From: Mimi Zohar <zohar@linux.ibm.com>
To: Ignaz Forster <iforster@suse.de>,
Goldwyn Rodrigues <rgoldwyn@suse.com>,
linux-integrity@vger.kernel.org,
Miklos Szeredi <miklos@szeredi.hu>,
linux-unionfs@vger.kernel.org
Subject: Re: EVM: Permission denied with overlayfs
Date: Tue, 18 Dec 2018 18:00:31 -0500 [thread overview]
Message-ID: <1545174031.4178.8.camel@linux.ibm.com> (raw)
In-Reply-To: <12c81a49-efca-d66c-2143-ae04ca248cce@suse.de>
Hi Ignaz,
On Tue, 2018-12-18 at 20:49 +0100, Ignaz Forster wrote:
> Hi,
>
> as a follow up to my attempts to use overlayfs on an IMA protected
> system[1] I've now tried to also enable EVM. From what I understand this
> should - at least in theory - be possible: EVM will call
> d_backing_inode(dentry), which I thought would get the inode of the
> underlying file system[2], and use that for HMAC verification.
>
> In practice simply trying to access an existing file will fail with
> "Permission denied" already. In the corresponding audit log I can see
> the file access (failed with "invalid-HMAC"), but with an inode number
> unknown to me - stat returns a completely different number for the file
> in the lower and target dir.
>
> For testing purposes I added a new hashing algorithm to
> evm_ima_xattr_type which will not add the file system specific
> attributes (inode number, generation, file system uuid) to the hash -
> just like EVM_XATTR_PORTABLE_DIGSIG, but with the hashes generated by
> the kernel. Files created with this signature can be read correctly,
> though writing the files will still fail.
>
> Unfortunately I'm out of ideas what is happening here. If anybody wants
> to have a look at this: Any help would be appreciated.
>
> Kind Regards,
> Ignaz
>
> [1] https://www.spinics.net/lists/linux-integrity/msg03593.html
> [2] https://www.kernel.org/doc/htmldocs/filesystems/API-d-backing-inode.html
>
After creating a file on the overlay, I wasn't able to access it from
the overlay, but was able to access it from "upper". Both "stat" and
"getfattr -m ^security" returned exactly the same things for both
pathnames. However, the ino in the audit log was different.
After modifying evm_calc_hmac_or_hash(), replacing d_backing_inode()
with d_real_inode(), the hmac properly calculated for both the overlay
and the upper pathnames.
Something must have changed in d_backing_inode().
Mimi
next prev parent reply other threads:[~2018-12-18 23:00 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-18 19:49 EVM: Permission denied with overlayfs Ignaz Forster
2018-12-18 23:00 ` Mimi Zohar [this message]
2018-12-19 15:39 ` Mimi Zohar
2018-12-19 16:38 ` Amir Goldstein
2018-12-19 18:34 ` Mimi Zohar
2018-12-19 20:39 ` Amir Goldstein
2018-12-20 3:42 ` Goldwyn Rodrigues
2018-12-20 7:15 ` Amir Goldstein
2018-12-19 16:56 ` James Bottomley
2018-12-19 18:15 ` Mimi Zohar
2018-12-19 19:34 ` James Bottomley
2018-12-19 20:12 ` Amir Goldstein
2018-12-19 21:02 ` Mimi Zohar
2018-12-19 22:08 ` James Bottomley
2018-12-20 14:55 ` Mimi Zohar
2018-12-20 19:24 ` James Bottomley
2018-12-19 22:11 ` James Bottomley
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