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Subject: Re: [PATCH] fs: Return raw xattr for security.* if there is size
 disagreement with LSMs
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Paul Moore <paul@paul-moore.com>
Cc:     Roberto Sassu <roberto.sassu@huawei.com>,
        Stefan Berger <stefanb@linux.ibm.com>,
        "viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
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        "casey@schaufler-ca.com" <casey@schaufler-ca.com>,
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        "linux-integrity@vger.kernel.org" <linux-integrity@vger.kernel.org>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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Date:   Fri, 18 Jun 2021 12:04:21 -0400
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On Thu, 2021-06-17 at 23:18 -0400, Paul Moore wrote:
> On Thu, Jun 17, 2021 at 11:28 AM Mimi Zohar <zohar@linux.ibm.com> wrote:
> > On Thu, 2021-06-17 at 07:09 +0000, Roberto Sassu wrote:
> 
> ...
> 
> > > An alternative would be to do the EVM verification twice if the
> > > first time didn't succeed (with vfs_getxattr_alloc() and with the
> > > new function that behaves like vfs_getxattr()).
> >
> > Unfortunately, I don't see an alternative.
> 
> ... and while unfortunate, the impact should be non-existant if you
> are using the right tools to label files or ensuring that you are
> formatting labels properly if doing it by hand.
> 
> Handling a corner case is good, but I wouldn't add a lot of code
> complexity trying to optimize it.

From userspace it's really difficult to understand the EVM signature
verification failure is due to the missing NULL.

Roberto, I just pushed the "evm: output EVM digest calculation info"
patch to the next-integrity-testing branch, which includes some
debugging.   Instead of this patch, which returns the raw xattr data,
how about adding additional debugging info in evm_calc_hmac_or_hash()
indicating the size discrepancy between the raw xattr and the LSM
returned xattr?

thanks,

Mimi