On Sun, Dec 22, 2024 at 07:38:44PM +0100, Paolo Bonzini wrote:
> On 11/4/24 09:43, Yan Zhao wrote:
> > Remove the RCU-protected attribute from slot->gmem.file. No need to use RCU
> > primitives rcu_assign_pointer()/synchronize_rcu() to update this pointer.
> >
> > - slot->gmem.file is updated in 3 places:
> > kvm_gmem_bind(), kvm_gmem_unbind(), kvm_gmem_release().
> > All of them are protected by kvm->slots_lock.
> >
> > - slot->gmem.file is read in 2 paths:
> > (1) kvm_gmem_populate
> > kvm_gmem_get_file
> > __kvm_gmem_get_pfn
> >
> > (2) kvm_gmem_get_pfn
> > kvm_gmem_get_file
> > __kvm_gmem_get_pfn
> >
> > Path (1) kvm_gmem_populate() requires holding kvm->slots_lock, so
> > slot->gmem.file is protected by the kvm->slots_lock in this path.
> >
> > Path (2) kvm_gmem_get_pfn() does not require holding kvm->slots_lock.
> > However, it's also not guarded by rcu_read_lock() and rcu_read_unlock().
> > So synchronize_rcu() in kvm_gmem_unbind()/kvm_gmem_release() actually
> > will not wait for the readers in kvm_gmem_get_pfn() due to lack of RCU
> > read-side critical section.
> >
> > The path (2) kvm_gmem_get_pfn() is safe without RCU protection because:
> > a) kvm_gmem_bind() is called on a new memslot, before the memslot is
> > visible to kvm_gmem_get_pfn().
> > b) kvm->srcu ensures that kvm_gmem_unbind() and freeing of a memslot
> > occur after the memslot is no longer visible to kvm_gmem_get_pfn().
> > c) get_file_active() ensures that kvm_gmem_get_pfn() will not access the
> > stale file if kvm_gmem_release() sets it to NULL. This is because if
> > kvm_gmem_release() occurs before kvm_gmem_get_pfn(), get_file_active()
> > will return NULL; if get_file_active() does not return NULL,
> > kvm_gmem_release() should not occur until after kvm_gmem_get_pfn()
> > releases the file reference.
>
> Thanks for the analysis, I added some notes:
Thank you for adding those notes!
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 4ec2564c0d0f..c788d0bd952a 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -602,6 +602,11 @@ struct kvm_memory_slot {
> #ifdef CONFIG_KVM_PRIVATE_MEM
> struct {
> + /*
> + * 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;
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 9d9bf3d033bd..411ff7224caa 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -261,6 +261,12 @@ 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);
> @@ -508,8 +514,8 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
> /*
> * memslots of flag KVM_MEM_GUEST_MEMFD are immutable to change, so
> - * kvm_gmem_bind() must occur on a new memslot.
> - * Readers are guaranteed to see this new file.
> + * 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.
> */
> WRITE_ONCE(slot->gmem.file, file);
> slot->gmem.pgoff = start;
> @@ -547,6 +554,11 @@ 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);
> +
> + /*
> + * 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);
> Queued to kvm-coco-queue.
>
> Paolo
>
