Re: [PATCH] KVM: arm64: Don't free hyp pages with pKVM on GICv2

[Marc Zyngier <maz@kernel.org>]

On Thu, 26 Jun 2025 11:10:14 +0100,
Quentin Perret <qperret@google.com> wrote:
> 
> Marc reported that enabling protected mode on a device with GICv2
> doesn't fail gracefully as one would expect, and leads to a host
> kernel crash.
> 
> As it turns out, the first half of pKVM init happens before the vgic
> probe, and so by the time we find out we have a GICv2 we're already
> committed to keeping the pKVM vectors installed at EL2 -- pKVM rejects
> stub HVCs for obvious security reasons. However, the error path on KVM
> init leads to teardown_hyp_mode() which unconditionally frees hypervisor
> allocations (including the EL2 stacks and per-cpu pages) under the
> assumption that a previous cpu_hyp_uninit() execution has reset the
> vectors back to the stubs, which is false with pKVM.
> 
> Interestingly, host stage-2 protection is not enabled yet at this point,
> so this use-after-free may go unnoticed for a while. The issue becomes
> more obvious after the finalize_pkvm() call.
> 
> Fix this by keeping track of the CPUs on which pKVM is initialized in
> the kvm_hyp_initialized per-cpu variable, and use it from
> teardown_hyp_mode() to skip freeing pages that are in fact used.
> 
> Fixes: a770ee80e662 ("KVM: arm64: pkvm: Disable GICv2 support")
> Reported-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Quentin Perret <qperret@google.com>

I just gave it a go on a Juno-r1, and it didn't die! Must be perfect!
:) I'll stash it on top of Mostafa's patch.

Thanks!

	M.

-- 
Without deviation from the norm, progress is not possible.