[PATCH v6 3/3] ima: Use kstat.ctime as a fallback for change detection

[Frederick Lawler <fred@cloudflare.com>]

IMA performs unnecessary measurements on files in file systems
that do not set STATX_CHANGE_COOKIE in the vfs_getattr_nosec()'s
result mask.

Commit 1cf7e834a6fb ("xfs: switch to multigrain timestamps") introduced
multigrain timestamps to XFS, and this made XFS no longer report an
inode's i_version used by ima_collect_measurement() for change detection.

Additionally, ima_check_last_writer() & integrity_inode_attributes_change()
think there's a change for a file, when there may have not been.

Most file systems already use ctime for change detection, therefore, make
IMA fall back to compare against ctime when a file system does not report
an i_version to the kstat.change_cookie.

If neither fields are reported by vfs_getattr_nosec(), assume the file
changed.

timespec64_to_ns() is chosen to avoid adding extra storage to
integrity_inode_attributes by leveraging the existing version field.

XFS is just one example. There may be more file systems that adopt
multigrain timestamps in the future.

Link: https://lore.kernel.org/all/aTspr4_h9IU4EyrR@CMGLRV3
Fixes: 1cf7e834a6fb ("xfs: switch to multigrain timestamps")
Suggested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Frederick Lawler <fred@cloudflare.com>
---
 include/linux/integrity.h         |  6 +++++-
 security/integrity/ima/ima_api.c  | 11 ++++++++---
 security/integrity/ima/ima_main.c |  2 +-
 3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/include/linux/integrity.h b/include/linux/integrity.h
index 382c783f0fa3ae4a938cdf9559291ba1903a378e..ec2c94907f417c4a71ecce29ac79edac9bc2c6f8 100644
--- a/include/linux/integrity.h
+++ b/include/linux/integrity.h
@@ -10,6 +10,7 @@
 #include <linux/fs.h>
 #include <linux/iversion.h>
 #include <linux/kernel.h>
+#include <linux/time64.h>
 
 enum integrity_status {
 	INTEGRITY_PASS = 0,
@@ -58,6 +59,9 @@ integrity_inode_attrs_stat_changed
 	if (stat->result_mask & STATX_CHANGE_COOKIE)
 		return stat->change_cookie != attrs->version;
 
+	if (stat->result_mask & STATX_CTIME)
+		return timespec64_to_ns(&stat->ctime) != (s64)attrs->version;
+
 	return true;
 }
 
@@ -84,7 +88,7 @@ integrity_inode_attrs_changed(const struct integrity_inode_attributes *attrs,
 	 * only for IMA if vfs_getattr_nosec() fails.
 	 */
 	if (!file || vfs_getattr_nosec(&file->f_path, &stat,
-				       STATX_CHANGE_COOKIE,
+				       STATX_CHANGE_COOKIE | STATX_CTIME,
 				       AT_STATX_SYNC_AS_STAT))
 		return !IS_I_VERSION(inode) ||
 		       !inode_eq_iversion(inode, attrs->version);
diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
index c35ea613c9f8d404ba4886e3b736c3bab29d1668..e47d6281febc15a0ac1bd2ea1d28fea4d0cd5c58 100644
--- a/security/integrity/ima/ima_api.c
+++ b/security/integrity/ima/ima_api.c
@@ -272,10 +272,15 @@ int ima_collect_measurement(struct ima_iint_cache *iint, struct file *file,
 	 * to an initial measurement/appraisal/audit, but was modified to
 	 * assume the file changed.
 	 */
-	result = vfs_getattr_nosec(&file->f_path, &stat, STATX_CHANGE_COOKIE,
+	result = vfs_getattr_nosec(&file->f_path, &stat,
+				   STATX_CHANGE_COOKIE | STATX_CTIME,
 				   AT_STATX_SYNC_AS_STAT);
-	if (!result && (stat.result_mask & STATX_CHANGE_COOKIE))
-		i_version = stat.change_cookie;
+	if (!result) {
+		if (stat.result_mask & STATX_CHANGE_COOKIE)
+			i_version = stat.change_cookie;
+		else if (stat.result_mask & STATX_CTIME)
+			i_version = timespec64_to_ns(&stat.ctime);
+	}
 	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 8cb17c9d446caaa5a98f5ec8f027c17ba7babca8..776db158b0bd8a0d053729ac0cc15af8b6020a98 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -199,7 +199,7 @@ static void ima_check_last_writer(struct ima_iint_cache *iint,
 					    &iint->atomic_flags);
 		if ((iint->flags & IMA_NEW_FILE) ||
 		    vfs_getattr_nosec(&file->f_path, &stat,
-				      STATX_CHANGE_COOKIE,
+				      STATX_CHANGE_COOKIE | STATX_CTIME,
 				      AT_STATX_SYNC_AS_STAT) ||
 		    integrity_inode_attrs_stat_changed(&iint->real_inode,
 						       &stat)) {

-- 
2.43.0