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Fix a long-standing flaw in x86's TDP MMU where unloading roots on a vCPU can

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result in the root being freed even though the root is completely valid and
 can be reused as-is (with a TLB flush).
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Merge tag 'kvm-x86-mmu-6.4-2' of https://github.com/kvm-x86/linux into HEAD

Fix a long-standing flaw in x86's TDP MMU where unloading roots on a vCPU can
result in the root being freed even though the root is completely valid and
can be reused as-is (with a TLB flush).
This commit is contained in:
Paolo Bonzini 2023-05-05 06:12:36 -04:00
commit 29b38e7650
1 changed files with 56 additions and 65 deletions
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121
arch/x86/kvm/mmu/tdp_mmu.c
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@ -40,7 +40,17 @@ static __always_inline bool kvm_lockdep_assert_mmu_lock_held(struct kvm *kvm,
void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
{
/* Also waits for any queued work items. */
/*
* Invalidate all roots, which besides the obvious, schedules all roots
* for zapping and thus puts the TDP MMU's reference to each root, i.e.
* ultimately frees all roots.
*/
kvm_tdp_mmu_invalidate_all_roots(kvm);
/*
* Destroying a workqueue also first flushes the workqueue, i.e. no
* need to invoke kvm_tdp_mmu_zap_invalidated_roots().
*/
destroy_workqueue(kvm->arch.tdp_mmu_zap_wq);
WARN_ON(atomic64_read(&kvm->arch.tdp_mmu_pages));
@ -116,16 +126,6 @@ static void tdp_mmu_schedule_zap_root(struct kvm *kvm, struct kvm_mmu_page *root
queue_work(kvm->arch.tdp_mmu_zap_wq, &root->tdp_mmu_async_work);
}
static inline bool kvm_tdp_root_mark_invalid(struct kvm_mmu_page *page)
{
union kvm_mmu_page_role role = page->role;
role.invalid = true;
/* No need to use cmpxchg, only the invalid bit can change. */
role.word = xchg(&page->role.word, role.word);
return role.invalid;
}
void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
bool shared)
{
@ -134,45 +134,12 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
return;
WARN_ON(!is_tdp_mmu_page(root));
/*
* The root now has refcount=0. It is valid, but readers already
* cannot acquire a reference to it because kvm_tdp_mmu_get_root()
* rejects it. This remains true for the rest of the execution
* of this function, because readers visit valid roots only
* (except for tdp_mmu_zap_root_work(), which however
* does not acquire any reference itself).
*
* Even though there are flows that need to visit all roots for
* correctness, they all take mmu_lock for write, so they cannot yet
* run concurrently. The same is true after kvm_tdp_root_mark_invalid,
* since the root still has refcount=0.
*
* However, tdp_mmu_zap_root can yield, and writers do not expect to
* see refcount=0 (see for example kvm_tdp_mmu_invalidate_all_roots()).
* So the root temporarily gets an extra reference, going to refcount=1
* while staying invalid. Readers still cannot acquire any reference;
* but writers are now allowed to run if tdp_mmu_zap_root yields and
* they might take an extra reference if they themselves yield.
* Therefore, when the reference is given back by the worker,
* there is no guarantee that the refcount is still 1. If not, whoever
* puts the last reference will free the page, but they will not have to
* zap the root because a root cannot go from invalid to valid.
* The TDP MMU itself holds a reference to each root until the root is
* explicitly invalidated, i.e. the final reference should be never be
* put for a valid root.
*/
if (!kvm_tdp_root_mark_invalid(root)) {
refcount_set(&root->tdp_mmu_root_count, 1);
/*
* Zapping the root in a worker is not just "nice to have";
* it is required because kvm_tdp_mmu_invalidate_all_roots()
* skips already-invalid roots. If kvm_tdp_mmu_put_root() did
* not add the root to the workqueue, kvm_tdp_mmu_zap_all_fast()
* might return with some roots not zapped yet.
*/
tdp_mmu_schedule_zap_root(kvm, root);
return;
}
KVM_BUG_ON(!is_tdp_mmu_page(root) || !root->role.invalid, kvm);
spin_lock(&kvm->arch.tdp_mmu_pages_lock);
list_del_rcu(&root->link);
@ -320,7 +287,14 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
root = tdp_mmu_alloc_sp(vcpu);
tdp_mmu_init_sp(root, NULL, 0, role);
refcount_set(&root->tdp_mmu_root_count, 1);
/*
* TDP MMU roots are kept until they are explicitly invalidated, either
* by a memslot update or by the destruction of the VM. Initialize the
* refcount to two; one reference for the vCPU, and one reference for
* the TDP MMU itself, which is held until the root is invalidated and
* is ultimately put by tdp_mmu_zap_root_work().
*/
refcount_set(&root->tdp_mmu_root_count, 2);
spin_lock(&kvm->arch.tdp_mmu_pages_lock);
list_add_rcu(&root->link, &kvm->arch.tdp_mmu_roots);
@ -946,32 +920,49 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
/*
* Mark each TDP MMU root as invalid to prevent vCPUs from reusing a root that
* is about to be zapped, e.g. in response to a memslots update. The actual
* zapping is performed asynchronously, so a reference is taken on all roots.
* Using a separate workqueue makes it easy to ensure that the destruction is
* performed before the "fast zap" completes, without keeping a separate list
* of invalidated roots; the list is effectively the list of work items in
* the workqueue.
* zapping is performed asynchronously. Using a separate workqueue makes it
* easy to ensure that the destruction is performed before the "fast zap"
* completes, without keeping a separate list of invalidated roots; the list is
* effectively the list of work items in the workqueue.
*
* Get a reference even if the root is already invalid, the asynchronous worker
* assumes it was gifted a reference to the root it processes. Because mmu_lock
* is held for write, it should be impossible to observe a root with zero refcount,
* i.e. the list of roots cannot be stale.
*
* This has essentially the same effect for the TDP MMU
* as updating mmu_valid_gen does for the shadow MMU.
* Note, the asynchronous worker is gifted the TDP MMU's reference.
* See kvm_tdp_mmu_get_vcpu_root_hpa().
*/
void kvm_tdp_mmu_invalidate_all_roots(struct kvm *kvm)
{
struct kvm_mmu_page *root;
lockdep_assert_held_write(&kvm->mmu_lock);
list_for_each_entry(root, &kvm->arch.tdp_mmu_roots, link) {
if (!root->role.invalid &&
!WARN_ON_ONCE(!kvm_tdp_mmu_get_root(root))) {
/*
* mmu_lock must be held for write to ensure that a root doesn't become
* invalid while there are active readers (invalidating a root while
* there are active readers may or may not be problematic in practice,
* but it's uncharted territory and not supported).
*
* Waive the assertion if there are no users of @kvm, i.e. the VM is
* being destroyed after all references have been put, or if no vCPUs
* have been created (which means there are no roots), i.e. the VM is
* being destroyed in an error path of KVM_CREATE_VM.
*/
if (IS_ENABLED(CONFIG_PROVE_LOCKING) &&
refcount_read(&kvm->users_count) && kvm->created_vcpus)
lockdep_assert_held_write(&kvm->mmu_lock);
/*
* As above, mmu_lock isn't held when destroying the VM! There can't
* be other references to @kvm, i.e. nothing else can invalidate roots
* or be consuming roots, but walking the list of roots does need to be
* guarded against roots being deleted by the asynchronous zap worker.
*/
rcu_read_lock();
list_for_each_entry_rcu(root, &kvm->arch.tdp_mmu_roots, link) {
if (!root->role.invalid) {
root->role.invalid = true;
tdp_mmu_schedule_zap_root(kvm, root);
}
}
rcu_read_unlock();
}
/*