[PATCH v3] ima: detect changes to the backing overlay file

[Mimi Zohar <zohar@xxxxxxxxxxxxx>]

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

Instead of always re-evaluating the file's integrity, detect a change
to the backing file, by comparing the cached file metadata with the
backing file's metadata.  Verifying just the i_version has not changed
is insufficient.  In addition save and compare the i_ino and s_dev
as well.

Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
---
Changelog:
- Changes made based on Amir's review:
v2: Use s_dev, not i_rdev. Limit setting real_ino, real_dev.
v1: Removal of unnecessary overlay magic test. Verify i_version, i_ino
and i_rdev haven't changed.

 fs/overlayfs/super.c              |  2 +-
 security/integrity/ima/ima_api.c  |  5 +++++
 security/integrity/ima/ima_main.c | 16 +++++++++++++++-
 security/integrity/integrity.h    |  2 ++
 4 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 3fa2416264a4..c71d185980c0 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1489,7 +1489,7 @@ 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..597ea0c4d72f 100644
--- a/security/integrity/ima/ima_api.c
+++ b/security/integrity/ima/ima_api.c
@@ -243,6 +243,7 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
 {
 	const char *audit_cause = "failed";
 	struct inode *inode = file_inode(file);
+	struct inode *real_inode = d_real_inode(file_dentry(file));
 	const char *filename = file->f_path.dentry->d_name.name;
 	struct ima_max_digest_data hash;
 	struct kstat stat;
@@ -302,6 +303,10 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
 	iint->ima_hash = tmpbuf;
 	memcpy(iint->ima_hash, &hash, length);
 	iint->version = i_version;
+	if (real_inode != inode) {
+		iint->real_ino = real_inode->i_ino;
+		iint->real_dev = real_inode->i_sb->s_dev;
+	}
 
 	/* Possibly temporary failure due to type of read (eg. O_DIRECT) */
 	if (!result)
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 365db0e43d7c..cc1217ac2c6f 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 +285,19 @@ static int process_measurement(struct file *file, const struct cred *cred,
 		iint->measured_pcrs = 0;
 	}
 
+	/* Detect and re-evaluate changes made to the backing file. */
+	backing_inode = d_real_inode(file_dentry(file));
+	if (backing_inode != inode &&
+	    (action & IMA_DO_MASK) && (iint->flags & IMA_DONE_MASK)) {
+		if (!IS_I_VERSION(backing_inode) ||
+		    backing_inode->i_sb->s_dev != iint->real_dev ||
+		    backing_inode->i_ino != iint->real_ino ||
+		    !inode_eq_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)
diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
index d7553c93f5c0..9561db7cf6b4 100644
--- a/security/integrity/integrity.h
+++ b/security/integrity/integrity.h
@@ -164,6 +164,8 @@ struct integrity_iint_cache {
 	unsigned long flags;
 	unsigned long measured_pcrs;
 	unsigned long atomic_flags;
+	unsigned long real_ino;
+	dev_t real_dev;
 	enum integrity_status ima_file_status:4;
 	enum integrity_status ima_mmap_status:4;
 	enum integrity_status ima_bprm_status:4;
-- 
2.39.3