Re: [PATCH v5 15/16] ima: Move dentries into ima_namespace

Stefan Berger
2021-12-09T15:01:16Z

On 12/9/21 09:41, Christian Brauner wrote:
> On Thu, Dec 09, 2021 at 03:37:49PM +0100, Christian Brauner wrote:
>> On Thu, Dec 09, 2021 at 03:34:28PM +0100, Christian Brauner wrote:
>>> On Wed, Dec 08, 2021 at 05:18:17PM -0500, Stefan Berger wrote:
>>>> Move the dentries into the ima_namespace for reuse by virtualized
>>>> SecurityFS. Implement function freeing the dentries in order of
>>>> files and symlinks before directories.
>>>>
>>>> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
>>>> ---
>>> This doesn't work as implemented, I think.
>>>
>>> What I would have preferred and what I tried to explain in the earlier
>>> review was:
>>> Keep the dentry stashing global since it is only needed for init_ima_ns.
>>> Then struct ima_namespace becomes way smaller and simpler.
>>> If you do that then it makes sense to remove the additional dget() in
>>> securityfs_create_dentry() for non-init_ima_ns.
>>> Then you can rely on auto-cleanup in .kill_sb() or on
>>> ima_securityfs_init() failure and you only need to call
>>> ima_fs_ns_free_dentries() if ns != init_ima_ns.
> s/ns != init_ima_ns/ns == init_ima_ns/
>
>>> IIuc, it seems you're currently doing one dput() too many since you're
>>> calling securityfs_remove() in the error path for non-init_ima_ns which
>>> relies on the previous increased dget() which we removed.

I thought that securityfs_remove() will now simply influence when a
dentry is removed and freed. If we call it in the error cleanup path in
non-init_user_ns case it would go away right there and leave nothing to
do for .kill_sb() while an additional dget() would require the cleanup
as well but do another cleanup then in .kill_sb() since that brings the
reference count to 0 via the dput()s that it does. Am I wrong on this?

>> If you really want to move the dentry stashing into struct ima_namespace
>> even though it's really unnecessary then you may as well not care about
>> the auto-cleanup and keep that additional ima_fs_ns_free_dentries(ns)
>> call in .kill_sb(). But I really think not dragging dentry stashing into
>> struct ima_namespace is the correct way to go about this.

I moved the dentries into the ima_namespace so that each namespace holds
a pointer to the dentries it owns and isolates them. We certainly
wouldn't want to have IMA namespaces write over the current static
variables and create a mess with what these are pointing to (
https://elixir.bootlin.com/linux/latest/source/security/integrity/ima/ima_fs.c#L359
) and possible race conditions when doing parallel initialization (if
that's possible at all). This also reduces the code size and we don't
need two different implementations for init_user_ns and
non-init_user_ns. So I don't quite understand whey we wouldn't want to
have the dentries isolated via ima_namespace?

>>
>>>> include/linux/ima.h | 13 ++++++
>>>> security/integrity/ima/ima_fs.c | 72 ++++++++++++++++++---------------
>>>> 2 files changed, 52 insertions(+), 33 deletions(-)
>>>>
>>>> diff --git a/include/linux/ima.h b/include/linux/ima.h
>>>> index 3aaf6e806db4..4dd64e318b15 100644
>>>> --- a/include/linux/ima.h
>>>> +++ b/include/linux/ima.h
>>>> @@ -220,6 +220,17 @@ struct ima_h_table {
>>>> struct hlist_head queue[IMA_MEASURE_HTABLE_SIZE];
>>>> };
>>>>
>>>> +enum {
>>>> + IMAFS_DENTRY_DIR = 0,
>>>> + IMAFS_DENTRY_SYMLINK,
>>>> + IMAFS_DENTRY_BINARY_RUNTIME_MEASUREMENTS,
>>>> + IMAFS_DENTRY_ASCII_RUNTIME_MEASUREMENTS,
>>>> + IMAFS_DENTRY_RUNTIME_MEASUREMENTS_COUNT,
>>>> + IMAFS_DENTRY_VIOLATIONS,
>>>> + IMAFS_DENTRY_IMA_POLICY,
>>>> + IMAFS_DENTRY_LAST
>>>> +};
>>>> +
>>>> struct ima_namespace {
>>>> struct kref kref;
>>>> struct user_namespace *user_ns;
>>>> @@ -266,6 +277,8 @@ struct ima_namespace {
>>>> struct mutex ima_write_mutex;
>>>> unsigned long ima_fs_flags;
>>>> int valid_policy;
>>>> +
>>>> + struct dentry *dentry[IMAFS_DENTRY_LAST];
>>>> };
>>>>
>>>> extern struct ima_namespace init_ima_ns;
>>>> diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
>>>> index a749a3e79304..3810d11fb463 100644
>>>> --- a/security/integrity/ima/ima_fs.c
>>>> +++ b/security/integrity/ima/ima_fs.c
>>>> @@ -360,14 +360,6 @@ static ssize_t ima_write_policy(struct file *file, const char __user *buf,
>>>> return result;
>>>> }
>>>>
>>>> -static struct dentry *ima_dir;
>>>> -static struct dentry *ima_symlink;
>>>> -static struct dentry *binary_runtime_measurements;
>>>> -static struct dentry *ascii_runtime_measurements;
>>>> -static struct dentry *runtime_measurements_count;
>>>> -static struct dentry *violations;
>>>> -static struct dentry *ima_policy;
>>>> -
>>>> enum ima_fs_flags {
>>>> IMA_FS_BUSY,
>>>> };
>>>> @@ -437,8 +429,8 @@ static int ima_release_policy(struct inode *inode, struct file *file)
>>>>
>>>> ima_update_policy(ns);
>>>> #if !defined(CONFIG_IMA_WRITE_POLICY) && !defined(CONFIG_IMA_READ_POLICY)
>>>> - securityfs_remove(ima_policy);
>>>> - ima_policy = NULL;
>>>> + securityfs_remove(ns->dentry[IMAFS_DENTRY_IMA_POLICY]);
>>>> + ns->dentry[IMAFS_DENTRY_IMA_POLICY] = NULL;
>>>> #elif defined(CONFIG_IMA_WRITE_POLICY)
>>>> clear_bit(IMA_FS_BUSY, &ns->ima_fs_flags);
>>>> #elif defined(CONFIG_IMA_READ_POLICY)
>>>> @@ -455,58 +447,72 @@ static const struct file_operations ima_measure_policy_ops = {
>>>> .llseek = generic_file_llseek,
>>>> };
>>>>
>>>> -int __init ima_fs_init(void)
>>>> +static void ima_fs_ns_free_dentries(struct ima_namespace *ns)
>>>> {
>>>> - ima_dir = securityfs_create_dir("ima", integrity_dir);
>>>> - if (IS_ERR(ima_dir))
>>>> + int i;
>>>> +
>>>> + for (i = IMAFS_DENTRY_LAST - 1; i >= 0; i--)
>>>> + securityfs_remove(ns->dentry[i]);
>>>> +
>>>> + memset(ns->dentry, 0, sizeof(ns->dentry));
>>>> +}
>>>> +
>>>> +static int __init ima_securityfs_init(struct user_namespace *user_ns)
>>>> +{
>>>> + struct ima_namespace *ns = user_ns->ima_ns;
>>>> + struct dentry *ima_dir;
>>>> +
>>>> + ns->dentry[IMAFS_DENTRY_DIR] = securityfs_create_dir("ima", integrity_dir);
>>>> + if (IS_ERR(ns->dentry[IMAFS_DENTRY_DIR]))
>>>> return -1;
>>>> + ima_dir = ns->dentry[IMAFS_DENTRY_DIR];
>>>>
>>>> - ima_symlink = securityfs_create_symlink("ima", NULL, "integrity/ima",
>>>> - NULL);
>>>> - if (IS_ERR(ima_symlink))
>>>> + ns->dentry[IMAFS_DENTRY_SYMLINK] =
>>>> + securityfs_create_symlink("ima", NULL, "integrity/ima", NULL);
>>>> + if (IS_ERR(ns->dentry[IMAFS_DENTRY_SYMLINK]))
>>>> goto out;
>>>>
>>>> - binary_runtime_measurements =
>>>> + ns->dentry[IMAFS_DENTRY_BINARY_RUNTIME_MEASUREMENTS] =
>>>> securityfs_create_file("binary_runtime_measurements",
>>>> S_IRUSR | S_IRGRP, ima_dir, NULL,
>>>> &ima_measurements_ops);
>>>> - if (IS_ERR(binary_runtime_measurements))
>>>> + if (IS_ERR(ns->dentry[IMAFS_DENTRY_BINARY_RUNTIME_MEASUREMENTS]))
>>>> goto out;
>>>>
>>>> - ascii_runtime_measurements =
>>>> + ns->dentry[IMAFS_DENTRY_ASCII_RUNTIME_MEASUREMENTS] =
>>>> securityfs_create_file("ascii_runtime_measurements",
>>>> S_IRUSR | S_IRGRP, ima_dir, NULL,
>>>> &ima_ascii_measurements_ops);
>>>> - if (IS_ERR(ascii_runtime_measurements))
>>>> + if (IS_ERR(ns->dentry[IMAFS_DENTRY_ASCII_RUNTIME_MEASUREMENTS]))
>>>> goto out;
>>>>
>>>> - runtime_measurements_count =
>>>> + ns->dentry[IMAFS_DENTRY_RUNTIME_MEASUREMENTS_COUNT] =
>>>> securityfs_create_file("runtime_measurements_count",
>>>> S_IRUSR | S_IRGRP, ima_dir, NULL,
>>>> &ima_measurements_count_ops);
>>>> - if (IS_ERR(runtime_measurements_count))
>>>> + if (IS_ERR(ns->dentry[IMAFS_DENTRY_RUNTIME_MEASUREMENTS_COUNT]))
>>>> goto out;
>>>>
>>>> - violations =
>>>> + ns->dentry[IMAFS_DENTRY_VIOLATIONS] =
>>>> securityfs_create_file("violations", S_IRUSR | S_IRGRP,
>>>> ima_dir, NULL, &ima_htable_violations_ops);
>>>> - if (IS_ERR(violations))
>>>> + if (IS_ERR(ns->dentry[IMAFS_DENTRY_VIOLATIONS]))
>>>> goto out;
>>>>
>>>> - ima_policy = securityfs_create_file("policy", POLICY_FILE_FLAGS,
>>>> + ns->dentry[IMAFS_DENTRY_IMA_POLICY] =
>>>> + securityfs_create_file("policy", POLICY_FILE_FLAGS,
>>>> ima_dir, NULL,
>>>> &ima_measure_policy_ops);
>>>> - if (IS_ERR(ima_policy))
>>>> + if (IS_ERR(ns->dentry[IMAFS_DENTRY_IMA_POLICY]))
>>>> goto out;
>>>>
>>>> return 0;
>>>> out:
>>>> - securityfs_remove(violations);
>>>> - securityfs_remove(runtime_measurements_count);
>>>> - securityfs_remove(ascii_runtime_measurements);
>>>> - securityfs_remove(binary_runtime_measurements);
>>>> - securityfs_remove(ima_symlink);
>>>> - securityfs_remove(ima_dir);
>>>> - securityfs_remove(ima_policy);
>>>> + ima_fs_ns_free_dentries(ns);
>>>> return -1;
>>>> }
>>>> +
>>>> +int __init ima_fs_init(void)
>>>> +{
>>>> + return ima_securityfs_init(&init_user_ns);
>>>> +}
>>>> --
>>>> 2.31.1
>>>>
>>>>