[PATCH v2] ima: fallback to using i_version to detect file change

[Mimi Zohar <zohar@linux.ibm.com>]

Commit db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the i_version")
replaced detecting file change based on i_version with
STATX_CHANGE_COOKIE.

On filesystems without STATX_CHANGE_COOKIE enabled, revert back to
detecting file change based on i_version.

On filesystems which do not support either, assume the file changed.

Reported-by: Roberto Sassu <roberto.sassu@huawei.com>
Fixes: db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the i_version")
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
---
Changelog v2:
- Use the real_inode's iversion to detect file change on overlayfs
- Add Roberto's Reported-by tag

 security/integrity/ima/ima_api.c  | 13 +++++++----
 security/integrity/ima/ima_main.c | 39 ++++++++++++++++++++++++-------
 2 files changed, 40 insertions(+), 12 deletions(-)

diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
index c35ea613c9f8..28cf1fe07f8f 100644
--- a/security/integrity/ima/ima_api.c
+++ b/security/integrity/ima/ima_api.c
@@ -267,15 +267,20 @@ int ima_collect_measurement(struct ima_iint_cache *iint, struct file *file,
 		goto out;
 
 	/*
-	 * Detecting file change is based on i_version. On filesystems
-	 * which do not support i_version, support was originally limited
-	 * to an initial measurement/appraisal/audit, but was modified to
-	 * assume the file changed.
+	 * Detect file change based on STATX_CHANGE_COOKIE, when supported,
+	 * and fallback to detecting file change based on i_version.
+	 *
+	 * On filesystems which did not support i_version, support was
+	 * originally limited to an initial measurement/appraisal/audit,
+	 * but was later modified to assume the file changed.
 	 */
 	result = vfs_getattr_nosec(&file->f_path, &stat, STATX_CHANGE_COOKIE,
 				   AT_STATX_SYNC_AS_STAT);
 	if (!result && (stat.result_mask & STATX_CHANGE_COOKIE))
 		i_version = stat.change_cookie;
+	else if (IS_I_VERSION(real_inode))
+		i_version = inode_peek_iversion(real_inode);
+
 	hash.hdr.algo = algo;
 	hash.hdr.length = hash_digest_size[algo];
 
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 1d6229b156fb..4fc383479847 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -180,6 +180,34 @@ static void ima_rdwr_violation_check(struct file *file,
 				  "invalid_pcr", "open_writers");
 }
 
+/*
+ * Detect file change based on STATX_CHANGE_COOKIE, when supported, and
+ * fallback to detecting file change based on i_version. On filesystems
+ * which do not support either, assume the file changed.
+ */
+static bool ima_detect_file_change(struct ima_iint_cache *iint,
+				   struct inode *inode, struct file *file)
+{
+	struct kstat stat;
+	int result;
+
+	result = vfs_getattr_nosec(&file->f_path, &stat, STATX_CHANGE_COOKIE,
+				   AT_STATX_SYNC_AS_STAT);
+
+	if (!result && stat.result_mask & STATX_CHANGE_COOKIE &&
+	    stat.change_cookie != iint->real_inode.version)
+		return true;
+	else if (!(stat.result_mask & STATX_CHANGE_COOKIE) &&
+		 IS_I_VERSION(inode) &&
+		 !(inode_eq_iversion(inode, iint->real_inode.version)))
+		return true;
+	else if (!(stat.result_mask & STATX_CHANGE_COOKIE) &&
+		 !(IS_I_VERSION(inode)))
+		return true;
+
+	return false;
+}
+
 static void ima_check_last_writer(struct ima_iint_cache *iint,
 				  struct inode *inode, struct file *file)
 {
@@ -191,18 +219,13 @@ static void ima_check_last_writer(struct ima_iint_cache *iint,
 
 	mutex_lock(&iint->mutex);
 	if (atomic_read(&inode->i_writecount) == 1) {
-		struct kstat stat;
-
 		clear_bit(IMA_EMITTED_OPENWRITERS, &iint->atomic_flags);
 
 		update = test_and_clear_bit(IMA_UPDATE_XATTR,
 					    &iint->atomic_flags);
-		if ((iint->flags & IMA_NEW_FILE) ||
-		    vfs_getattr_nosec(&file->f_path, &stat,
-				      STATX_CHANGE_COOKIE,
-				      AT_STATX_SYNC_AS_STAT) ||
-		    !(stat.result_mask & STATX_CHANGE_COOKIE) ||
-		    stat.change_cookie != iint->real_inode.version) {
+
+		if (iint->flags & IMA_NEW_FILE ||
+		    ima_detect_file_change(iint, inode, file)) {
 			iint->flags &= ~(IMA_DONE_MASK | IMA_NEW_FILE);
 			iint->measured_pcrs = 0;
 			if (update)
-- 
2.53.0