lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <ZXdDvgbjSNo78zM2@do-x1extreme>
Date:   Mon, 11 Dec 2023 11:15:42 -0600
From:   Seth Forshee <sforshee@...nel.org>
To:     Roberto Sassu <roberto.sassu@...weicloud.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        Amir Goldstein <amir73il@...il.com>, miklos@...redi.hu,
        linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        zohar@...ux.ibm.com, 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>
Subject: Re: [RFC][PATCH] overlayfs: Redirect xattr ops on security.evm to
 security.evm_overlayfs

On Mon, Dec 11, 2023 at 04:41:46PM +0100, Roberto Sassu wrote:
> On Mon, 2023-12-11 at 09:36 -0600, Seth Forshee wrote:
> > On Mon, Dec 11, 2023 at 03:56:06PM +0100, Roberto Sassu wrote:
> > > Ok, I will try.
> > > 
> > > I explain first how EVM works in general, and then why EVM does not
> > > work with overlayfs.
> > > 
> > > EVM gets called before there is a set/removexattr operation, and after,
> > > if that operation is successful. Before the set/removexattr operation
> > > EVM calculates the HMAC on current inode metadata (i_ino, i_generation,
> > > i_uid, i_gid, i_mode, POSIX ACLs, protected xattrs). Finally, it
> > > compares the calculated HMAC with the one in security.evm.
> > > 
> > > If the verification and the set/removexattr operation are successful,
> > > EVM calculates again the HMAC (in the post hooks) based on the updated
> > > inode metadata, and sets security.evm with the new HMAC.
> > > 
> > > The problem is the combination of: overlayfs inodes have different
> > > metadata than the lower/upper inodes; overlayfs calls the VFS to
> > > set/remove xattrs.
> > 
> > I don't know all of the inner workings of overlayfs in detail, but is it
> > not true that whatever metadata an overlayfs mount presents for a given
> > inode is stored in the lower and/or upper filesystem inodes? If the
> > metadata for those inodes is verified with EVM, why is it also necessary
> > to verify the metadata at the overlayfs level? If some overlayfs
> > metadata is currently omitted from the checks on the lower/upper inodes,
> > is there any reason EVM couldn't start including that its checksums?
> 
> Currently, the metadata where there is a misalignment are:
> i_generation, s_uuid, (i_ino?). Maybe there is more?
> 
> If metadata are aligned, there is no need to store two separate HMACs.

I can only think of three possible sources for the metadata overlayfs
presents:

 1. It comes directly from the underlying filesystems
 2. overlayfs synthesizes if from the underlying filesystem data
 3. It's purely generated at runtime

Are there others?

1 and 2 should be covered by EVM on the underlying filesystems. If 3 is
happening then it seems like hashing that data is just confirming that
overlayfs consistently generates the same values for that data, and
verifying code behavior doesn't seem in-scope for EVM.

Powered by blists - more mailing lists

Powered by Openwall GNU/*/Linux Powered by OpenVZ