From: Mengchi Cheng <mengcc@amazon.com>
To: <roberto.sassu@huaweicloud.com>
Cc: <bpf@vger.kernel.org>, <casey@schaufler-ca.com>,
<dmitry.kasatkin@gmail.com>, <eparis@parisplace.org>,
<jmorris@namei.org>, <kamatam@amazon.com>,
<keescook@chromium.org>, <kpsingh@kernel.org>,
<linux-integrity@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
<linux-security-module@vger.kernel.org>,
<linux-unionfs@vger.kernel.org>, <mengcc@amazon.com>,
<miklos@szeredi.hu>, <nicolas.bouchinet@clip-os.org>,
<paul@paul-moore.com>, <reiserfs-devel@vger.kernel.org>,
<roberto.sassu@huawei.com>, <selinux@vger.kernel.org>,
<serge@hallyn.com>, <stephen.smalley.work@gmail.com>,
<yoonjaeh@amazon.com>, <zohar@linux.ibm.com>
Subject: Re: [PATCH] Smack modifications for: security: Allow all LSMs to provide xattrs for inode_init_security hook
Date: Wed, 19 Apr 2023 12:25:16 -0700 [thread overview]
Message-ID: <20230419192516.757220-1-mengcc@amazon.com> (raw)
In-Reply-To: <0fccab67e496f10f4ee7bf2220e70a655013935f.camel@huaweicloud.com>
>
> I got some errors during xattr removal, so not sure if my patch was
> working properly or not (it happened also without it, didn't
> investigate more).
>
> However, I saw another discussion related to transmute:
>
> https://lore.kernel.org/linux-security-module/20230419002338.566487-1-mengcc@amazon.com/
>
> I add the people in CC.
>
> The steps described were so easy to understand and executed, I tried
> without and with overlayfs.
>
> Without:
>
> # echo "_ system rwxatl" > /sys/fs/smackfs/load2
> # mkdir /data
> # chsmack -a "system" /data
> # chsmack -t /data
> # mkdir -p /data/dir1/dir2
> # chsmack /data/dir1
> /data/dir1 access="system" transmute="TRUE"
> # chsmack /data/dir1/dir2
> /data/dir1/dir2 access="system" transmute="TRUE"
>
> It seems to work, right?
>
> With overlay fs it didn't work, same result as the one Mengchi
> reported. Since Mengchi's solution was to set SMK_INODE_CHANGED, and I
> want to get rid of it, I thought to investigate more.
>
> Looking at smack_dentry_create_files_as(), I see that the label of the
> process is overwritten with the label of the transmuting directory.
>
> That causes smack_inode_init_security() to lookup the transmuting rule
> on the overridden credential, and not on the original one.
>
> In the example above, it means that, when overlayfs is creating the new
> inode, the label of the process is system, not _. So no transmute
> permission, and also the xattr will not be added, as observed by
> Mengchi.
>
> Hopefully I undertood the code, so in this particular case we would not
> need to override the label of the process in smack_dentry_create_files_
> as().
>
> If you see smack_inode_init_security():
>
> struct smack_known *skp = smk_of_current();
> struct smack_known *isp = smk_of_inode(inode);
> struct smack_known *dsp = smk_of_inode(dir);
>
> [...]
>
> if (may > 0 && ((may & MAY_TRANSMUTE) != 0) &&
> smk_inode_transmutable(dir)) {
> isp = dsp;
> [...]
>
> xattr->value = kstrdup(isp->smk_known, GFP_NOFS);
>
> This code is telling, if there is a transmute rule, and the directory
> is transmuting, set the label of the new inode to the label of the
> directory. That should be already the result that we wanted to obtain.
>
> The current code should have been doing it by overriding the label of
> the process in smack_dentry_create_files_as() with the label of the
> parent directory, and letting the inode being created with the
> overridden label of the process. The transmute xattr is not set due to
> the problem described above.
>
> So, as a quick test, I kept this patch with the change to xattr2->name,
> and skipped the label override in smack_dentry_create_files_as(). It
> worked, I get the same result as without overlayfs. Wondering if the
> process label override is necessary in other cases.
If I understand correctly, removing the if block below is what you suggested.
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index cfcbb748da25..a867288e9de9 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -4769,8 +4769,8 @@ static int smack_dentry_create_files_as(struct dentry *dentry, int mode,
* providing access is transmuting use the containing
* directory label instead of the process label.
*/
- if (may > 0 && (may & MAY_TRANSMUTE))
- ntsp->smk_task = isp->smk_inode;
+// if (may > 0 && (may & MAY_TRANSMUTE))
+// ntsp->smk_task = isp->smk_inode;
}
return 0;
}
This way will have issue in the following situation on the vanila kernel.
data in the lowerdir has "_" label before overlay and dir1 is already
created in the lowerdir.
# chsmack /data
/data access="_"
# chsmack /data/dir1
/data/dir1 access="system" transmute="TRUE"
Apply overlay on data directory and set the smack rule in the same way.
data has the same smack label.
# chsmack /data
/data access="system" transmute="TRUE"
After that, remove dir1 and mkdir dir1 again. dir1 did not get the correct
label.
# rm -r /data/dir1
# mkdir -p /data/dir1
# chsmack /data/dir1
/data/dir1 access="_"
Since I am not very familiar your change. Could you help check with your
patch will this issue also happen?
Best,
Mengchi
>
> Roberto
next prev parent reply other threads:[~2023-04-19 19:25 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-31 12:32 [PATCH v10 0/4] evm: Do HMAC of multiple per LSM xattrs for new inodes Roberto Sassu
2023-03-31 12:32 ` [PATCH v10 1/4] reiserfs: Add security prefix to xattr name in reiserfs_security_write() Roberto Sassu
2023-04-04 18:25 ` Paul Moore
2023-03-31 12:32 ` [PATCH v10 2/4] security: Allow all LSMs to provide xattrs for inode_init_security hook Roberto Sassu
2023-04-04 18:54 ` Paul Moore
2023-04-05 2:08 ` Casey Schaufler
2023-04-05 9:43 ` Roberto Sassu
2023-04-05 19:59 ` Paul Moore
2023-04-05 20:43 ` Casey Schaufler
2023-04-05 20:49 ` Paul Moore
2023-04-05 21:07 ` Casey Schaufler
2023-04-06 9:14 ` Roberto Sassu
2023-04-06 16:17 ` Casey Schaufler
2023-04-06 9:08 ` Roberto Sassu
2023-04-11 7:22 ` Mimi Zohar
2023-04-11 7:53 ` Roberto Sassu
2023-04-11 16:42 ` Casey Schaufler
2023-04-11 17:23 ` [PATCH] Smack modifications for: " Roberto Sassu
2023-04-11 17:54 ` Casey Schaufler
2023-04-12 7:22 ` Roberto Sassu
2023-04-12 20:29 ` Casey Schaufler
2023-04-13 7:11 ` Roberto Sassu
2023-04-17 16:41 ` Casey Schaufler
2023-04-18 7:05 ` Roberto Sassu
2023-04-18 16:02 ` Casey Schaufler
2023-04-19 13:46 ` Roberto Sassu
2023-04-19 19:25 ` Mengchi Cheng [this message]
2023-04-20 8:48 ` Roberto Sassu
2023-05-08 12:29 ` Roberto Sassu
2023-05-09 23:44 ` Mengchi Cheng
2023-05-09 23:56 ` Casey Schaufler
2023-04-19 21:00 ` Casey Schaufler
2023-04-20 8:50 ` Roberto Sassu
2023-04-20 10:44 ` Mimi Zohar
2023-04-20 14:10 ` Roberto Sassu
2023-04-11 17:25 ` [PATCH v10 2/4] " Roberto Sassu
2023-04-11 17:40 ` Casey Schaufler
2023-03-31 12:32 ` [PATCH v10 3/4] evm: Align evm_inode_init_security() definition with LSM infrastructure Roberto Sassu
2023-04-04 18:56 ` Paul Moore
2023-04-11 7:22 ` Mimi Zohar
2023-04-11 7:58 ` Roberto Sassu
2023-03-31 12:32 ` [PATCH v10 4/4] evm: Support multiple LSMs providing an xattr Roberto Sassu
2023-04-11 7:22 ` Mimi Zohar
2023-04-03 10:36 ` [PATCH v10 0/4] evm: Do HMAC of multiple per LSM xattrs for new inodes Mimi Zohar
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