From: Michal Luczaj <mhal@xxxxxxx>
commit afb2acb2e3a32e4d56f7fbd819769b98ed1b7520 upstream.
In kvm_vm_ioctl_create_vcpu(), add vcpu to vcpu_array iff it's safe to
access vcpu via kvm_get_vcpu() and kvm_for_each_vcpu(), i.e. when there's
no failure path requiring vcpu removal and destruction. Such order is
important because vcpu_array accessors may end up referencing vcpu at
vcpu_array[0] even before online_vcpus is set to 1.
When online_vcpus=0, any call to kvm_get_vcpu() goes through
array_index_nospec() and ends with an attempt to xa_load(vcpu_array, 0):
int num_vcpus = atomic_read(&kvm->online_vcpus);
i = array_index_nospec(i, num_vcpus);
return xa_load(&kvm->vcpu_array, i);
Similarly, when online_vcpus=0, a kvm_for_each_vcpu() does not iterate over
an "empty" range, but actually [0, ULONG_MAX]:
xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0, \
(atomic_read(&kvm->online_vcpus) - 1))
In both cases, such online_vcpus=0 edge case, even if leading to
unnecessary calls to XArray API, should not be an issue; requesting
unpopulated indexes/ranges is handled by xa_load() and xa_for_each_range().
However, this means that when the first vCPU is created and inserted in
vcpu_array *and* before online_vcpus is incremented, code calling
kvm_get_vcpu()/kvm_for_each_vcpu() already has access to that first vCPU.
This should not pose a problem assuming that once a vcpu is stored in
vcpu_array, it will remain there, but that's not the case:
kvm_vm_ioctl_create_vcpu() first inserts to vcpu_array, then requests a
file descriptor. If create_vcpu_fd() fails, newly inserted vcpu is removed
from the vcpu_array, then destroyed:
vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
r = xa_insert(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, GFP_KERNEL_ACCOUNT);
kvm_get_kvm(kvm);
r = create_vcpu_fd(vcpu);
if (r < 0) {
xa_erase(&kvm->vcpu_array, vcpu->vcpu_idx);
kvm_put_kvm_no_destroy(kvm);
goto unlock_vcpu_destroy;
}
atomic_inc(&kvm->online_vcpus);
This results in a possible race condition when a reference to a vcpu is
acquired (via kvm_get_vcpu() or kvm_for_each_vcpu()) moments before said
vcpu is destroyed.
Signed-off-by: Michal Luczaj <mhal@xxxxxxx>
Message-Id: <20230510140410.1093987-2-mhal@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: c5b077549136 ("KVM: Convert the kvm->vcpus array to a xarray", 2021-12-08)
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
virt/kvm/kvm_main.c | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3959,18 +3959,19 @@ static int kvm_vm_ioctl_create_vcpu(stru
}
vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
- r = xa_insert(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, GFP_KERNEL_ACCOUNT);
- BUG_ON(r == -EBUSY);
+ r = xa_reserve(&kvm->vcpu_array, vcpu->vcpu_idx, GFP_KERNEL_ACCOUNT);
if (r)
goto unlock_vcpu_destroy;
/* Now it's all set up, let userspace reach it */
kvm_get_kvm(kvm);
r = create_vcpu_fd(vcpu);
- if (r < 0) {
- xa_erase(&kvm->vcpu_array, vcpu->vcpu_idx);
- kvm_put_kvm_no_destroy(kvm);
- goto unlock_vcpu_destroy;
+ if (r < 0)
+ goto kvm_put_xa_release;
+
+ if (KVM_BUG_ON(!!xa_store(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, 0), kvm)) {
+ r = -EINVAL;
+ goto kvm_put_xa_release;
}
/*
@@ -3985,6 +3986,9 @@ static int kvm_vm_ioctl_create_vcpu(stru
kvm_create_vcpu_debugfs(vcpu);
return r;
+kvm_put_xa_release:
+ kvm_put_kvm_no_destroy(kvm);
+ xa_release(&kvm->vcpu_array, vcpu->vcpu_idx);
unlock_vcpu_destroy:
mutex_unlock(&kvm->lock);
kvm_dirty_ring_free(&vcpu->dirty_ring);
