diff options
Diffstat (limited to '')
| -rw-r--r-- | include/linux/kvm_host.h | 7 | ||||
| -rw-r--r-- | virt/kvm/guest_memfd.c | 34 |
2 files changed, 27 insertions, 14 deletions
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 401439bb21e3..9df216943eb4 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -596,7 +596,12 @@ struct kvm_memory_slot { #ifdef CONFIG_KVM_PRIVATE_MEM struct { - struct file __rcu *file; + /* + * Writes protected by kvm->slots_lock. Acquiring a + * reference via kvm_gmem_get_file() is protected by + * either kvm->slots_lock or kvm->srcu. + */ + struct file *file; pgoff_t pgoff; } gmem; #endif diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index 47a9f68f7b24..18c87feaf68f 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -259,15 +259,19 @@ static int kvm_gmem_release(struct inode *inode, struct file *file) * dereferencing the slot for existing bindings needs to be protected * against memslot updates, specifically so that unbind doesn't race * and free the memslot (kvm_gmem_get_file() will return NULL). + * + * Since .release is called only when the reference count is zero, + * after which file_ref_get() and get_file_active() fail, + * kvm_gmem_get_pfn() cannot be using the file concurrently. + * file_ref_put() provides a full barrier, and get_file_active() the + * matching acquire barrier. */ mutex_lock(&kvm->slots_lock); filemap_invalidate_lock(inode->i_mapping); xa_for_each(&gmem->bindings, index, slot) - rcu_assign_pointer(slot->gmem.file, NULL); - - synchronize_rcu(); + WRITE_ONCE(slot->gmem.file, NULL); /* * All in-flight operations are gone and new bindings can be created. @@ -296,8 +300,7 @@ static inline struct file *kvm_gmem_get_file(struct kvm_memory_slot *slot) /* * Do not return slot->gmem.file if it has already been closed; * there might be some time between the last fput() and when - * kvm_gmem_release() clears slot->gmem.file, and you do not - * want to spin in the meanwhile. + * kvm_gmem_release() clears slot->gmem.file. */ return get_file_active(&slot->gmem.file); } @@ -508,11 +511,11 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot, } /* - * No synchronize_rcu() needed, any in-flight readers are guaranteed to - * be see either a NULL file or this new file, no need for them to go - * away. + * memslots of flag KVM_MEM_GUEST_MEMFD are immutable to change, so + * kvm_gmem_bind() must occur on a new memslot. Because the memslot + * is not visible yet, kvm_gmem_get_pfn() is guaranteed to see the file. */ - rcu_assign_pointer(slot->gmem.file, file); + WRITE_ONCE(slot->gmem.file, file); slot->gmem.pgoff = start; xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL); @@ -548,8 +551,12 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot) filemap_invalidate_lock(file->f_mapping); xa_store_range(&gmem->bindings, start, end - 1, NULL, GFP_KERNEL); - rcu_assign_pointer(slot->gmem.file, NULL); - synchronize_rcu(); + + /* + * synchronize_srcu(&kvm->srcu) ensured that kvm_gmem_get_pfn() + * cannot see this memslot. + */ + WRITE_ONCE(slot->gmem.file, NULL); filemap_invalidate_unlock(file->f_mapping); fput(file); @@ -561,11 +568,12 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file, pgoff_t index, kvm_pfn_t *pfn, bool *is_prepared, int *max_order) { + struct file *gmem_file = READ_ONCE(slot->gmem.file); struct kvm_gmem *gmem = file->private_data; struct folio *folio; - if (file != slot->gmem.file) { - WARN_ON_ONCE(slot->gmem.file); + if (file != gmem_file) { + WARN_ON_ONCE(gmem_file); return ERR_PTR(-EFAULT); } |