linux-kernel - Re: [PATCH] ima: don't clear IMA_DIGSIG flag when setting non-IMA xattr

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Message-ID: <gw7xopwfzrp5qatwqa4n4i3miztbvbdolfaa2jxslb5pktc7fs@56r7n6feu55m>
Date: Mon, 15 Sep 2025 12:06:14 +0800
From: Coiby Xu <coxu@...hat.com>
To: Mimi Zohar <zohar@...ux.ibm.com>
Cc: linux-integrity@...r.kernel.org, 
	Roberto Sassu <roberto.sassu@...wei.com>, Dmitry Kasatkin <dmitry.kasatkin@...il.com>, 
	Eric Snowberg <eric.snowberg@...cle.com>, Paul Moore <paul@...l-moore.com>, 
	James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>, 
	"open list:SECURITY SUBSYSTEM" <linux-security-module@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ima: don't clear IMA_DIGSIG flag when setting non-IMA
 xattr

On Wed, Sep 10, 2025 at 08:21:33AM -0400, Mimi Zohar wrote:
>On Wed, 2025-09-10 at 09:36 +0800, Coiby Xu wrote:
>> On Mon, Sep 08, 2025 at 04:58:05PM -0400, Mimi Zohar wrote:
>> > On Mon, 2025-09-08 at 10:53 -0400, Mimi Zohar wrote:
>> > > Hi Coiby,
>> > >
>> > > On Mon, 2025-09-08 at 19:12 +0800, Coiby Xu wrote:
>> > > > >
>> > > > > Even without an IMA appraise policy, the security xattrs are written out to the
>> > > > > filesystem, but the IMA_DIGSIG flag is not cached.
>> > > >
>> > > > It seems I miss some context for the above sentence. If no IMA policy is
>> > > > configured, no ima_iint_cache will be created. If you mean non-appraisal
>> > > > policy, will not caching IMA_DIGSIG flag cause any problem?
>> > >
>> > > Sorry.  What I was trying to say is that your test program illustrates the
>> > > problem both with or without any of the boot command line options as you
>> > > suggested - "ima_appraise=fix evm=fix ima_policy=appraise_tcb".  Writing some
>> > > other security xattr is a generic problem, whether the file is in policy or not,
>> > > whether IMA or EVM are in fix mode or not.  The rpm-plugin-ima should install
>> > > the IMA signature regardless.
>> >
>> > My mistake.  An appraise policy indeed needs to be defined for the file
>> > signature to be replaced with a file hash.
>>
>> Thanks for the clarification! rpm-plugin-ima does try to install IMA
>> signature as shown from the following strace output,
>
>Agreed. I was referring to the SELinux label, which would be installed for new
>files, but not necessarily re-installed on existing files.  The test program
>simplified testing.  Thank you.

My pleasure! Note reinstalling a package using dnf/rpm is equivalent to
installing a new package in terms of this issue. Because according to
the strace output and rpm's source code, when reinstalling a package,
the following steps happens, taking lnstat as an example,

1. A temporary file "lnstat;68aee3f4" is created
2. Read the content from RPM and write it to lnstat;68aee3f4
3. Set file permission
4. Set security.ima by rpm-plugin-ima
5. Set security.selinux by rpm-plugin-selinux
6. Rename "lnstat;68aee3f4" to lnstat

And here's the strace output,

     # strace rpm --reinstall ip*.rpm
     openat(11, "lnstat;68aee3f4", O_WRONLY|O_CREAT|O_EXCL, 0200) = 12
     dup(12)                                 = 13
     write(13, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0'\0\0\0\0\0\0"..., 19256) = 19256
     close(13)                               = 0
     getuid()                                = 0
     fchown(12, 0, 0)                        = 0
     fchmod(12, 0755)                        = 0
     getuid()                                = 0
     utimensat(12, NULL, [{tv_sec=1734480000, tv_nsec=0} /* 2024-12-17T19:00:00-0500 */, {tv_sec=1734480000, tv_nsec=0} /* 2024-12-17T19:00:00-0500 */], 0) = 0
     fsetxattr(12, "security.ima", "\3\2\4\3232\4I\0f0d\0020O\231\341q\323Q\322\235\341\7\323\224\205\2104\24\241\331#"..., 111, 0) = 0
     fsetxattr(12, "security.selinux", "system_u:object_r:bin_t:s0", 27, 0) = 0
     close(12)                               = 0
     ...
     renameat(11, "lnstat;68aee3f4", 11, "lnstat") = 0

>
>Mimi
>

-- 
Best regards,
Coiby