Merge patch series "fs: refuse O_TMPFILE creation with an unmapped fsuid or fsgid"

Christian Brauner <brauner@kernel.org> says:

vfs_tmpfile() is the only object-creation path in the VFS that never
checked whether the caller's fsuid and fsgid map into the filesystem.

On an idmapped mount whose idmapping does not cover the caller's
fs{u,g}id, the ->tmpfile() instance initializes the new inode through
inode_init_owner(), where mapped_fsuid()/mapped_fsgid() return
INVALID_UID/INVALID_GID.  The tmpfile ends up owned by (uid_t)-1, and
because it is created I_LINKABLE it can subsequently be spliced into the
namespace with linkat(2).

Every other creation path already refuses this: may_o_create() (O_CREAT)
and may_create_dentry() (mkdir, mknod, symlink, link) bail out with
-EOVERFLOW via fsuidgid_has_mapping() precisely so that an object cannot
be created with an owner the filesystem cannot represent.  O_TMPFILE is
no exception and must hold the same guarantee.

Patch 1 adds the missing fsuidgid_has_mapping() check to vfs_tmpfile().
It is a no-op on non-idmapped mounts -- there the caller's fs{u,g}id
always map in the superblock's user namespace -- and only takes effect
on an idmapped mount that does not map the caller.  It applies to every
filesystem that sets FS_ALLOW_IDMAP and implements ->tmpfile() (tmpfs,
ext4, btrfs, xfs, f2fs, ...), and to overlayfs, whose upper-layer
tmpfile creation funnels through vfs_tmpfile() via
backing_tmpfile_open().

Patch 2 adds a selftest that idmaps a detached tmpfs mount and checks
both directions: an unmapped caller is refused with -EOVERFLOW, while a
mapped caller can create an O_TMPFILE, link it into the namespace, and
have its ownership round-trip through the mount idmap.

* patches from https://patch.msgid.link/20260615-work-idmapped-tmpfile-v1-0-754a94d81f83@kernel.org:
  selftests/filesystems: test O_TMPFILE creation on idmapped mounts
  fs: refuse O_TMPFILE creation with an unmapped fsuid or fsgid

Link: https://patch.msgid.link/20260615-work-idmapped-tmpfile-v1-0-754a94d81f83@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>