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Subject[PATCH v2] KVM: X86: Grab KVM's srcu lock when accessing hv assist page
From: wanpeng li <wanpengli@tencent.com>

For the duration of mapping eVMCS, it derefences ->memslots without holding
->srcu or ->slots_lock when accessing hv assist page. This patch fixes it by
moving nested_sync_vmcs12_to_shadow to prepare_guest_switch, where the SRCU
is already taken.

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/vmx.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 9a66648..6bd6ca4 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1214,6 +1214,9 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)

vmx_set_host_fs_gs(host_state, fs_sel, gs_sel, fs_base, gs_base);
vmx->guest_state_loaded = true;
+
+ if (vmx->nested.need_vmcs12_to_shadow_sync)
+ nested_sync_vmcs12_to_shadow(vcpu);
}

static void vmx_prepare_switch_to_host(struct vcpu_vmx *vmx)
@@ -6480,9 +6483,6 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
vmcs_write32(PLE_WINDOW, vmx->ple_window);
}

- if (vmx->nested.need_vmcs12_to_shadow_sync)
- nested_sync_vmcs12_to_shadow(vcpu);
-
if (kvm_register_is_dirty(vcpu, VCPU_REGS_RSP))
vmcs_writel(GUEST_RSP, vcpu->arch.regs[VCPU_REGS_RSP]);
if (kvm_register_is_dirty(vcpu, VCPU_REGS_RIP))
--
2.7.4
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