[PATCH 03/10] ovl: handle idmapped mounts in ovl_setattr()

Pass the mount's struct mnt_idmap to setattr_prepare() so that the
permission checks for a chown/chmod performed through an idmapped
overlay mount are evaluated in the mount's id space.

The ownership requested in @attr is expressed relative to the overlay
mount idmap.  Before forwarding the change to the upper layer via
ovl_do_notify_change() - whose notify_change() applies the upper layer
idmap in turn - rebase ia_vfsuid/ia_vfsgid into the overlay's own id
space, i.e. the same space as the overlay inode's i_{u,g}id established
by ovl_copyattr().  Without this rebase the upper layer would interpret
the caller's mount-relative id as an upper-relative one and store the
wrong owner on disk, or reject it with -EOVERFLOW.

from_vfsuid() returns INVALID_UID for an id that the overlay mount
idmap does not map; that invalid id is carried faithfully into the
forwarded iattr and rejected by the upper notify_change() via
vfsuid_has_fsmapping(), so no bogus owner can be written.

No functional change until FS_ALLOW_IDMAP is set on ovl_fs_type; until
then the overlay mount idmap is &nop_mnt_idmap and from_vfsuid() is the
identity.

Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
 fs/overlayfs/inode.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index f59db57dfd55..33734ca971e1 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -26,10 +26,18 @@ int ovl_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 	bool full_copy_up = false;
 	struct dentry *upperdentry;
 
-	err = setattr_prepare(&nop_mnt_idmap, dentry, attr);
+	err = setattr_prepare(idmap, dentry, attr);
 	if (err)
 		return err;
 
+	/* Rebase ownership from the mount idmap into overlay id space. */
+	if (attr->ia_valid & ATTR_UID)
+		attr->ia_vfsuid = VFSUIDT_INIT(from_vfsuid(idmap,
+				i_user_ns(d_inode(dentry)), attr->ia_vfsuid));
+	if (attr->ia_valid & ATTR_GID)
+		attr->ia_vfsgid = VFSGIDT_INIT(from_vfsgid(idmap,
+				i_user_ns(d_inode(dentry)), attr->ia_vfsgid));
+
 	if (attr->ia_valid & ATTR_SIZE) {
 		/* Truncate should trigger data copy up as well */
 		full_copy_up = true;

-- 
2.47.3