linux-kernel - Re: [PATCH v6 08/11] evm: Allow setxattr() and setattr() for unmodified metadata

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Message-ID: <735bae46f0772b40ef6ecfb3c6fe0267b3ebbee8.camel@linux.ibm.com>
Date:   Tue, 11 May 2021 10:11:49 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>, mjg59@...gle.com
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v6 08/11] evm: Allow setxattr() and setattr() for
 unmodified metadata

Hi Roberto,

On Wed, 2021-05-05 at 13:33 +0200, Roberto Sassu wrote:
> With the patch to allow xattr/attr operations if a portable signature
> verification fails, cp and tar can copy all xattrs/attrs so that at the
> end of the process verification succeeds.
> 
> However, it might happen that the xattrs/attrs are already set to the
> correct value (taken at signing time) and signature verification succeeds
> before the copy has completed. For example, an archive might contains files
> owned by root and the archive is extracted by root.
> 
> Then, since portable signatures are immutable, all subsequent operations
> fail (e.g. fchown()), even if the operation is legitimate (does not alter
> the current value).
> 
> This patch avoids this problem by reporting successful operation to user
> space when that operation does not alter the current value of xattrs/attrs.

I must be missing something.  If both the IMA and EVM status flags are
reset after xattr or attr modification, do we really need to prevent
any metadata - same or different - changes?  Both evm_protect_xattr()
and evm_inode_setattr() would need to be modified to allow
INTEGRITY_PASS_IMMUTABLE.

thanks,

Mimi