[PATCH 2/4] KVM: arm64: Handle PSCI resets before userspace touches vCPU state

From: Oliver Upton <oupton@google.com>
To: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu
Cc: Marc Zyngier <maz@kernel.org>, Peter Shier <pshier@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	Jing Zhang <jingzhangos@google.com>,
	Raghavendra Rao Anata <rananta@google.com>,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Oliver Upton <oupton@google.com>
Subject: [PATCH 2/4] KVM: arm64: Handle PSCI resets before userspace touches vCPU state
Date: Wed, 18 Aug 2021 08:50:45 +0000	[thread overview]
Message-ID: <20210818085047.1005285-3-oupton@google.com> (raw)
In-Reply-To: <20210818085047.1005285-1-oupton@google.com>

The CPU_ON PSCI call takes a payload that KVM uses to configure a
destination vCPU to run. This payload is non-architectural state and not
exposed through any existing UAPI. Effectively, we have a race between
CPU_ON and userspace saving/restoring a guest: if the target vCPU isn't
ran again before the VMM saves its state, the requested PC and context
ID are lost. When restored, the target vCPU will be runnable and start
executing at its old PC.

We can avoid this race by making sure the reset payload is serviced
before userspace can access a vCPU's state. This is, of course, a hairy
ugly hack. A benefit of such a hack, though, is that we've managed to
massage the reset state into the architected state, thereby making it
migratable without forcing userspace to play our game with a UAPI
addition.

Fixes: 358b28f09f0a ("arm/arm64: KVM: Allow a VCPU to fully reset itself")
Signed-off-by: Oliver Upton <oupton@google.com>
---
I really hate this, but my imagination is failing me on any other way to
cure the race without cluing in userspace. Any ideas?

 arch/arm64/kvm/arm.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 0de4b41c3706..6b124c29c663 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1216,6 +1216,15 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		if (copy_from_user(&reg, argp, sizeof(reg)))
 			break;
 
+		/*
+		 * ugly hack. We could owe a reset due to PSCI and not yet
+		 * serviced it. Prevent userspace from reading/writing state
+		 * that will be clobbered by the eventual handling of the reset
+		 * bit.
+		 */
+		if (kvm_check_request(KVM_REQ_VCPU_RESET, vcpu))
+			kvm_reset_vcpu(vcpu);
+
 		if (ioctl == KVM_SET_ONE_REG)
 			r = kvm_arm_set_reg(vcpu, &reg);
 		else
-- 
2.33.0.rc1.237.g0d66db33f3-goog

