[RFC][PATCH v2 2/9] ima: Nest iint mutex for DIGEST_LIST_CHECK hook

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

Invoking digest_cache_get() inside the iint->mutex critical region can
cause deadlocks due to the fact that IMA can be recursively invoked for
reading the digest list. The deadlock would occur if the digest_cache LSM
attempts to read the same inode that is already locked by IMA.

However, since the digest_cache LSM makes sure that the above situation
never happens, as it checks the inodes, it is safe to call
digest_cache_get() inside the critical region and nest the iint->mutex
when the DIGEST_LIST_CHECK hook is executed.

Add a lockdep subclass to the iint->mutex, that is 0 if the IMA hook
executed is not DIGEST_LIST_CHECK, and 1 when it is. Since lockdep allows
nesting with higher classes and subclasses, that effectively eliminates the
warning about the unsafe lock.

Pass the new lockdep subclass (nested variable) from ima_inode_get() to
ima_iint_init_always() and ima_iint_lockdep_annotate().

Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
---
 security/integrity/ima/ima.h      |  2 +-
 security/integrity/ima/ima_iint.c | 17 +++++++++++------
 security/integrity/ima/ima_main.c |  6 +++---
 3 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index cea4517e73ab..c9140a57b591 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -216,7 +216,7 @@ static inline void ima_inode_set_iint(const struct inode *inode,
 }
 
 struct ima_iint_cache *ima_iint_find(struct inode *inode);
-struct ima_iint_cache *ima_inode_get(struct inode *inode);
+struct ima_iint_cache *ima_inode_get(struct inode *inode, bool nested);
 void ima_inode_free(struct inode *inode);
 void __init ima_iintcache_init(void);
 
diff --git a/security/integrity/ima/ima_iint.c b/security/integrity/ima/ima_iint.c
index e7c9c216c1c6..c98a30815c8a 100644
--- a/security/integrity/ima/ima_iint.c
+++ b/security/integrity/ima/ima_iint.c
@@ -39,9 +39,12 @@ struct ima_iint_cache *ima_iint_find(struct inode *inode)
  * files both on overlayfs and on underlying fs, we need to annotate the iint
  * mutex to avoid lockdep false positives related to IMA + overlayfs.
  * See ovl_lockdep_annotate_inode_mutex_key() for more details.
+ *
+ * In addition to that, safely ignored nested locks for digest lists, since
+ * the digest_cache LSM prevents circular dependencies.
  */
 static inline void ima_iint_lockdep_annotate(struct ima_iint_cache *iint,
-					     struct inode *inode)
+					     struct inode *inode, bool nested)
 {
 #ifdef CONFIG_LOCKDEP
 	static struct lock_class_key ima_iint_mutex_key[IMA_MAX_NESTING];
@@ -51,12 +54,13 @@ static inline void ima_iint_lockdep_annotate(struct ima_iint_cache *iint,
 	if (WARN_ON_ONCE(depth < 0 || depth >= IMA_MAX_NESTING))
 		depth = 0;
 
-	lockdep_set_class(&iint->mutex, &ima_iint_mutex_key[depth]);
+	lockdep_set_class_and_subclass(&iint->mutex, &ima_iint_mutex_key[depth],
+				       nested);
 #endif
 }
 
 static void ima_iint_init_always(struct ima_iint_cache *iint,
-				 struct inode *inode)
+				 struct inode *inode, bool nested)
 {
 	iint->ima_hash = NULL;
 	iint->version = 0;
@@ -69,7 +73,7 @@ static void ima_iint_init_always(struct ima_iint_cache *iint,
 	iint->ima_creds_status = INTEGRITY_UNKNOWN;
 	iint->measured_pcrs = 0;
 	mutex_init(&iint->mutex);
-	ima_iint_lockdep_annotate(iint, inode);
+	ima_iint_lockdep_annotate(iint, inode, nested);
 }
 
 static void ima_iint_free(struct ima_iint_cache *iint)
@@ -82,13 +86,14 @@ static void ima_iint_free(struct ima_iint_cache *iint)
 /**
  * ima_inode_get - Find or allocate an iint associated with an inode
  * @inode: Pointer to the inode
+ * @nested: Whether or not the iint->mutex lock can be nested
  *
  * Find an iint associated with an inode, and allocate a new one if not found.
  * Caller must lock i_mutex.
  *
  * Return: An iint on success, NULL on error.
  */
-struct ima_iint_cache *ima_inode_get(struct inode *inode)
+struct ima_iint_cache *ima_inode_get(struct inode *inode, bool nested)
 {
 	struct ima_iint_cache *iint;
 
@@ -100,7 +105,7 @@ struct ima_iint_cache *ima_inode_get(struct inode *inode)
 	if (!iint)
 		return NULL;
 
-	ima_iint_init_always(iint, inode);
+	ima_iint_init_always(iint, inode, nested);
 
 	inode->i_flags |= S_IMA;
 	ima_inode_set_iint(inode, iint);
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 780627b0cde7..18285fc8ac07 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -248,7 +248,7 @@ static int process_measurement(struct file *file, const struct cred *cred,
 	inode_lock(inode);
 
 	if (action) {
-		iint = ima_inode_get(inode);
+		iint = ima_inode_get(inode, func == DIGEST_LIST_CHECK);
 		if (!iint)
 			rc = -ENOMEM;
 	}
@@ -699,7 +699,7 @@ static void ima_post_create_tmpfile(struct mnt_idmap *idmap,
 		return;
 
 	/* Nothing to do if we can't allocate memory */
-	iint = ima_inode_get(inode);
+	iint = ima_inode_get(inode, false);
 	if (!iint)
 		return;
 
@@ -731,7 +731,7 @@ static void ima_post_path_mknod(struct mnt_idmap *idmap, struct dentry *dentry)
 		return;
 
 	/* Nothing to do if we can't allocate memory */
-	iint = ima_inode_get(inode);
+	iint = ima_inode_get(inode, false);
 	if (!iint)
 		return;
 
-- 
2.34.1