[RFC][PATCH] overlayfs: Redirect xattr ops on security.evm to security.evm_overlayfs

[Mimi Zohar]

On Thu, 2023-12-14 at 20:06 +0200, Amir Goldstein wrote:
> > > > There is another problem, when delayed copy is used. The content comes
> > > > from one source, metadata from another.
> > > >
> > > > I initially created test-file-lower on the lower directory
> > > > (overlayfs/data), before mounting overlayfs. After mount on
> > > > overlayfs/mnt:
> > > >
> > > > # getfattr -m - -e hex -d overlayfs/mnt/test-file-lower
> > > > # file: overlayfs/mnt/test-file-lower
> > > > security.evm=0x02c86ec91a4c0cf024537fd24347b780b90973402e
> > > > security.ima=0x0404f2ca1bb6c7e907d06dafe4687e579fce76b37e4e93b7605022da52e6ccc26fd2
> > > > security.selinux=0x73797374656d5f753a6f626a6563745f723a756e6c6162656c65645f743a733000
> > > >
> > > > # chcon -t unconfined_t overlayfs/mnt/test-file-lower
> > > >
> > > > After this, IMA creates an empty file in the upper directory
> > > > (overlayfs/root/data), and writes security.ima at file close.
> > > > Unfortunately, this is what is presented from overlayfs, which is not
> > > > in sync with the content.
> > > >
> > > > # getfattr -m - -e hex -d overlayfs/mnt/test-file-lower
> > > > # file: overlayfs/mnt/test-file-lower
> > > > security.evm=0x021d71e7df78c36745e3b651ce29cb9f47dc301248
> > > > security.ima=0x04048855508aade16ec573d21e6a485dfd0a7624085c1a14b5ecdd6485de0c6839a4
> > > > security.selinux=0x73797374656d5f753a6f626a6563745f723a756e636f6e66696e65645f743a733000
> > > >
> > > > # sha256sum overlayfs/mnt/test-file-lower
> > > > f2ca1bb6c7e907d06dafe4687e579fce76b37e4e93b7605022da52e6ccc26fd2  overlayfs/mnt/test-file-lower
> > > >
> > > > # sha256sum overlayfs/root/data/test-file-lower
> > > > 8855508aade16ec573d21e6a485dfd0a7624085c1a14b5ecdd6485de0c6839a4  overlayfs/root/data/test-file-lower (upperdir)
> > > >
> > > > We would need to use the lower security.ima until the copy is made, but
> > > > at the same time we need to keep the upper valid (with all xattrs) so
> > > > that IMA can update the next time overlayfs requests that.
> > > >
> > >
> > > Yap.
> > >
> > > As Seth wrote, overlayfs is a combination of upper and lower.
> > > The information that IMA needs should be accessible from either lower
> > > or upper, but sometimes we will need to make the right choice.
> > >
> > > The case of security.ima is similar to that of st_blocks -
> > > it is a data-related metadata, so it needs to be taken from the lowerdata inode
> > > (not even the lower inode). See example of getting STATX_BLOCKS
> > > in ovl_getattr().
> > >
> > > I would accept a patch that special cases security.ima in ovl_xattr_get()
> > > and gets it from ovl_i_path_lowerdata(), which would need to be
> > > factored out of ovl_path_lowerdata().
> > >
> > > I would also accept filtering out security.{ima,evm} from
> > >
> > > But I would only accept it if I know that IMA is not trying to write the
> > > security.ima xattr when closing an overlayfs file, only when closing the
> > > real underlying upper file.
> >
> > I don't see how that would be possible.  As far as I'm aware, the
> > correlation is between the overlay and the underlying lower/uppper
> > file, not the other way around.  How could a close on the underlying
> > file trigger IMA on an overlay file?
> >
> 
> Well, you are right. it cannot.
> 
> What I meant is that close of overlayfs file should NOT open and read
> the overlayfs file and recalculate security.ima to store in overlayfs inode
> because close of overlayfs file will follow a close of the upper file that
> should recalculate and store security.ima in the upper inode.
> 
> It is possible that a close of an overlayfs file will update the security
> state of the overlayfs inode by copying the security state from the
> upper inode.

Thank you for the explanation.

Basically IMA should differentiate between file close on the underlying
upper/lower file and the overlay file.  Since IMA doesn't define
inode_copy_up_xattr, security.ima will be copied up.  Re-calculating
security.ima on the overlay is unnecessary.

> But then again, I could be misunderstanding the IMA workflows
> and it could be more complicated than I try to present it.
> This is the reason that I requested the documentation of how
> IMA+overlayfs is *expected* to work.

Ok

Mimi