linux-kernel - [PATCH] ima: detect changes to the backing overlay file

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Date:   Tue, 24 Oct 2023 08:08:45 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     linux-integrity@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Mimi Zohar <zohar@...ux.ibm.com>,
        Raul Rangel <rrangel@...omium.org>,
        Eric Snowberg <eric.snowberg@...cle.com>,
        Amir Goldstein <amir73il@...il.com>
Subject: [PATCH] ima: detect changes to the backing overlay file

Commit 18b44bc5a672 ("ovl: Always reevaluate the file signature for
IMA") forced signature re-evaulation on every file access.

IMA does not detect changes made to the backing overlay file.  Instead
of always re-evaluating the file's integrity, detect a change to the
backing overlay file, by comparing the i_version, as stored in the iint,
with the backing file's i_version.

Signed-off-by: Mimi Zohar <zohar@...ux.ibm.com>
---
 fs/overlayfs/super.c              |  2 +-
 security/integrity/ima/ima_api.c  |  1 +
 security/integrity/ima/ima_main.c | 21 ++++++++++++++++++++-
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index def266b5e2a3..4d9137ba2293 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1482,7 +1482,7 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 		ovl_trusted_xattr_handlers;
 	sb->s_fs_info = ofs;
 	sb->s_flags |= SB_POSIXACL;
-	sb->s_iflags |= SB_I_SKIP_SYNC | SB_I_IMA_UNVERIFIABLE_SIGNATURE;
+	sb->s_iflags |= SB_I_SKIP_SYNC;
 
 	err = -ENOMEM;
 	root_dentry = ovl_get_root(sb, ctx->upper.dentry, oe);
diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
index 452e80b541e5..d1c718ef9295 100644
--- a/security/integrity/ima/ima_api.c
+++ b/security/integrity/ima/ima_api.c
@@ -272,6 +272,7 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
 				   AT_STATX_SYNC_AS_STAT);
 	if (!result && (stat.result_mask & STATX_CHANGE_COOKIE))
 		i_version = stat.change_cookie;
+
 	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 365db0e43d7c..7c8aac81d16e 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -25,6 +25,7 @@
 #include <linux/xattr.h>
 #include <linux/ima.h>
 #include <linux/fs.h>
+#include <linux/iversion.h>
 
 #include "ima.h"
 
@@ -207,7 +208,7 @@ static int process_measurement(struct file *file, const struct cred *cred,
 			       u32 secid, char *buf, loff_t size, int mask,
 			       enum ima_hooks func)
 {
-	struct inode *inode = file_inode(file);
+	struct inode *backing_inode, *inode = file_inode(file);
 	struct integrity_iint_cache *iint = NULL;
 	struct ima_template_desc *template_desc = NULL;
 	char *pathbuf = NULL;
@@ -284,6 +285,24 @@ static int process_measurement(struct file *file, const struct cred *cred,
 		iint->measured_pcrs = 0;
 	}
 
+	/*
+	 * IMA does not detect changes made to the backing overlay file.
+	 * If the backing file's i_version is greater than the overlay
+	 * file's i_version or the backing file doesn't support iversion,
+	 * clear the cache to force the file's integrity to be re-evaluated.
+	 */
+	if (inode->i_sb->s_magic == 0x794c7630 &&
+	    (action & IMA_DO_MASK) && (iint->flags & IMA_DONE_MASK)) {
+		backing_inode = d_real_inode(file_dentry(file));
+		if (backing_inode != inode) {
+			if (!IS_I_VERSION(backing_inode) ||
+			    inode_peek_iversion(backing_inode) > iint->version) {
+				iint->flags &= ~IMA_DONE_MASK;
+				iint->measured_pcrs = 0;
+			}
+		}
+	}
+
 	/* Determine if already appraised/measured based on bitmask
 	 * (IMA_MEASURE, IMA_MEASURED, IMA_XXXX_APPRAISE, IMA_XXXX_APPRAISED,
 	 *  IMA_AUDIT, IMA_AUDITED)
-- 
2.39.3