On 12/11/21 04:50, Christian Brauner wrote:
On Fri, Dec 10, 2021 at 08:57:11AM -0500, Stefan Berger wrote:
there anything that would prevent us from setns()'ing to that target user
namespace so that we would now see that of a user namespace that we are not
allowed to see?
If you're really worried about someone being able to access a securityfs
instance whose userns doesn't match the userns the securityfs instance
was mounted in there are multiple ways to fix it. The one that I tend to
prefer is:
From e0ff6a8dcc573763568e685dd70d1547efd68df9 Mon Sep 17 00:00:00 2001
From: Christian Brauner <christian.brauner@xxxxxxxxxx>
Date: Fri, 10 Dec 2021 11:47:37 +0100
Subject: !!!! HERE BE DRAGONS - COMPLETELY UNTESTED !!!!
securityfs: only allow access to securityfs from within same namespace
Limit opening of securityfs files to callers located in the same namespace.
---
security/inode.c | 33 +++++++++++++++++++++++++++++++--
1 file changed, 31 insertions(+), 2 deletions(-)
diff --git a/security/inode.c b/security/inode.c
index eaccba7017d9..9eaf757c08cb 100644
--- a/security/inode.c
+++ b/security/inode.c
@@ -80,6 +80,35 @@ static struct file_system_type fs_type = {
.fs_flags = FS_USERNS_MOUNT,
};
+static int securityfs_permission(struct user_namespace *mnt_userns,
+ struct inode *inode, int mask)
+{
+ int err;
+
+ err = generic_permission(&init_user_ns, inode, mask);
+ if (!err) {
+ if (inode->i_sb->s_user_ns != current_user_ns())
+ err = -EACCES;
+ }
+
+ return err;
+}
+
+const struct inode_operations securityfs_dir_inode_operations = {
+ .permission = securityfs_permission,
+ .lookup = simple_lookup,
+};
+
+const struct file_operations securityfs_dir_operations = {
+ .permission = securityfs_permission,
This interface function on file operations doesn't exist.
I'll use the inode_operations and also hook it to the root dentry of the
super_block. Then there's no need to have this check on symlinks and
files...
+ .open = dcache_dir_open,
+ .release = dcache_dir_close,
+ .llseek = dcache_dir_lseek,
+ .read = generic_read_dir,
+ .iterate_shared = dcache_readdir,
+ .fsync = noop_fsync,
+};
+
/**
* securityfs_create_dentry - create a dentry in the securityfs filesystem
*
@@ -167,8 +196,8 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode,
inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
inode->i_private = data;
if (S_ISDIR(mode)) {
- inode->i_op = &simple_dir_inode_operations;
- inode->i_fop = &simple_dir_operations;
+ inode->i_op = &securityfs_dir_inode_operations;
+ inode->i_fop = &securityfs_dir_operations;
inc_nlink(inode);
inc_nlink(dir);
} else if (S_ISLNK(mode)) {
