ima: limit the number of open-writers integrity violations

Each time a file in policy, that is already opened for write, is opened
for read, an open-writers integrity violation audit message is emitted
and a violation record is added to the IMA measurement list. This
occurs even if an open-writers violation has already been recorded.

Limit the number of open-writers integrity violations for an existing
file open for write to one.  After the existing file open for write
closes (__fput), subsequent open-writers integrity violations may be
emitted.

Cc: [email protected] # applies cleanly up to linux-6.6
Tested-by: Stefan Berger <[email protected]>
Reviewed-by: Petr Vorel <[email protected]>
Tested-by: Petr Vorel <[email protected]>
Reviewed-by: Roberto Sassu <[email protected]>
Signed-off-by: Mimi Zohar <[email protected]>

Author: mimizohar

security/integrity/ima/ima.h

@@ -182,6 +182,7 @@ struct ima_kexec_hdr {
 #define IMA_CHANGE_ATTR		2
 #define IMA_DIGSIG		3
 #define IMA_MUST_MEASURE	4
+#define IMA_EMITTED_OPENWRITERS	5
 
 /* IMA integrity metadata associated with an inode */
 struct ima_iint_cache {

security/integrity/ima/ima_main.c

@@ -137,8 +137,13 @@ static void ima_rdwr_violation_check(struct file *file,
 	} else {
 		if (must_measure)
 			set_bit(IMA_MUST_MEASURE, &iint->atomic_flags);
-		if (inode_is_open_for_write(inode) && must_measure)
-			send_writers = true;
+
+		/* Limit number of open_writers violations */
+		if (inode_is_open_for_write(inode) && must_measure) {
+			if (!test_and_set_bit(IMA_EMITTED_OPENWRITERS,
+					      &iint->atomic_flags))
+				send_writers = true;
+		}
 	}
 
 	if (!send_tomtou && !send_writers)
@@ -167,6 +172,8 @@ static void ima_check_last_writer(struct ima_iint_cache *iint,
 	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) ||