[PATCH v5 4/6] KVM: guest_memfd: Split bind() into prepare()+commit() phases

[Sean Christopherson <seanjc@google.com>]

Split binding a memslot to a guest_memfd instance into prepare() and
commit() phases so that KVM can separate preparing the memslot from binding
the memslot to the gmem instance, i.e. from committing the memslot.  This
will allow waiting to commit the memslot+gmem binding until the memslot is
fully prepared, which is necessary as the memslot becomes reachable when
the binding is created.

As a bonus, drop the unwind-on-failure from the commit phase (other than
nullifying the bindings), as the only reason bind() did the full unwind is
because it technically didn't own the memslot, i.e. "needed" to leave
memslot in the same state it started in.

No functional change intended (the unwinding down on bind() failure was
effectively dead code since KVM simply deletes the memslot on failure,
i.e. there was nothing that could actually observe the unwind).

Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/guest_memfd.c | 68 ++++++++++++++++++++++++------------------
 virt/kvm/guest_memfd.h | 19 ++++++++----
 virt/kvm/kvm_main.c    | 18 ++++++++++-
 3 files changed, 70 insertions(+), 35 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index c094611f7c7a..80932f4ec4a3 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -641,15 +641,14 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 	return __kvm_gmem_create(kvm, size, flags);
 }
 
-int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
-		  unsigned int fd, uoff_t offset)
+int kvm_gmem_prepare_memory_region(struct kvm *kvm, struct kvm_memory_slot *slot,
+				   unsigned int fd, uoff_t offset)
 {
 	uoff_t size = slot->npages << PAGE_SHIFT;
-	unsigned long start, end;
 	struct gmem_file *f;
 	struct inode *inode;
 	struct file *file;
-	int r = -EINVAL;
+
 
 	BUILD_BUG_ON(sizeof(gpa_t) != sizeof(offset));
 	BUILD_BUG_ON(sizeof(gfn_t) != sizeof(slot->gmem.pgoff));
@@ -673,44 +672,55 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 	if (!PAGE_ALIGNED(offset) || offset + size > i_size_read(inode))
 		goto err;
 
-	filemap_invalidate_lock(inode->i_mapping);
-
-	start = offset >> PAGE_SHIFT;
-	end = start + slot->npages;
-
-	if (!xa_empty(&f->bindings) &&
-	    xa_find(&f->bindings, &start, end - 1, XA_PRESENT)) {
-		r = -EEXIST;
-		filemap_invalidate_unlock(inode->i_mapping);
-		goto err;
-	}
-
 	/*
 	 * 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.
 	 */
 	WRITE_ONCE(slot->gmem.file, file);
-	slot->gmem.pgoff = start;
+	slot->gmem.pgoff = offset >> PAGE_SHIFT;
 	if (kvm_gmem_supports_mmap(inode))
 		slot->flags |= KVM_MEMSLOT_GMEM_ONLY;
 
-	r = xa_err(xa_store_range(&f->bindings, start, end - 1, slot, GFP_KERNEL));
-	if (r) {
-		xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL);
-		slot->gmem.file = NULL;
-		slot->gmem.pgoff = 0;
-		slot->flags &= ~KVM_MEMSLOT_GMEM_ONLY;
-	}
-	filemap_invalidate_unlock(inode->i_mapping);
-
 	/*
-	 * Drop the reference to the file, even on success.  The file pins KVM,
-	 * not the other way 'round.  Active bindings are invalidated if the
-	 * file is closed before memslots are destroyed.
+	 * Gift the caller a reference to the file.  The reference will be
+	 * dropped after bindings are established, or if installing the new
+	 * memslot ultimately fails.
 	 */
+	return 0;
+
 err:
 	fput(file);
+	return -EINVAL;
+}
+
+int kvm_gmem_commit_memory_region(struct kvm *kvm, struct kvm_memory_slot *slot)
+{
+	struct gmem_file *f = slot->gmem.file->private_data;
+	struct inode *inode = file_inode(slot->gmem.file);
+	unsigned long start, end;
+	int r;
+
+	if (WARN_ON_ONCE(slot->gmem.file->f_op != &kvm_gmem_fops))
+		return -EIO;
+
+	filemap_invalidate_lock(inode->i_mapping);
+
+	start = slot->gmem.pgoff;
+	end = start + slot->npages;
+
+	if (!xa_empty(&f->bindings) &&
+	    xa_find(&f->bindings, &start, end - 1, XA_PRESENT)) {
+		filemap_invalidate_unlock(inode->i_mapping);
+		return -EEXIST;
+	}
+
+	r = xa_err(xa_store_range(&f->bindings, start, end - 1, slot, GFP_KERNEL));
+	if (r)
+		xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL);
+
+	filemap_invalidate_unlock(inode->i_mapping);
+
 	return r;
 }
 
diff --git a/virt/kvm/guest_memfd.h b/virt/kvm/guest_memfd.h
index 0f9c6f840838..01bd359d27e3 100644
--- a/virt/kvm/guest_memfd.h
+++ b/virt/kvm/guest_memfd.h
@@ -8,8 +8,9 @@
 int kvm_gmem_init(struct module *module);
 void kvm_gmem_exit(void);
 int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args);
-int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
-		  unsigned int fd, uoff_t offset);
+int kvm_gmem_prepare_memory_region(struct kvm *kvm, struct kvm_memory_slot *slot,
+				   unsigned int fd, uoff_t offset);
+int kvm_gmem_commit_memory_region(struct kvm *kvm, struct kvm_memory_slot *slot);
 void kvm_gmem_unbind(struct kvm_memory_slot *slot);
 #else
 static inline int kvm_gmem_init(struct module *module)
@@ -17,9 +18,17 @@ static inline int kvm_gmem_init(struct module *module)
 	return 0;
 }
 static inline void kvm_gmem_exit(void) {};
-static inline int kvm_gmem_bind(struct kvm *kvm,
-					 struct kvm_memory_slot *slot,
-					 unsigned int fd, uoff_t offset)
+
+static inline int kvm_gmem_prepare_memory_region(struct kvm *kvm,
+						 struct kvm_memory_slot *slot,
+						 unsigned int fd, uoff_t offset)
+{
+	WARN_ON_ONCE(1);
+	return -EIO;
+}
+
+static inline int kvm_gmem_commit_memory_region(struct kvm *kvm,
+						struct kvm_memory_slot *slot)
 {
 	WARN_ON_ONCE(1);
 	return -EIO;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ccfd5f5102a5..45b509f4e54b 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2117,7 +2117,23 @@ static int kvm_set_memory_region(struct kvm *kvm,
 	new->flags = mem->flags;
 	new->userspace_addr = mem->userspace_addr;
 	if (change == KVM_MR_CREATE && (mem->flags & KVM_MEM_GUEST_MEMFD)) {
-		r = kvm_gmem_bind(kvm, new, mem->guest_memfd, mem->guest_memfd_offset);
+		r = kvm_gmem_prepare_memory_region(kvm, new, mem->guest_memfd,
+						   mem->guest_memfd_offset);
+		if (r)
+			goto out;
+
+		r = kvm_gmem_commit_memory_region(kvm, new);
+
+		/*
+		 * Drop the reference to the file, even on success.  The file
+		 * pins KVM, not the other way 'round.  Active bindings are
+		 * invalidated if the file is closed before memslots are
+		 * destroyed.
+		 */
+#ifdef CONFIG_KVM_GUEST_MEMFD
+		 fput(new->gmem.file);
+#endif
+
 		if (r)
 			goto out;
 	}
-- 
2.55.0.1082.g2b9226bbc0-goog