[PATCH RESEND] KVM: X86: Grab KVM's srcu lock when accessing hv assist page

[Wanpeng Li]

From: Wanpeng Li <wanpengli@xxxxxxxxxxx>

Acquire kvm->srcu for the duration of mapping eVMCS to fix a bug where accessing
hv assist page derefences ->memslots without holding ->srcu or ->slots_lock.

It can be reproduced by running KVM's evmcs_test selftest.

  =============================
  WARNING: suspicious RCU usage
  5.6.0-rc1+ #53 Tainted: G        W IOE
  -----------------------------
  ./include/linux/kvm_host.h:623 suspicious rcu_dereference_check() usage!

  other info that might help us debug this:

   rcu_scheduler_active = 2, debug_locks = 1
  1 lock held by evmcs_test/8507:
   #0: ffff9ddd156d00d0 (&vcpu->mutex){+.+.}, at:
kvm_vcpu_ioctl+0x85/0x680 [kvm]

  stack backtrace:
  CPU: 6 PID: 8507 Comm: evmcs_test Tainted: G        W IOE     5.6.0-rc1+ #53
  Hardware name: Dell Inc. OptiPlex 7040/0JCTF8, BIOS 1.4.9 09/12/2016
  Call Trace:
   dump_stack+0x68/0x9b
   kvm_read_guest_cached+0x11d/0x150 [kvm]
   kvm_hv_get_assist_page+0x33/0x40 [kvm]
   nested_enlightened_vmentry+0x2c/0x60 [kvm_intel]
   nested_vmx_handle_enlightened_vmptrld.part.52+0x32/0x1c0 [kvm_intel]
   nested_sync_vmcs12_to_shadow+0x439/0x680 [kvm_intel]
   vmx_vcpu_run+0x67a/0xe60 [kvm_intel]
   vcpu_enter_guest+0x35e/0x1bc0 [kvm]
   kvm_arch_vcpu_ioctl_run+0x40b/0x670 [kvm]
   kvm_vcpu_ioctl+0x370/0x680 [kvm]
   ksys_ioctl+0x235/0x850
   __x64_sys_ioctl+0x16/0x20
   do_syscall_64+0x77/0x780
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
---
 arch/x86/kvm/vmx/nested.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 657c2ed..a68a69d 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -1994,14 +1994,18 @@ static int
nested_vmx_handle_enlightened_vmptrld(struct kvm_vcpu *vcpu,
 void nested_sync_vmcs12_to_shadow(struct kvm_vcpu *vcpu)
 {
     struct vcpu_vmx *vmx = to_vmx(vcpu);
+    int idx;

     /*
      * hv_evmcs may end up being not mapped after migration (when
      * L2 was running), map it here to make sure vmcs12 changes are
      * properly reflected.
      */
-    if (vmx->nested.enlightened_vmcs_enabled && !vmx->nested.hv_evmcs)
+    if (vmx->nested.enlightened_vmcs_enabled && !vmx->nested.hv_evmcs) {
+        idx = srcu_read_lock(&vcpu->kvm->srcu);
         nested_vmx_handle_enlightened_vmptrld(vcpu, false);
+        srcu_read_unlock(&vcpu->kvm->srcu, idx);
+    }

     if (vmx->nested.hv_evmcs) {
         copy_vmcs12_to_enlightened(vmx);
--
2.7.4