On Thu, Dec 13, 2018 at 03:09:55PM -0500, Stephen Smalley wrote: > On 12/13/18 1:54 PM, Vivek Goyal wrote: > > On Thu, Dec 13, 2018 at 11:12:31AM -0500, Stephen Smalley wrote: > > > > [..] > > > > > > Can you elaborate a bit more on how this is leaking data through overlay > > > > > > mount. If it is, then why accessing file on lower is not equivalent of > > > > > > leaking of data. > > > > > > > > > > In the container use case, retaining the lower label on copy-up for a > > > > > context-mounted overlay permits a process in the container to leak the > > > > > container data out to host files not labeled with the container label and > > > > > thus potentially accessible to other containers or host processes. > > > > > > > > > The > > > > > container process appears to just be writing to files labeled with the > > > > > container label via the overlay, but the written data and/or metadata is > > > > > directly accessible through the lower label, which is likely readable to > > > > > all/many containers and host processes. > > > > > > > > > > In the multi-level security (MLS) use case, an analogy would a situation > > > > > where you have an unclassified lower dir with some content to be shared > > > > > read-only across all levels, and an overlay is context-mounted at each level > > > > > with a corresponding upper dir and work dir private to that level. If a > > > > > client process at secret performs a write to a file via the secret overlay, > > > > > and if the written data is stored in a file in the upper dir that inherits > > > > > the label from the lower file (unclassified), then the secret process can > > > > > leak data to unclassified processes at will, violating the MLS policy. > > > > > > > > For the case of devices, its already happening. One might change metadata > > > > of a device (hence trigger copy up). Now all writes to upper device file > > > > from secret process still go to same underlying device and are still > > > > readable from lower device file. > > > > > > This is an argument for not copying up device files IMHO, not for preserving > > > the lower label on them. > > > > How do we handle metadata change to device node (like timestamp, ownership > > change) without copy up. > > Do we need to support such metadata changes to device nodes through an > overlay mount? Is that required for some legitimate purpose (and if so, > what is the use case?)? If not, just deny it up front. Much simpler and no > potential for a leak. This will be overlay specific behavior and further from POSIX like filesystem behavior. Don't know which workloads depend on changing ownership of devices of changing metadata of devices. Thanks Vivek
