Commit 146f76cc authored Jul 04, 2020 by Marc Zyngier

KVM: arm64: PMU: Fix per-CPU access in preemptible context



Commit 07da1ffa ("KVM: arm64: Remove host_cpu_context
member from vcpu structure") has, by removing the host CPU
context pointer, exposed that kvm_vcpu_pmu_restore_guest
is called in preemptible contexts:

[  266.932442] BUG: using smp_processor_id() in preemptible [00000000] code: qemu-system-aar/779
[  266.939721] caller is debug_smp_processor_id+0x20/0x30
[  266.944157] CPU: 2 PID: 779 Comm: qemu-system-aar Tainted: G            E     5.8.0-rc3-00015-g8d4aa58b2fe3 #1374
[  266.954268] Hardware name: amlogic w400/w400, BIOS 2020.04 05/22/2020
[  266.960640] Call trace:
[  266.963064]  dump_backtrace+0x0/0x1e0
[  266.966679]  show_stack+0x20/0x30
[  266.969959]  dump_stack+0xe4/0x154
[  266.973338]  check_preemption_disabled+0xf8/0x108
[  266.977978]  debug_smp_processor_id+0x20/0x30
[  266.982307]  kvm_vcpu_pmu_restore_guest+0x2c/0x68
[  266.986949]  access_pmcr+0xf8/0x128
[  266.990399]  perform_access+0x8c/0x250
[  266.994108]  kvm_handle_sys_reg+0x10c/0x2f8
[  266.998247]  handle_exit+0x78/0x200
[  267.001697]  kvm_arch_vcpu_ioctl_run+0x2ac/0xab8

Note that the bug was always there, it is only the switch to
using percpu accessors that made it obvious.
The fix is to wrap these accesses in a preempt-disabled section,
so that we sample a coherent context on trap from the guest.

Fixes: 435e53fb ("arm64: KVM: Enable VHE support for :G/:H perf event modifiers")
Cc:: Andrew Murray <amurray@thegoodpenguin.co.uk>
Signed-off-by: Marc Zyngier <maz@kernel.org>

parent a3f574cd

arch/arm64/kvm/pmu.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -159,7 +159,10 @@ static void kvm_vcpu_pmu_disable_el0(unsigned long events)
		}

		/*
		* On VHE ensure that only guest events have EL0 counting enabled
		* On VHE ensure that only guest events have EL0 counting enabled.
		* This is called from both vcpu_{load,put} and the sysreg handling.
		* Since the latter is preemptible, special care must be taken to
		* disable preemption.
		*/
		void kvm_vcpu_pmu_restore_guest(struct kvm_vcpu *vcpu)
		{
		@@ -169,12 +172,14 @@ void kvm_vcpu_pmu_restore_guest(struct kvm_vcpu *vcpu)
		if (!has_vhe())
		return;

		preempt_disable();
		host = this_cpu_ptr(&kvm_host_data);
		events_guest = host->pmu_events.events_guest;
		events_host = host->pmu_events.events_host;

		kvm_vcpu_pmu_enable_el0(events_guest);
		kvm_vcpu_pmu_disable_el0(events_host);
		preempt_enable();
		}

		/*

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