[PATCH] x86/virt: Fix RCU lockdep splat in emergency virt callback path

[Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>]

x86_virt_invoke_kvm_emergency_callback() reaches rcu_dereference()
through machine_crash_shutdown() with IRQs disabled but with RCU not
necessarily watching, which triggers a suspicious RCU usage splat on
debug kernels (CONFIG_PROVE_RCU=y) during panic/kdump:

WARNING: suspicious RCU usage
arch/x86/virt/hw.c:52 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by tee/11119:
 #0: ffff8881fa32c440 (sb_writers#3){.+.+}-{0:0}, at: ksys_write

Call Trace:
 <TASK>
 dump_stack_lvl+0x84/0xd0
 lockdep_rcu_suspicious.cold+0x37/0x8f
 x86_virt_invoke_kvm_emergency_callback+0x5f/0x70
 x86_svm_emergency_disable_virtualization_cpu+0x2a/0x30
 x86_virt_emergency_disable_virtualization_cpu+0x6b/0x90
 native_machine_crash_shutdown+0x72/0x170
 __crash_kexec+0x137/0x280
 panic+0xce/0xd0
 sysrq_handle_crash+0x1f/0x20
 __handle_sysrq.cold+0x192/0x335
 write_sysrq_trigger+0x8c/0xc0
 proc_reg_write+0x1c3/0x3c0
 vfs_write+0x1d0/0xf80
 ksys_write+0x116/0x250
 do_syscall_64+0x11c/0x1480
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
 </TASK>

The RCU usage is correct: writers
(x86_virt_{register,unregister}_emergency_callback()) serialize via
rcu_assign_pointer() + synchronize_rcu(), while the reader on the
emergency path runs with IRQs disabled (the only caller is
x86_virt_emergency_disable_virtualization_cpu(), which has
lockdep_assert_irqs_disabled()), which is a valid classic-RCU read-side
critical section.

Use rcu_dereference_check() with irqs_disabled() to silence the splat
without weakening the protection.

Reproducible on a debug kernel (CONFIG_PROVE_LOCKING=y, CONFIG_PROVE_RCU=y)
with kvm_amd or kvm_intel loaded by triggering kdump:

  echo c > /proc/sysrq-trigger

Fixes: 428afac5a8ea ("KVM: x86: Move bulk of emergency virtualizaton logic to virt subsystem")
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
---
 arch/x86/virt/hw.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/virt/hw.c b/arch/x86/virt/hw.c
index f647557d38ac..57eebc99299d 100644
--- a/arch/x86/virt/hw.c
+++ b/arch/x86/virt/hw.c
@@ -49,7 +49,13 @@ static void x86_virt_invoke_kvm_emergency_callback(void)
 {
 	cpu_emergency_virt_cb *kvm_callback;
 
-	kvm_callback = rcu_dereference(kvm_emergency_callback);
+	/*
+	 * Callers invoke this with IRQs disabled (see
+	 * x86_virt_emergency_disable_virtualization_cpu()), which is a valid
+	 * RCU read-side critical section. Tell lockdep so it doesn't complain
+	 * during panic/reboot paths.
+	 */
+	kvm_callback = rcu_dereference_check(kvm_emergency_callback, irqs_disabled());
 	if (kvm_callback)
 		kvm_callback();
 }
-- 
2.54.0