[Virtio-fs] Securing file handles

[Miklos Szeredi]

Hi,

Thanks for the good summary.

Another aspect is what the file handle will be used for:

 a) allowing server to close O_PATH descriptors any time because they
can be reconstructed using the file handle

 b) allowing NFS export on client, or just name_to_handle_at(2)
open_by_handle_at(2).

The requirements are slightly different, since file handles used for
(a) do not have to persist after a guest reboot (since the VFS cache
referencing those handles is gone).  While (b) requires persistence
after a reboot.

Yet another issue is global CAP_DAC_READ_SEARCH required by the server
for file handle decode.

Taking this into account, I think the final solution has to be in the
host kernel.   E.g. it seems okay to allow user namespace owner to
decode file handles on filesystems it actually owns.   That would not
generally help us, though, since virtiofs will want to export root
owned fs as well.

Addition of a MAC header to the file handle by name_to_handle_at(2)
could solve some or all of the above problems.  The question is where
the key comes from and what the security implications are.

A per-process (e.g. associated with task->files, generated by the
kernel on demand and discarded on process exit) key would suffice to
replace O_PATH descriptors.  In this case the only difference between
keeping the O_PATH fd open and

  name_to_handle_at(opathfd, &handle);
  close(opathfd);
  opathfd=open_by_handle_at(&handle);

would be that the resulting fd might point to a disconnected dentry
and hence would result in incomplete path information under
/proc/self/fd/.  Need to think hard about whether this has any
security implications for unprivileged users.

Adding key management would solve the other aspects, but would also
possibly open up holes for accessing arbitrary files, so this would
need to be done carefully.

Adding subtree checking to the kernel is also a possibility (i.e.
limit the opened fd to the subtree of the bind mount).   It would have
the advantage of not resulting in disconnected dentries, but
disadvantage of not working if the file was moved across directories.

Thanks,
Miklos