexandre Chartre wrote:

I think that's precisely what makes ASI and PTI different and independent.
PTI is just about switching between userland and kernel page-tables, while
ASI is about switching page-table inside the kernel. You can have ASI without
having PTI. You can also use ASI for kernel threads so for code that won't
be triggered from userland and so which won't involve PTI.

PTI is not mapping kernel space to avoid speculation crap (meltdown).
ASI is not mapping part of kernel space to avoid (different) speculation crap (MDS).

See how very similar they are?

Furthermore, to recover SMT for userspace (under MDS) we not only need
core-scheduling but core-scheduling per address space. And ASI was
specifically designed to help mitigate the trainwreck just described.

By explicitly exposing (hopefully harmless) part of the kernel to MDS,
we reduce the part that needs core-scheduling and thus reduce the rate
the SMT siblngs need to sync up/schedule.

But looking at it that way, it makes no sense to retain 3 address
spaces, namely:

user / kernel exposed / kernel private.

Specifically, it makes no sense to expose part of the kernel through MDS
but not through Meltdow. Therefore we can merge the user and kernel
exposed address spaces.

The goal of ASI is to provide a reduced address space which exclude sensitive
data. A user process (for example a database daemon, a web server, or a vmm
like qemu) will likely have sensitive data mapped in its user address space.
Such data shouldn't be mapped with ASI because it can potentially leak to the
sibling hyperthread. For example, if an hyperthread is running a VM then the
VM could potentially access user sensitive data if they are mapped on the
sibling hyperthread with ASI.

The current approach is assuming that anything in the user address space
can be sensitive, and so the user address space shouldn't be mapped in ASI.

It looks like what you are suggesting could be an optimization when creating
an ASI for a process which has no sensitive data (this could be an option to
specify when creating an ASI, for example).


And then we've fully replaced PTI.

So no, they're not orthogonal.