On Tue, Feb 27, 2024 at 10:49:32AM +0100, David Hildenbrand wrote:

On 26.02.24 18:27, Elliot Berman wrote:

quoted

On Mon, Feb 26, 2024 at 12:53:48PM +0100, David Hildenbrand wrote:

quoted

On 26.02.24 12:06, Christoph Hellwig wrote:

quoted

The point is that we can't we just allow modules to unmap data from
the kernel mapping, no matter how noble your intentions are.

I absolutely agree.

Hi David and Chirstoph,

Are your preferences that we should make Gunyah builtin only or should add
fixing up S2 PTW errors (or something else)?

Having that built into the kernel certainly does sound better than exposing
that functionality to arbitrary OOT modules. But still, this feels like it
is using a "too-low-level" interface.

What are your thoughts about fixing up the stage-2 fault instead? I
think this gives mmu-based isolation a slight speed boost because we
avoid modifying kernel mapping. The hypervisor driver (KVM or Gunyah)
knows that the page isn't mapped. Whether we get S2 or S1 fault, the
kernel is likely going to crash, except in the rare case where we want
to fix the exception. In that case, we can modify the S2 fault handler
to call fixup_exception() when appropriate.

quoted

Also, do you extend that preference to modifying S2 mappings? This would
require any hypervisor driver that supports confidential compute
usecases to only ever be builtin.

Is your concern about unmapping data from kernel mapping, then module
being unloaded, and then having no way to recover the mapping? Would a
permanent module be better? The primary reason we were wanting to have
it as module was to avoid having driver in memory if you're not a Gunyah
guest.

What I didn't grasp from this patch description: is the area where a driver
would unmap/remap that memory somehow known ahead of time and limited?

How would the driver obtain that memory it would try to unmap/remap the
direct map of? Simply allocate some pages and then unmap the direct map?

That's correct.

For example, we do have mm/secretmem.c, where we unmap the directmap on
allocation and remap when freeing a page. A nice abstraction on alloc/free,
so one cannot really do a lot of harm.

Further, we enlightened the remainder of the system about secretmem, such
that we can detect that the directmap is no longer there. As one example,
see the secretmem_active() check in kernel/power/hibernate.c.

I'll take a look at this. guest_memfd might be able to use PM notifiers here
instead, but I'll dig in the archives to see why secretmem isn't using that.

A similar abstraction would make sense (I remember a discussion about having
secretmem functionality in guest_memfd, would that help?), but the question
is "which" memory you want to unmap the direct map of, and how the driver
became "owner" of that memory such that it would really be allowed to mess
with the directmap.

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