From: David Woodhouse <dwmw@xxxxxxxxxxxx>
The Xen interrupt injection for event channels relies on accessing the
guest's vcpu_info structure in __kvm_xen_has_interrupt(), through a
gfn_to_hva_cache.
This requires the srcu lock to be held, which is mostly the case except
for this code path:
[ 11.822877] WARNING: suspicious RCU usage
[ 11.822965] -----------------------------
[ 11.823013] include/linux/kvm_host.h:664 suspicious rcu_dereference_check() usage!
[ 11.823131]
[ 11.823131] other info that might help us debug this:
[ 11.823131]
[ 11.823196]
[ 11.823196] rcu_scheduler_active = 2, debug_locks = 1
[ 11.823253] 1 lock held by dom:0/90:
[ 11.823292] #0: ffff998956ec8118 (&vcpu->mutex){+.+.}, at: kvm_vcpu_ioctl+0x85/0x680
[ 11.823379]
[ 11.823379] stack backtrace:
[ 11.823428] CPU: 2 PID: 90 Comm: dom:0 Kdump: loaded Not tainted 5.4.34+ #5
[ 11.823496] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
[ 11.823612] Call Trace:
[ 11.823645] dump_stack+0x7a/0xa5
[ 11.823681] lockdep_rcu_suspicious+0xc5/0x100
[ 11.823726] __kvm_xen_has_interrupt+0x179/0x190
[ 11.823773] kvm_cpu_has_extint+0x6d/0x90
[ 11.823813] kvm_cpu_accept_dm_intr+0xd/0x40
[ 11.823853] kvm_vcpu_ready_for_interrupt_injection+0x20/0x30
< post_kvm_run_save() inlined here >
[ 11.823906] kvm_arch_vcpu_ioctl_run+0x135/0x6a0
[ 11.823947] kvm_vcpu_ioctl+0x263/0x680
Fixes: 40da8ccd724f ("KVM: x86/xen: Add event channel interrupt vector upcall")
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
There are potentially other ways of doing this, by shuffling the tail
of kvm_arch_vcpu_ioctl_run() around a little and holding the lock once
there instead of taking it within vcpu_run(). But the call to
post_kvm_run_save() occurs even on the error paths, and it gets complex
to untangle. This is the simple approach.
This doesn't show up when I enable event channel delivery in
xen_shinfo_test because we have pic_in_kernel() there. I'll fix the
tests not to hardcode that, as I expand the event channel support and
add more testing of it.
arch/x86/kvm/x86.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f0874c3d12ce..cd42d58008f7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8783,9 +8783,17 @@ static void post_kvm_run_save(struct kvm_vcpu *vcpu)
kvm_run->cr8 = kvm_get_cr8(vcpu);
kvm_run->apic_base = kvm_get_apic_base(vcpu);
+
+ /*
+ * The call to kvm_ready_for_interrupt_injection() may end up in
+ * kvm_xen_has_interrupt() which may require the srcu lock to be
+ * held to protect against changes in the vcpu_info address.
+ */
+ vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
kvm_run->ready_for_interrupt_injection =
pic_in_kernel(vcpu->kvm) ||
kvm_vcpu_ready_for_interrupt_injection(vcpu);
+ srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
if (is_smm(vcpu))
kvm_run->flags |= KVM_RUN_X86_SMM;
--
2.25.1
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