fs: rework listmount() implementation

Linus pointed out that there's error handling and naming issues in the
that we should rewrite:

* Perform the access checks for the buffer before actually doing any
  work instead of doing it during the iteration.
* Rename the arguments to listmount() and do_listmount() to clarify what
  the arguments are used for.
* Get rid of the pointless ctr variable and overflow checking.
* Get rid of the pointless speculation check.

Link: https://lore.kernel.org/r/CAHk-=wjh6Cypo8WC-McXgSzCaou3UXccxB+7PVeSuGR8AjCphg@mail.gmail.comSuggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -5042,13 +5042,12 @@ static struct mount *listmnt_next(struct mount *curr)
 	return node_to_mount(rb_next(&curr->mnt_node));
 }
 
-static ssize_t do_listmount(struct mount *first, struct path *orig, u64 mnt_id,
-			    u64 __user *buf, size_t bufsize,
-			    const struct path *root)
+static ssize_t do_listmount(struct mount *first, struct path *orig,
+			    u64 mnt_parent_id, u64 __user *mnt_ids,
+			    size_t nr_mnt_ids, const struct path *root)
 {
 	struct mount *r;
-	ssize_t ctr;
-	int err;
+	ssize_t ret;
 
 	/*
 	 * Don't trigger audit denials. We just want to determine what
@@ -5058,50 +5057,57 @@ static ssize_t do_listmount(struct mount *first, struct path *orig, u64 mnt_id,
 	    !ns_capable_noaudit(&init_user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
-	err = security_sb_statfs(orig->dentry);
-	if (err)
-		return err;
+	ret = security_sb_statfs(orig->dentry);
+	if (ret)
+		return ret;
 
-	for (ctr = 0, r = first; r && ctr < bufsize; r = listmnt_next(r)) {
-		if (r->mnt_id_unique == mnt_id)
+	for (ret = 0, r = first; r && nr_mnt_ids; r = listmnt_next(r)) {
+		if (r->mnt_id_unique == mnt_parent_id)
 			continue;
 		if (!is_path_reachable(r, r->mnt.mnt_root, orig))
 			continue;
-		ctr = array_index_nospec(ctr, bufsize);
-		if (put_user(r->mnt_id_unique, buf + ctr))
+		if (put_user(r->mnt_id_unique, mnt_ids))
 			return -EFAULT;
-		if (check_add_overflow(ctr, 1, &ctr))
-			return -ERANGE;
+		mnt_ids++;
+		nr_mnt_ids--;
+		ret++;
 	}
-	return ctr;
+	return ret;
 }
 
-SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req,
-		u64 __user *, buf, size_t, bufsize, unsigned int, flags)
+SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req, u64 __user *,
+		mnt_ids, size_t, nr_mnt_ids, unsigned int, flags)
 {
 	struct mnt_namespace *ns = current->nsproxy->mnt_ns;
 	struct mnt_id_req kreq;
 	struct mount *first;
 	struct path root, orig;
-	u64 mnt_id, last_mnt_id;
+	u64 mnt_parent_id, last_mnt_id;
+	const size_t maxcount = (size_t)-1 >> 3;
 	ssize_t ret;
 
 	if (flags)
 		return -EINVAL;
 
+	if (unlikely(nr_mnt_ids > maxcount))
+		return -EFAULT;
+
+	if (!access_ok(mnt_ids, nr_mnt_ids * sizeof(*mnt_ids)))
+		return -EFAULT;
+
 	ret = copy_mnt_id_req(req, &kreq);
 	if (ret)
 		return ret;
-	mnt_id = kreq.mnt_id;
+	mnt_parent_id = kreq.mnt_id;
 	last_mnt_id = kreq.param;
 
 	down_read(&namespace_sem);
 	get_fs_root(current->fs, &root);
-	if (mnt_id == LSMT_ROOT) {
+	if (mnt_parent_id == LSMT_ROOT) {
 		orig = root;
 	} else {
 		ret = -ENOENT;
-		orig.mnt  = lookup_mnt_in_ns(mnt_id, ns);
+		orig.mnt = lookup_mnt_in_ns(mnt_parent_id, ns);
 		if (!orig.mnt)
 			goto err;
 		orig.dentry = orig.mnt->mnt_root;
@@ -5111,7 +5117,7 @@ SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req,
 	else
 		first = mnt_find_id_at(ns, last_mnt_id + 1);
 
-	ret = do_listmount(first, &orig, mnt_id, buf, bufsize, &root);
+	ret = do_listmount(first, &orig, mnt_parent_id, mnt_ids, nr_mnt_ids, &root);
 err:
 	path_put(&root);
 	up_read(&namespace_sem);

include/linux/syscalls.h

@@ -414,7 +414,7 @@ asmlinkage long sys_statmount(const struct mnt_id_req __user *req,
 			      struct statmount __user *buf, size_t bufsize,
 			      unsigned int flags);
 asmlinkage long sys_listmount(const struct mnt_id_req __user *req,
-			      u64 __user *buf, size_t bufsize,
+			      u64 __user *mnt_ids, size_t nr_mnt_ids,
 			      unsigned int flags);
 asmlinkage long sys_truncate(const char __user *path, long length);
 asmlinkage long sys_ftruncate(unsigned int fd, unsigned long length);