fs: use guard for namespace_sem in statmount()

Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -5040,17 +5040,70 @@ static int copy_statmount_to_user(struct kstatmount *s)
 	return 0;
 }
 
-static int do_statmount(struct kstatmount *s)
+static struct mount *listmnt_next(struct mount *curr, bool reverse)
 {
-	struct mount *m = real_mount(s->mnt);
-	struct mnt_namespace *ns = m->mnt_ns;
+	struct rb_node *node;
+
+	if (reverse)
+		node = rb_prev(&curr->mnt_node);
+	else
+		node = rb_next(&curr->mnt_node);
+
+	return node_to_mount(node);
+}
+
+static int grab_requested_root(struct mnt_namespace *ns, struct path *root)
+{
+	struct mount *first;
+
+	rwsem_assert_held(&namespace_sem);
+
+	/* We're looking at our own ns, just use get_fs_root. */
+	if (ns == current->nsproxy->mnt_ns) {
+		get_fs_root(current->fs, root);
+		return 0;
+	}
+
+	/*
+	 * We have to find the first mount in our ns and use that, however it
+	 * may not exist, so handle that properly.
+	 */
+	if (RB_EMPTY_ROOT(&ns->mounts))
+		return -ENOENT;
+
+	first = listmnt_next(ns->root, false);
+	if (!first)
+		return -ENOENT;
+	root->mnt = mntget(&first->mnt);
+	root->dentry = dget(root->mnt->mnt_root);
+	return 0;
+}
+
+static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
+			struct mnt_namespace *ns)
+{
+	struct path root __free(path_put) = {};
+	struct mount *m;
 	int err;
 
+	/* Has the namespace already been emptied? */
+	if (mnt_ns_id && RB_EMPTY_ROOT(&ns->mounts))
+		return -ENOENT;
+
+	s->mnt = lookup_mnt_in_ns(mnt_id, ns);
+	if (!s->mnt)
+		return -ENOENT;
+
+	err = grab_requested_root(ns, &root);
+	if (err)
+		return err;
+
 	/*
 	 * Don't trigger audit denials. We just want to determine what
 	 * mounts to show users.
 	 */
-	if (!is_path_reachable(m, m->mnt.mnt_root, &s->root) &&
+	m = real_mount(s->mnt);
+	if (!is_path_reachable(m, m->mnt.mnt_root, &root) &&
 	    !ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
@@ -5058,6 +5111,7 @@ static int do_statmount(struct kstatmount *s)
 	if (err)
 		return err;
 
+	s->root = root;
 	if (s->mask & STATMOUNT_SB_BASIC)
 		statmount_sb_basic(s);
 
@@ -5096,6 +5150,9 @@ static inline bool retry_statmount(const long ret, size_t *seq_size)
 	return true;
 }
 
+#define STATMOUNT_STRING_REQ (STATMOUNT_MNT_ROOT | STATMOUNT_MNT_POINT | \
+			      STATMOUNT_FS_TYPE)
+
 static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 			      struct statmount __user *buf, size_t bufsize,
 			      size_t seq_size)
@@ -5107,10 +5164,18 @@ static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 	ks->mask = kreq->param;
 	ks->buf = buf;
 	ks->bufsize = bufsize;
-	ks->seq.size = seq_size;
-	ks->seq.buf = kvmalloc(seq_size, GFP_KERNEL_ACCOUNT);
-	if (!ks->seq.buf)
-		return -ENOMEM;
+
+	if (ks->mask & STATMOUNT_STRING_REQ) {
+		if (bufsize == sizeof(ks->sm))
+			return -EOVERFLOW;
+
+		ks->seq.buf = kvmalloc(seq_size, GFP_KERNEL_ACCOUNT);
+		if (!ks->seq.buf)
+			return -ENOMEM;
+
+		ks->seq.size = seq_size;
+	}
+
 	return 0;
 }
 
@@ -5138,45 +5203,6 @@ static int copy_mnt_id_req(const struct mnt_id_req __user *req,
 	return 0;
 }
 
-static struct mount *listmnt_next(struct mount *curr, bool reverse)
-{
-	struct rb_node *node;
-
-	if (reverse)
-		node = rb_prev(&curr->mnt_node);
-	else
-		node = rb_next(&curr->mnt_node);
-
-	return node_to_mount(node);
-}
-
-static int grab_requested_root(struct mnt_namespace *ns, struct path *root)
-{
-	struct mount *first;
-
-	rwsem_assert_held(&namespace_sem);
-
-	/* We're looking at our own ns, just use get_fs_root. */
-	if (ns == current->nsproxy->mnt_ns) {
-		get_fs_root(current->fs, root);
-		return 0;
-	}
-
-	/*
-	 * We have to find the first mount in our ns and use that, however it
-	 * may not exist, so handle that properly.
-	 */
-	if (RB_EMPTY_ROOT(&ns->mounts))
-		return -ENOENT;
-
-	first = listmnt_next(ns->root, false);
-	if (!first)
-		return -ENOENT;
-	root->mnt = mntget(&first->mnt);
-	root->dentry = dget(root->mnt->mnt_root);
-	return 0;
-}
-
 /*
  * If the user requested a specific mount namespace id, look that up and return
  * that, or if not simply grab a passive reference on our mount namespace and
@@ -5195,9 +5221,8 @@ SYSCALL_DEFINE4(statmount, const struct mnt_id_req __user *, req,
 		unsigned int, flags)
 {
 	struct mnt_namespace *ns __free(mnt_ns_release) = NULL;
-	struct vfsmount *mnt;
+	struct kstatmount *ks __free(kfree) = NULL;
 	struct mnt_id_req kreq;
-	struct kstatmount ks;
 	/* We currently support retrieval of 3 strings. */
 	size_t seq_size = 3 * PATH_MAX;
 	int ret;
@@ -5217,40 +5242,21 @@ SYSCALL_DEFINE4(statmount, const struct mnt_id_req __user *, req,
 	    !ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))
 		return -ENOENT;
 
+	ks = kmalloc(sizeof(*ks), GFP_KERNEL_ACCOUNT);
+	if (!ks)
+		return -ENOMEM;
+
 retry:
-	ret = prepare_kstatmount(&ks, &kreq, buf, bufsize, seq_size);
+	ret = prepare_kstatmount(ks, &kreq, buf, bufsize, seq_size);
 	if (ret)
 		return ret;
 
-	down_read(&namespace_sem);
-	/* Has the namespace already been emptied? */
-	if (kreq.mnt_ns_id && RB_EMPTY_ROOT(&ns->mounts)) {
-		up_read(&namespace_sem);
-		kvfree(ks.seq.buf);
-		return -ENOENT;
-	}
-
-	mnt = lookup_mnt_in_ns(kreq.mnt_id, ns);
-	if (!mnt) {
-		up_read(&namespace_sem);
-		kvfree(ks.seq.buf);
-		return -ENOENT;
-	}
-
-	ks.mnt = mnt;
-	ret = grab_requested_root(ns, &ks.root);
-	if (ret) {
-		up_read(&namespace_sem);
-		kvfree(ks.seq.buf);
-		return ret;
-	}
-	ret = do_statmount(&ks);
-	path_put(&ks.root);
-	up_read(&namespace_sem);
+	scoped_guard(rwsem_read, &namespace_sem)
+		ret = do_statmount(ks, kreq.mnt_id, kreq.mnt_ns_id, ns);
 
 	if (!ret)
-		ret = copy_statmount_to_user(&ks);
-	kvfree(ks.seq.buf);
+		ret = copy_statmount_to_user(ks);
+	kvfree(ks->seq.buf);
 	if (retry_statmount(ret, &seq_size))
 		goto retry;
 	return ret;