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Subject[PATCH 5.3 084/105] KVM: x86: Grab KVMs srcu lock when setting nested state
From: Sean Christopherson <>

commit ad5996d9a0e8019c3ae5151e687939369acfe044 upstream.

Acquire kvm->srcu for the duration of ->set_nested_state() to fix a bug
where nVMX derefences ->memslots without holding ->srcu or ->slots_lock.

The other half of nested migration, ->get_nested_state(), does not need
to acquire ->srcu as it is a purely a dump of internal KVM (and CPU)
state to userspace.

Detected as an RCU lockdep splat that is 100% reproducible by running
KVM's state_test selftest with CONFIG_PROVE_LOCKING=y. Note that the
failing function, kvm_is_visible_gfn(), is only checking the validity of
a gfn, it's not actually accessing guest memory (which is more or less
unsupported during vmx_set_nested_state() due to incorrect MMU state),
i.e. vmx_set_nested_state() itself isn't fundamentally broken. In any
case, setting nested state isn't a fast path so there's no reason to go
out of our way to avoid taking ->srcu.

WARNING: suspicious RCU usage
5.4.0-rc7+ #94 Not tainted
include/linux/kvm_host.h:626 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/10939:
#0: ffff88826ffcb800 (&vcpu->mutex){+.+.}, at: kvm_vcpu_ioctl+0x85/0x630 [kvm]

stack backtrace:
CPU: 1 PID: 10939 Comm: evmcs_test Not tainted 5.4.0-rc7+ #94
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
Call Trace:
kvm_is_visible_gfn+0x179/0x180 [kvm]
mmu_check_root+0x11/0x30 [kvm]
fast_cr3_switch+0x40/0x120 [kvm]
kvm_mmu_new_cr3+0x34/0x60 [kvm]
nested_vmx_load_cr3+0xbd/0x1f0 [kvm_intel]
nested_vmx_enter_non_root_mode+0xab8/0x1d60 [kvm_intel]
vmx_set_nested_state+0x256/0x340 [kvm_intel]
kvm_arch_vcpu_ioctl+0x491/0x11a0 [kvm]
kvm_vcpu_ioctl+0xde/0x630 [kvm]
RIP: 0033:0x7f59a2b95f47

Fixes: 8fcc4b5923af5 ("kvm: nVMX: Introduce KVM_CAP_NESTED_STATE")
Signed-off-by: Sean Christopherson <>
Signed-off-by: Paolo Bonzini <>
Signed-off-by: Greg Kroah-Hartman <>

arch/x86/kvm/x86.c | 3 +++
1 file changed, 3 insertions(+)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4333,6 +4333,7 @@ long kvm_arch_vcpu_ioctl(struct file *fi
struct kvm_nested_state __user *user_kvm_nested_state = argp;
struct kvm_nested_state kvm_state;
+ int idx;

r = -EINVAL;
if (!kvm_x86_ops->set_nested_state)
@@ -4356,7 +4357,9 @@ long kvm_arch_vcpu_ioctl(struct file *fi
&& !(kvm_state.flags & KVM_STATE_NESTED_GUEST_MODE))

+ idx = srcu_read_lock(&vcpu->kvm->srcu);
r = kvm_x86_ops->set_nested_state(vcpu, user_kvm_nested_state, &kvm_state);
+ srcu_read_unlock(&vcpu->kvm->srcu, idx);

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