[v3,5/6] ima: Defer fixing security.ima to __fput()

Commit Message

Roberto Sassu Jan. 22, 2025, 5:24 p.m. UTC

From: Roberto Sassu <[e-mail]>

IMA-Appraisal implements a fix mode, selectable from the kernel command
line by specifying ima_appraise=fix.

The fix mode is meant to be used in a TOFU (trust on first use) model,
where systems are supposed to work under controlled conditions before the
real enforcement starts.

Since the systems are under controlled conditions, it is assumed that the
files are not corrupted, and thus their current data digest can be trusted,
and written to security.ima.

When IMA-Appraisal is switched to enforcing mode, the security.ima value
collected during the fix mode is used as a reference value, and a mismatch
with the current value cause the access request to be denied.

However, since fixing security.ima is placed in ima_appraise_measurement()
during the integrity check, it requires the inode lock to be taken in
process_measurement(), in addition to ima_update_xattr() invoked at file
close.

Postpone the security.ima update to ima_check_last_writer(), by setting the
new atomic flag IMA_UPDATE_XATTR_FIX in the inode integrity metadata, in
ima_appraise_measurement(), if security.ima needs to be fixed. In this way,
the inode lock can be removed from process_measurement(). Also, set the
cause appropriately for the fix operation and for allowing access to new
and empty signed files.

Finally, update security.ima when IMA_UPDATE_XATTR_FIX is set, and when
there wasn't a previous security.ima update, which occurs if the process
closing the file descriptor is the last writer.

Deferring fixing security.ima has a side effect: metadata of files with an
invalid EVM HMAC cannot be updated until the file is close. In alternative
to waiting, it is also recommended to add 'evm=fix' in the kernel command
line to handle this case (recommendation added to kernel-parameters.txt as
well).

Signed-off-by: Roberto Sassu <[e-mail]>
---
 .../admin-guide/kernel-parameters.txt          |  3 +++
 security/integrity/ima/ima.h                   |  1 +
 security/integrity/ima/ima_appraise.c          |  7 +++++--
 security/integrity/ima/ima_main.c              | 18 +++++++++++-------
 4 files changed, 20 insertions(+), 9 deletions(-)

Patch

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index dc663c0ca670..07219a3a2ee5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2083,6 +2083,9 @@ 
 			Format: { "off" | "enforce" | "fix" | "log" }
 			default: "enforce"
 
+			ima_appraise=fix should be used in conjunction with
+			evm=fix, when also inode metadata should be fixed.
+
 	ima_appraise_tcb [IMA] Deprecated.  Use ima_policy= instead.
 			The builtin appraise policy appraises all files
 			owned by uid=0.
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index f96021637bcf..e1a3d1239bee 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -179,6 +179,7 @@  struct ima_kexec_hdr {
 #define IMA_CHANGE_ATTR		2
 #define IMA_DIGSIG		3
 #define IMA_MUST_MEASURE	4
+#define IMA_UPDATE_XATTR_FIX	5
 
 /* IMA integrity metadata associated with an inode */
 struct ima_iint_cache {
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 884a3533f7af..ec57b36925cf 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -576,8 +576,10 @@  int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
 		if ((ima_appraise & IMA_APPRAISE_FIX) && !try_modsig &&
 		    (!xattr_value ||
 		     xattr_value->type != EVM_IMA_XATTR_DIGSIG)) {
-			if (!ima_fix_xattr(dentry, iint))
-				status = INTEGRITY_PASS;
+			/* Fix by setting security.ima on file close. */
+			set_bit(IMA_UPDATE_XATTR_FIX, &iint->atomic_flags);
+			status = INTEGRITY_PASS;
+			cause = "fix";
 		}
 
 		/*
@@ -587,6 +589,7 @@  int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
 		if (inode->i_size == 0 && iint->flags & IMA_NEW_FILE &&
 		    test_bit(IMA_DIGSIG, &iint->atomic_flags)) {
 			status = INTEGRITY_PASS;
+			cause = "new-signed-file";
 		}
 
 		integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 0aed8f730c42..46adfd524dd8 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -158,13 +158,16 @@  static void ima_check_last_writer(struct ima_iint_cache *iint,
 				  struct inode *inode, struct file *file)
 {
 	fmode_t mode = file->f_mode;
-	bool update;
+	bool update = false, update_fix;
 
-	if (!(mode & FMODE_WRITE))
+	update_fix = test_and_clear_bit(IMA_UPDATE_XATTR_FIX,
+					&iint->atomic_flags);
+
+	if (!(mode & FMODE_WRITE) && !update_fix)
 		return;
 
 	ima_iint_lock(inode);
-	if (atomic_read(&inode->i_writecount) == 1) {
+	if ((mode & FMODE_WRITE) && atomic_read(&inode->i_writecount) == 1) {
 		struct kstat stat;
 
 		update = test_and_clear_bit(IMA_UPDATE_XATTR,
@@ -181,6 +184,10 @@  static void ima_check_last_writer(struct ima_iint_cache *iint,
 				ima_update_xattr(iint, file);
 		}
 	}
+
+	if (!update && update_fix)
+		ima_update_xattr(iint, file);
+
 	ima_iint_unlock(inode);
 }
 
@@ -378,13 +385,10 @@  static int process_measurement(struct file *file, const struct cred *cred,
 				      template_desc);
 	if (rc == 0 && (action & IMA_APPRAISE_SUBMASK)) {
 		rc = ima_check_blacklist(iint, modsig, pcr);
-		if (rc != -EPERM) {
-			inode_lock(inode);
+		if (rc != -EPERM)
 			rc = ima_appraise_measurement(func, iint, file,
 						      pathname, xattr_value,
 						      xattr_len, modsig);
-			inode_unlock(inode);
-		}
 		if (!rc)
 			rc = mmap_violation_check(func, file, &pathbuf,
 						  &pathname, filename);