VFS: introduce start_creating_noperm() and start_removing_noperm()

xfs, fuse, ipc/mqueue need variants of start_creating or start_removing
which do not check permissions.
This patch adds _noperm versions of these functions.

Note that do_mq_open() was only calling mntget() so it could call
path_put() - it didn't really need an extra reference on the mnt.
Now it doesn't call mntget() and uses end_creating() which does
the dput() half of path_put().

Also mq_unlink() previously passed
   d_inode(dentry->d_parent)
as the dir inode to vfs_unlink().  This is after locking
   d_inode(mnt->mnt_root)
These two inodes are the same, but normally calls use the textual
parent.
So I've changes the vfs_unlink() call to be given d_inode(mnt->mnt_root).

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>

--
changes since v2:
 - dir arg passed to vfs_unlink() in mq_unlink() changed to match
   the dir passed to lookup_noperm()
 - restore assignment to path->mnt even though the mntget() is removed.

Link: https://patch.msgid.link/20251113002050.676694-7-neilb@ownmail.net
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 12 insertions, 20 deletions

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 093551fe66a7..6d7610310003 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -913,13 +913,12 @@ static int do_mq_open(const char __user *u_name, int oflag, umode_t mode,
 		goto out_putname;
 
 	ro = mnt_want_write(mnt);	/* we'll drop it in any case */
-	inode_lock(d_inode(root));
-	path.dentry = lookup_noperm(&QSTR(name->name), root);
+	path.dentry = start_creating_noperm(root, &QSTR(name->name));
 	if (IS_ERR(path.dentry)) {
 		error = PTR_ERR(path.dentry);
 		goto out_putfd;
 	}
-	path.mnt = mntget(mnt);
+	path.mnt = mnt;
 	error = prepare_open(path.dentry, oflag, ro, mode, name, attr);
 	if (!error) {
 		struct file *file = dentry_open(&path, oflag, current_cred());
@@ -928,13 +927,12 @@ static int do_mq_open(const char __user *u_name, int oflag, umode_t mode,
 		else
 			error = PTR_ERR(file);
 	}
-	path_put(&path);
 out_putfd:
 	if (error) {
 		put_unused_fd(fd);
 		fd = error;
 	}
-	inode_unlock(d_inode(root));
+	end_creating(path.dentry, root);
 	if (!ro)
 		mnt_drop_write(mnt);
 out_putname:
@@ -957,7 +955,7 @@ SYSCALL_DEFINE1(mq_unlink, const char __user *, u_name)
 	int err;
 	struct filename *name;
 	struct dentry *dentry;
-	struct inode *inode = NULL;
+	struct inode *inode;
 	struct ipc_namespace *ipc_ns = current->nsproxy->ipc_ns;
 	struct vfsmount *mnt = ipc_ns->mq_mnt;
 
@@ -969,26 +967,20 @@ SYSCALL_DEFINE1(mq_unlink, const char __user *, u_name)
 	err = mnt_want_write(mnt);
 	if (err)
 		goto out_name;
-	inode_lock_nested(d_inode(mnt->mnt_root), I_MUTEX_PARENT);
-	dentry = lookup_noperm(&QSTR(name->name), mnt->mnt_root);
+	dentry = start_removing_noperm(mnt->mnt_root, &QSTR(name->name));
 	if (IS_ERR(dentry)) {
 		err = PTR_ERR(dentry);
-		goto out_unlock;
+		goto out_drop_write;
 	}
 
 	inode = d_inode(dentry);
-	if (!inode) {
-		err = -ENOENT;
-	} else {
-		ihold(inode);
-		err = vfs_unlink(&nop_mnt_idmap, d_inode(dentry->d_parent),
-				 dentry, NULL);
-	}
-	dput(dentry);
-
-out_unlock:
-	inode_unlock(d_inode(mnt->mnt_root));
+	ihold(inode);
+	err = vfs_unlink(&nop_mnt_idmap, d_inode(mnt->mnt_root),
+			 dentry, NULL);
+	end_removing(dentry);
 	iput(inode);
+
+out_drop_write:
 	mnt_drop_write(mnt);
 out_name:
 	putname(name);