Message-ID: <d6b43b5780770637a724d129c22d5212860f494a.camel@huaweicloud.com>
Date:   Thu, 14 Dec 2023 14:42:44 +0100
From:   Roberto Sassu <roberto.sassu@...weicloud.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Amir Goldstein <amir73il@...il.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        Seth Forshee <sforshee@...nel.org>, miklos@...redi.hu,
        linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        paul@...l-moore.com, stefanb@...ux.ibm.com, jlayton@...nel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Eric Snowberg <eric.snowberg@...cle.com>
Subject: Re: [RFC][PATCH] overlayfs: Redirect xattr ops on security.evm to
 security.evm_overlayfs

On Tue, 2023-12-12 at 10:27 -0500, Mimi Zohar wrote:
> On Tue, 2023-12-12 at 14:13 +0100, Roberto Sassu wrote:
> > On 12.12.23 11:44, Amir Goldstein wrote:
> > > On Tue, Dec 12, 2023 at 12:25 PM Roberto Sassu
> > > <roberto.sassu@...weicloud.com> wrote:
> > > > 
> > > > On 11.12.23 19:01, Christian Brauner wrote:
> > > > > > The second problem is that one security.evm is not enough. We need two,
> > > > > > to store the two different HMACs. And we need both at the same time,
> > > > > > since when overlayfs is mounted the lower/upper directories can be
> > > > > > still accessible.
> > > > > 
> > > > > "Changes to the underlying filesystems while part of a mounted overlay
> > > > > filesystem are not allowed. If the underlying filesystem is changed, the
> > > > > behavior of the overlay is undefined, though it will not result in a
> > > > > crash or deadlock."
> > > > > 
> > > > > https://docs.kernel.org/filesystems/overlayfs.html#changes-to-underlying-filesystems
> > > > > 
> > > > > So I don't know why this would be a problem.
> > > > 
> > > > + Eric Snowberg
> > > > 
> > > > Ok, that would reduce the surface of attack. However, when looking at:
> > > > 
> > > >        ovl: Always reevaluate the file signature for IMA
> > > > 
> > > >        Commit db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the
> > > > i_version")
> > > >        partially closed an IMA integrity issue when directly modifying a file
> > > >        on the lower filesystem.  If the overlay file is first opened by a
> > > > user
> > > >        and later the lower backing file is modified by root, but the extended
> > > >        attribute is NOT updated, the signature validation succeeds with
> > > > the old
> > > >        original signature.
> > > > 
> > > > Ok, so if the behavior of overlayfs is undefined if the lower backing
> > > > file is modified by root, do we need to reevaluate? Or instead would be
> > > > better to forbid the write from IMA (legitimate, I think, since the
> > > > behavior is documented)? I just saw that we have d_real_inode(), we can
> > > > use it to determine if the write should be denied.
> > > > 
> > > 
> > > There may be several possible legitimate actions in this case, but the
> > > overall concept IMO should be the same as I said about EVM -
> > > overlayfs does not need an IMA signature of its own, because it
> > > can use the IMA signature of the underlying file.
> > > 
> > > Whether overlayfs reads a file from lower fs or upper fs, it does not
> > > matter, the only thing that matters is that the underlying file content
> > > is attested when needed.
> > > 
> > > The only incident that requires special attention is copy-up.
> > > This is what the security hooks security_inode_copy_up() and
> > > security_inode_copy_up_xattr() are for.
> > > 
> > > When a file starts in state "lower" and has security.ima,evm xattrs
> > > then before a user changes the file, it is copied up to upper fs
> > > and suppose that security.ima,evm xattrs are copied as is?
> 
> For IMA copying up security.ima is fine.  Other than EVM portable
> signatures, security.evm contains filesystem specific metadata. 
> Copying security.evm up only works if the metadata is the same on both
> filesystems.  Currently the i_generation and i_sb->s_uuid are
> different.
> 
> > > When later the overlayfs file content is read from the upper copy
> > > the security.ima signature should be enough to attest that file content
> > > was not tampered with between going from "lower" to "upper".
> > > 
> > > security.evm may need to be fixed on copy up, but that should be
> > > easy to do with the security_inode_copy_up_xattr() hook. No?
> 
> Writing security.evm requires the existing security.evm to be valid. 
> After each security xattr in the protected list is modified,
> security.evm HMAC needs to be updated.  Perhaps calculating and writing
> security.evm could be triggered by security_inode_copy_up_xattr(). 
> Just copying a non-portable EVM signature wouldn't work, or for that
> matter copying an EVM HMAC with different filesystem metadata.

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.

Roberto

> > It is not yet clear to me. EVM will be seeing the creation of a new 
> > file, and for new files setting xattrs is already allowed.
> > 
> > Maybe the security_inode_copy_up*() would be useful for IMA/EVM to 
> > authorize writes by overlayfs, which would be otherwise denied to the 
> > others (according to my solution).
> > 
> > Still, would like to hear Mimi's opinion.
> 
> Thanks Roberto for all your work and analysis.  I'm still looking at
> security_inode_copy_up_xattr().
> 
> Mimi
> 

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