Re: [RFC 20/20] ima: Setup securityfs_ns for IMA namespace
flat view
From: Stefan Berger <stefanb@linux.ibm.com>
Date: 2021-12-01 18:14:20
Also in:
linux-integrity, lkml
On 12/1/21 12:56, James Bottomley wrote:
On Tue, 2021-11-30 at 11:06 -0500, Stefan Berger wrote: [...]quoted
+ +/* + * Fix the ownership (uid/gid) of the dentry's that couldn't be set at the + * time of their creation because the user namespace wasn't configured, yet. + */ +static void ima_fs_ns_fixup_uid_gid(struct ima_namespace *ns) +{ + struct inode *inode; + size_t i; + + if (ns->file_ownership_fixes_done || + ns->user_ns->uid_map.nr_extents == 0) + return; + + ns->file_ownership_fixes_done = true; + for (i = 0; i < IMAFS_DENTRY_LAST; i++) { + if (!ns->dentry[i]) + continue; + inode = ns->dentry[i]->d_inode; + inode->i_uid = make_kuid(ns->user_ns, 0); + inode->i_gid = make_kgid(ns->user_ns, 0); + } +} + +/* Fix the permissions when a file is opened */ +int ima_fs_ns_permission(struct user_namespace *mnt_userns, struct inode *inode, + int mask) +{ + ima_fs_ns_fixup_uid_gid(get_current_ns()); + return generic_permission(mnt_userns, inode, mask); +} + +const struct inode_operations ima_fs_ns_inode_operations = { + .lookup = simple_lookup, + .permission = ima_fs_ns_permission, +}; +In theory this uid/gid shifting should have already been done for you and all of the above code should be unnecessary. What is supposed to happen is that the mount of securityfs_ns in the new user namespace should pick up a superblock s_user_ns for that new user namespace. Now inode_alloc() uses i_uid_write(inode, 0) which maps back through the s_user_ns to obtain the owner of the user namespace. What can happen is that if you do the inode allocation before (or even without) writing to the uid_map file, it maps back through an empty map and ends up with -1 for i_uid ... is this what you're seeing?
I tried this with runc and a user namespace active mapping uid 1000 on the host to uid 0 in the container. There I run into the problem that all of the files and directories without the above work-around are mapped to 'nobody', just like all the files in sysfs in this case are also mapped to nobody. This code resolved the issue. sh-5.1# ls -l /sys/ total 0 drwxr-xr-x. 2 nobody nobody 0 Dec 1 18:06 block drwxr-xr-x. 28 nobody nobody 0 Dec 1 18:06 bus drwxr-xr-x. 54 nobody nobody 0 Dec 1 18:06 class drwxr-xr-x. 4 nobody nobody 0 Dec 1 18:06 dev drwxr-xr-x. 15 nobody nobody 0 Dec 1 18:06 devices drwxrwxrwt. 2 root root 40 Dec 1 18:06 firmware drwxr-xr-x. 9 nobody nobody 0 Dec 1 18:06 fs drwxr-xr-x. 16 nobody nobody 0 Dec 1 18:06 kernel drwxr-xr-x. 161 nobody nobody 0 Dec 1 18:06 module drwxr-xr-x. 3 nobody nobody 0 Dec 1 18:06 power sh-5.1# ls -l /sys/kernel/security/ total 0 lr--r--r--. 1 nobody nobody 0 Dec 1 18:06 ima -> integrity/ima drwxr-xr-x. 3 nobody nobody 0 Dec 1 18:06 integrity sh-5.1# ls -l /sys/kernel/security/ima/ total 0 -r--r-----. 1 root root 0 Dec 1 18:06 ascii_runtime_measurements -r--r-----. 1 root root 0 Dec 1 18:06 binary_runtime_measurements -rw-------. 1 root root 0 Dec 1 18:06 policy -r--r-----. 1 root root 0 Dec 1 18:06 runtime_measurements_count -r--r-----. 1 root root 0 Dec 1 18:06 violations The nobody's are obviously sufficient to cd into the directories, but for file accesses I wanted to see root and no changes to permissions. Stefan
James