• Linus Torvalds's avatar
    Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace · a867d734
    Linus Torvalds authored
    Pull userns vfs updates from Eric Biederman:
     "This tree contains some very long awaited work on generalizing the
      user namespace support for mounting filesystems to include filesystems
      with a backing store.  The real world target is fuse but the goal is
      to update the vfs to allow any filesystem to be supported.  This
      patchset is based on a lot of code review and testing to approach that
      While looking at what is needed to support the fuse filesystem it
      became clear that there were things like xattrs for security modules
      that needed special treatment.  That the resolution of those concerns
      would not be fuse specific.  That sorting out these general issues
      made most sense at the generic level, where the right people could be
      drawn into the conversation, and the issues could be solved for
      At a high level what this patchset does a couple of simple things:
       - Add a user namespace owner (s_user_ns) to struct super_block.
       - Teach the vfs to handle filesystem uids and gids not mapping into
         to kuids and kgids and being reported as INVALID_UID and
         INVALID_GID in vfs data structures.
      By assigning a user namespace owner filesystems that are mounted with
      only user namespace privilege can be detected.  This allows security
      modules and the like to know which mounts may not be trusted.  This
      also allows the set of uids and gids that are communicated to the
      filesystem to be capped at the set of kuids and kgids that are in the
      owning user namespace of the filesystem.
      One of the crazier corner casees this handles is the case of inodes
      whose i_uid or i_gid are not mapped into the vfs.  Most of the code
      simply doesn't care but it is easy to confuse the inode writeback path
      so no operation that could cause an inode write-back is permitted for
      such inodes (aka only reads are allowed).
      This set of changes starts out by cleaning up the code paths involved
      in user namespace permirted mounts.  Then when things are clean enough
      adds code that cleanly sets s_user_ns.  Then additional restrictions
      are added that are possible now that the filesystem superblock
      contains owner information.
      These changes should not affect anyone in practice, but there are some
      parts of these restrictions that are changes in behavior.
       - Andy's restriction on suid executables that does not honor the
         suid bit when the path is from another mount namespace (think
         /proc/[pid]/fd/) or when the filesystem was mounted by a less
         privileged user.
       - The replacement of the user namespace implicit setting of MNT_NODEV
         with implicitly setting SB_I_NODEV on the filesystem superblock
         Using SB_I_NODEV is a stronger form that happens to make this state
         user invisible.  The user visibility can be managed but it caused
         problems when it was introduced from applications reasonably
         expecting mount flags to be what they were set to.
      There is a little bit of work remaining before it is safe to support
      mounting filesystems with backing store in user namespaces, beyond
      what is in this set of changes.
       - Verifying the mounter has permission to read/write the block device
         during mount.
       - Teaching the integrity modules IMA and EVM to handle filesystems
         mounted with only user namespace root and to reduce trust in their
         security xattrs accordingly.
       - Capturing the mounters credentials and using that for permission
         checks in d_automount and the like.  (Given that overlayfs already
         does this, and we need the work in d_automount it make sense to
         generalize this case).
      Furthermore there are a few changes that are on the wishlist:
       - Get all filesystems supporting posix acls using the generic posix
         acls so that posix_acl_fix_xattr_from_user and
         posix_acl_fix_xattr_to_user may be removed.  [Maintainability]
       - Reducing the permission checks in places such as remount to allow
         the superblock owner to perform them.
       - Allowing the superblock owner to chown files with unmapped uids and
         gids to something that is mapped so the files may be treated
      I am not considering even obvious relaxations of permission checks
      until it is clear there are no more corner cases that need to be
      locked down and handled generically.
      Many thanks to Seth Forshee who kept this code alive, and putting up
      with me rewriting substantial portions of what he did to handle more
      corner cases, and for his diligent testing and reviewing of my
    * 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace: (30 commits)
      fs: Call d_automount with the filesystems creds
      fs: Update i_[ug]id_(read|write) to translate relative to s_user_ns
      evm: Translate user/group ids relative to s_user_ns when computing HMAC
      dquot: For now explicitly don't support filesystems outside of init_user_ns
      quota: Handle quota data stored in s_user_ns in quota_setxquota
      quota: Ensure qids map to the filesystem
      vfs: Don't create inodes with a uid or gid unknown to the vfs
      vfs: Don't modify inodes with a uid or gid unknown to the vfs
      cred: Reject inodes with invalid ids in set_create_file_as()
      fs: Check for invalid i_uid in may_follow_link()
      vfs: Verify acls are valid within superblock's s_user_ns.
      userns: Handle -1 in k[ug]id_has_mapping when !CONFIG_USER_NS
      fs: Refuse uid/gid changes which don't map into s_user_ns
      selinux: Add support for unprivileged mounts from user namespaces
      Smack: Handle labels consistently in untrusted mounts
      Smack: Add support for unprivileged mounts from user namespaces
      fs: Treat foreign mounts as nosuid
      fs: Limit file caps to the user namespace of the super block
      userns: Remove the now unnecessary FS_USERNS_DEV_MOUNT flag
      userns: Remove implicit MNT_NODEV fragility.
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