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:
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
