[RFC PATCH v2 2/2] ima: Fix detection of read/write violations on stacked filesystems

[Stefan Berger <stefanb@linux.ibm.com>]

On a stacked filesystem, when one process opens the file holding a file's
data (e.g., on upper or lower layer on overlayfs) then issue a violation
when another process opens the file for reading on the top layer (overlay
layer on overlayfs). This then provides similar behavior to the existing
case where a violation is generated when one process opens a file for
writing and another one opens the same file for reading.

Convert the current code so that it can handle the normal case as well
as the stacked filesystem case. Therefore, use d_real with parameter
D_REAL_FILEDATA to get the next dentry holding the file data. On a normal
filesystem this would be the dentry of the file and on a stacked filesystem
this could be an upper or lower dentry. Check the dentry's inode for
writes and if it has any issue the violation. Otherwise continue onto the
next dentry given the current dentry by again calling d_real. On a normal
filesystem this would return the same dentry as before and on a stacked
filesystem it would return the next-level dentry, so either the upper
or lower dentry of the next lower layer.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
---
 security/integrity/ima/ima_main.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index f04f43af651c..7d727c448dc7 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -122,7 +122,9 @@ static void ima_rdwr_violation_check(struct file *file,
 				     char *filename)
 {
 	struct inode *inode = file_inode(file);
+	struct dentry *fd_dentry, *d;
 	fmode_t mode = file->f_mode;
+	struct inode *fd_inode;
 	bool send_tomtou = false, send_writers = false;
 
 	if (mode & FMODE_WRITE) {
@@ -134,11 +136,20 @@ static void ima_rdwr_violation_check(struct file *file,
 						&iint->atomic_flags))
 				send_tomtou = true;
 		}
-	} else {
-		if (must_measure)
-			set_bit(IMA_MUST_MEASURE, &iint->atomic_flags);
-		if (inode_is_open_for_write(inode) && must_measure)
-			send_writers = true;
+	} else if (must_measure) {
+		set_bit(IMA_MUST_MEASURE, &iint->atomic_flags);
+
+		d = d_real(file_dentry(file), D_REAL_FILEDATA);
+		do {
+			fd_dentry = d;
+			fd_inode = d_inode(fd_dentry);
+			if (inode_is_open_for_write(fd_inode)) {
+				send_writers = true;
+				break;
+			}
+			/* next layer of stacked fs */
+			d = d_real(fd_dentry, D_REAL_FILEDATA);
+		} while (d != fd_dentry);
 	}
 
 	if (!send_tomtou && !send_writers)
-- 
2.43.0