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Subject[PATCH RESEND] KVM: X86: Grab KVM's srcu lock when accessing hv assist page
From: Wanpeng Li <wanpengli@tencent.com>

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@tencent.com>
---
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
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