acl: move idmapping handling into posix_acl_xattr_set()

The uapi POSIX ACL struct passed through the value argument during
setxattr() contains {g,u}id values encoded via ACL_{GROUP,USER} entries
that should actually be stored in the form of k{g,u}id_t (See [1] for a
long explanation of the issue.).

In 0c5fd887d2bb ("acl: move idmapped mount fixup into vfs_{g,s}etxattr()")
we took the mount's idmapping into account in order to let overlayfs
handle POSIX ACLs on idmapped layers correctly. The fixup is currently
performed directly in vfs_setxattr() which piles on top of the earlier
hackiness by handling the mount's idmapping and stuff the vfs{g,u}id_t
values into the uapi struct as well. While that is all correct and works
fine it's just ugly.

Now that we have introduced vfs_make_posix_acl() earlier move handling
idmapped mounts out of vfs_setxattr() and into the POSIX ACL handler
where it belongs.

Note that we also need to call vfs_make_posix_acl() for EVM which
interpretes POSIX ACLs during security_inode_setxattr(). Leave them a
longer comment for future reference.

All filesystems that support idmapped mounts via FS_ALLOW_IDMAP use the
standard POSIX ACL xattr handlers and are covered by this change. This
includes overlayfs which simply calls vfs_{g,s}etxattr().

The following filesystems use custom POSIX ACL xattr handlers: 9p, cifs,
ecryptfs, and ntfs3 (and overlayfs but we've covered that in the paragraph
above) and none of them support idmapped mounts yet.

Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org/ [1]
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>

1 files changed, 0 insertions, 9 deletions

diff --git a/include/linux/posix_acl_xattr.h b/include/linux/posix_acl_xattr.h
index 47eca15fd842..8163dd48c430 100644
--- a/include/linux/posix_acl_xattr.h
+++ b/include/linux/posix_acl_xattr.h
@@ -38,9 +38,6 @@ void posix_acl_fix_xattr_to_user(void *value, size_t size);
 void posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
 				     const struct inode *inode,
 				     void *value, size_t size);
-void posix_acl_setxattr_idmapped_mnt(struct user_namespace *mnt_userns,
-				     const struct inode *inode,
-				     void *value, size_t size);
 #else
 static inline void posix_acl_fix_xattr_from_user(void *value, size_t size)
 {
@@ -54,12 +51,6 @@ posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
 				size_t size)
 {
 }
-static inline void
-posix_acl_setxattr_idmapped_mnt(struct user_namespace *mnt_userns,
-				const struct inode *inode, void *value,
-				size_t size)
-{
-}
 #endif
 
 struct posix_acl *posix_acl_from_xattr(struct user_namespace *user_ns,