ima: limit the number of ToMToU integrity violations

Each time a file in policy, that is already opened for read, is opened
for write, a Time-of-Measure-Time-of-Use (ToMToU) integrity violation
audit message is emitted and a violation record is added to the IMA
measurement list.  This occurs even if a ToMToU violation has already
been recorded.

Limit the number of ToMToU integrity violations per file open for read.

Note: The IMA_MAY_EMIT_TOMTOU atomic flag must be set from the reader
side based on policy.  This may result in a per file open for read
ToMToU violation.

Since IMA_MUST_MEASURE is only used for violations, rename the atomic
IMA_MUST_MEASURE flag to IMA_MAY_EMIT_TOMTOU.

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

@@ -181,7 +181,7 @@ struct ima_kexec_hdr {
 #define IMA_UPDATE_XATTR	1
 #define IMA_CHANGE_ATTR		2
 #define IMA_DIGSIG		3
-#define IMA_MUST_MEASURE	4
+#define IMA_MAY_EMIT_TOMTOU	4
 #define IMA_EMITTED_OPENWRITERS	5
 
 /* IMA integrity metadata associated with an inode */

security/integrity/ima/ima_main.c

@@ -129,14 +129,15 @@ static void ima_rdwr_violation_check(struct file *file,
 		if (atomic_read(&inode->i_readcount) && IS_IMA(inode)) {
 			if (!iint)
 				iint = ima_iint_find(inode);
+
 			/* IMA_MEASURE is set from reader side */
-			if (iint && test_bit(IMA_MUST_MEASURE,
-						&iint->atomic_flags))
+			if (iint && test_and_clear_bit(IMA_MAY_EMIT_TOMTOU,
+						       &iint->atomic_flags))
 				send_tomtou = true;
 		}
 	} else {
 		if (must_measure)
-			set_bit(IMA_MUST_MEASURE, &iint->atomic_flags);
+			set_bit(IMA_MAY_EMIT_TOMTOU, &iint->atomic_flags);
 
 		/* Limit number of open_writers violations */
 		if (inode_is_open_for_write(inode) && must_measure) {