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KVM vcpu_array fixes and cleanups for 6.14: 

- Explicitly verify the target vCPU is online in kvm_get_vcpu() to fix a bug
    where KVM would return a pointer to a vCPU prior to it being fully online,
    and give kvm_for_each_vcpu() similar treatment to fix a similar flaw.
 
  - Wait for a vCPU to come online prior to executing a vCPU ioctl to fix a
    bug where userspace could coerce KVM into handling the ioctl on a vCPU that
    isn't yet onlined.
 
  - Gracefully handle xa_insert() failures even though such failuires should be
    impossible in practice.
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Merge tag 'kvm-x86-vcpu_array-6.14' of https://github.com/kvm-x86/linux into HEAD

KVM vcpu_array fixes and cleanups for 6.14:

 - Explicitly verify the target vCPU is online in kvm_get_vcpu() to fix a bug
   where KVM would return a pointer to a vCPU prior to it being fully online,
   and give kvm_for_each_vcpu() similar treatment to fix a similar flaw.

 - Wait for a vCPU to come online prior to executing a vCPU ioctl to fix a
   bug where userspace could coerce KVM into handling the ioctl on a vCPU that
   isn't yet onlined.

 - Gracefully handle xa_insert() failures even though such failuires should be
   impossible in practice.
pull/1134/head
Paolo Bonzini 2025-01-20 06:36:40 -05:00
parent 4e4f38f84e 01528db67f
commit 7a9164dc69
2 changed files with 65 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
include/linux/kvm_host.h
View File

@ -963,6 +963,15 @@ static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)
static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
{
int num_vcpus = atomic_read(&kvm->online_vcpus);
/*
* Explicitly verify the target vCPU is online, as the anti-speculation
* logic only limits the CPU's ability to speculate, e.g. given a "bad"
* index, clamping the index to 0 would return vCPU0, not NULL.
*/
if (i >= num_vcpus)
return NULL;
i = array_index_nospec(i, num_vcpus);
/* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu. */
@ -970,9 +979,10 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
return xa_load(&kvm->vcpu_array, i);
}
#define kvm_for_each_vcpu(idx, vcpup, kvm) \
xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0, \
(atomic_read(&kvm->online_vcpus) - 1))
#define kvm_for_each_vcpu(idx, vcpup, kvm) \
if (atomic_read(&kvm->online_vcpus)) \
xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0, \
(atomic_read(&kvm->online_vcpus) - 1))
static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
{

68
virt/kvm/kvm_main.c
View File

@ -4111,32 +4111,30 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
mutex_lock(&kvm->lock);
#ifdef CONFIG_LOCKDEP
/* Ensure that lockdep knows vcpu->mutex is taken *inside* kvm->lock */
mutex_lock(&vcpu->mutex);
mutex_unlock(&vcpu->mutex);
#endif
if (kvm_get_vcpu_by_id(kvm, id)) {
r = -EEXIST;
goto unlock_vcpu_destroy;
}
vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
r = xa_reserve(&kvm->vcpu_array, vcpu->vcpu_idx, GFP_KERNEL_ACCOUNT);
r = xa_insert(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, GFP_KERNEL_ACCOUNT);
WARN_ON_ONCE(r == -EBUSY);
if (r)
goto unlock_vcpu_destroy;
/* Now it's all set up, let userspace reach it */
/*
* Now it's all set up, let userspace reach it. Grab the vCPU's mutex
* so that userspace can't invoke vCPU ioctl()s until the vCPU is fully
* visible (per online_vcpus), e.g. so that KVM doesn't get tricked
* into a NULL-pointer dereference because KVM thinks the _current_
* vCPU doesn't exist. As a bonus, taking vcpu->mutex ensures lockdep
* knows it's taken *inside* kvm->lock.
*/
mutex_lock(&vcpu->mutex);
kvm_get_kvm(kvm);
r = create_vcpu_fd(vcpu);
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;
}
goto kvm_put_xa_erase;
/*
* Pairs with smp_rmb() in kvm_get_vcpu. Store the vcpu
@ -4144,15 +4142,17 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
*/
smp_wmb();
atomic_inc(&kvm->online_vcpus);
mutex_unlock(&vcpu->mutex);
mutex_unlock(&kvm->lock);
kvm_arch_vcpu_postcreate(vcpu);
kvm_create_vcpu_debugfs(vcpu);
return r;
kvm_put_xa_release:
kvm_put_xa_erase:
mutex_unlock(&vcpu->mutex);
kvm_put_kvm_no_destroy(kvm);
xa_release(&kvm->vcpu_array, vcpu->vcpu_idx);
xa_erase(&kvm->vcpu_array, vcpu->vcpu_idx);
unlock_vcpu_destroy:
mutex_unlock(&kvm->lock);
kvm_dirty_ring_free(&vcpu->dirty_ring);
@ -4277,6 +4277,33 @@ static int kvm_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
}
#endif
static int kvm_wait_for_vcpu_online(struct kvm_vcpu *vcpu)
{
struct kvm *kvm = vcpu->kvm;
/*
* In practice, this happy path will always be taken, as a well-behaved
* VMM will never invoke a vCPU ioctl() before KVM_CREATE_VCPU returns.
*/
if (likely(vcpu->vcpu_idx < atomic_read(&kvm->online_vcpus)))
return 0;
/*
* Acquire and release the vCPU's mutex to wait for vCPU creation to
* complete (kvm_vm_ioctl_create_vcpu() holds the mutex until the vCPU
* is fully online).
*/
if (mutex_lock_killable(&vcpu->mutex))
return -EINTR;
mutex_unlock(&vcpu->mutex);
if (WARN_ON_ONCE(!kvm_get_vcpu(kvm, vcpu->vcpu_idx)))
return -EIO;
return 0;
}
static long kvm_vcpu_ioctl(struct file *filp,
unsigned int ioctl, unsigned long arg)
{
@ -4292,6 +4319,15 @@ static long kvm_vcpu_ioctl(struct file *filp,
if (unlikely(_IOC_TYPE(ioctl) != KVMIO))
return -EINVAL;
/*
* Wait for the vCPU to be online before handling the ioctl(), as KVM
* assumes the vCPU is reachable via vcpu_array, i.e. may dereference
* a NULL pointer if userspace invokes an ioctl() before KVM is ready.
*/
r = kvm_wait_for_vcpu_online(vcpu);
if (r)
return r;
/*
* Some architectures have vcpu ioctls that are asynchronous to vcpu
* execution; mutex_lock() would break them.