[PATCH] KVM: x86: Take srcu lock in post_kvm_run_save()

[David Woodhouse <dwmw2@infradead.org>]

[-- Attachment #1: Type: text/plain, Size: 3122 bytes --]

From: David Woodhouse <dwmw@amazon.co.uk>

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@amazon.co.uk>
---

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


[-- Attachment #2: smime.p7s --]
[-- Type: application/pkcs7-signature, Size: 5174 bytes --]