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

Stefan Berger <stefanb@xxxxxxxxxxxxx> · Thu, 9 Dec 2021 10:00:59 -0500

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@xxxxxxxxxxxxx>
---
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








