FAILED: patch "[PATCH] ovl: Use ovl mounter's fsuid and fsgid in ovl_link()" failed to apply to 4.14-stable tree

The patch below does not apply to the 4.14-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Possible dependencies:

5b0db51215e8 ("ovl: Use ovl mounter's fsuid and fsgid in ovl_link()") 471ec5dcf4e7 ("ovl: struct cattr cleanups")
6cf00764b008 ("ovl: strip debug argument from ovl_do_ helpers") e7dd0e71348c ("ovl: whiteout index when union nlink drops to zero") 016b720f5558 ("ovl: index directories on copy up for NFS export") fbd2d2074bde ("ovl: index all non-dir on copy up for NFS export") 24b33ee104ec ("ovl: create ovl_need_index() helper")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 5b0db51215e895a361bc63132caa7cca36a53d6a Mon Sep 17 00:00:00 2001
From: Zhang Tianci <[email protected]>
Date: Thu, 1 Sep 2022 16:29:29 +0800
Subject: [PATCH] ovl: Use ovl mounter's fsuid and fsgid in ovl_link()

There is a wrong case of link() on overlay:

  $ mkdir /lower /fuse /merge
  $ mount -t fuse /fuse
  $ mkdir /fuse/upper /fuse/work
  $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,\
    workdir=work
  $ touch /merge/file
  $ chown bin.bin /merge/file // the file's caller becomes "bin"
  $ ln /merge/file /merge/lnkfile

Then we will get an error(EACCES) because fuse daemon checks the link()'s caller is "bin", it denied this request.

In the changing history of ovl_link(), there are two key commits:

The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which overrides the cred's fsuid/fsgid using the new inode. The new inode's owner is initialized by inode_init_owner(), and inode->fsuid is assigned to the current user. So the override fsuid becomes the current user. We know link() is actually modifying the directory, so the caller must have the MAY_WRITE permission on the directory. The current caller may should have this permission. This is acceptable to use the caller's fsuid.

The second is commit 51f7e52dc943 ("ovl: share inode for hard link") which removed the inode creation in ovl_link(). This commit move inode_init_owner() into ovl_create_object(), so the ovl_link() just give the old inode to ovl_create_or_link(). Then the override fsuid becomes the old inode's fsuid, neither the caller nor the overlay's mounter! So this is incorrect.

Fix this bug by using ovl mounter's fsuid/fsgid to do underlying fs's link().

index 6b03457f72bb..c3032cef391e 100644
@@ -592,28 +592,42 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 			goto out_revert_creds;
 	}
 
-	err = -ENOMEM;
-	override_cred = prepare_creds();
-	if (override_cred) {
+	if (!attr->hardlink) {
+		err = -ENOMEM;
+		override_cred = prepare_creds();
+		if (!override_cred)
+			goto out_revert_creds;
+		/*
+		 * In the creation cases(create, mkdir, mknod, symlink),
+		 * ovl should transfer current's fs{u,g}id to underlying
+		 * fs. Because underlying fs want to initialize its new
+		 * inode owner using current's fs{u,g}id. And in this
+		 * case, the @inode is a new inode that is initialized
+		 * in inode_init_owner() to current's fs{u,g}id. So use
+		 * the inode's i_{u,g}id to override the cred's fs{u,g}id.
+		 *
+		 * But in the other hardlink case, ovl_link() does not
+		 * create a new inode, so just use the ovl mounter's
+		 * fs{u,g}id.
+		 */
 		override_cred->fsuid = inode->i_uid;
 		override_cred->fsgid = inode->i_gid;
-		if (!attr->hardlink) {
-			err = security_dentry_create_files_as(dentry,
-					attr->mode, &dentry->d_name, old_cred,
-					override_cred);
-			if (err) {
-				put_cred(override_cred);
-				goto out_revert_creds;
-			}
+		err = security_dentry_create_files_as(dentry,
+				attr->mode, &dentry->d_name, old_cred,
+				override_cred);
+		if (err) {
+			put_cred(override_cred);
+			goto out_revert_creds;
 		}
 		put_cred(override_creds(override_cred));
 		put_cred(override_cred);
-
-		if (!ovl_dentry_is_whiteout(dentry))
-			err = ovl_create_upper(dentry, inode, attr);
-		else
-			err = ovl_create_over_whiteout(dentry, inode, attr);
 	}
+
+	if (!ovl_dentry_is_whiteout(dentry))
+		err = ovl_create_upper(dentry, inode, attr);
+	else
+		err = ovl_create_over_whiteout(dentry, inode, attr);
+
 out_revert_creds:
 	revert_creds(old_cred);
 	return err;