[PATCH 6.12 237/567] fsnotify: do not generate ACCESS/MODIFY events on child for special files

Greg Kroah-Hartman
2026-01-06T17:27:24Z

From: Amir Goldstein <amir73il@gmail.com>

commit 635bc4def026a24e071436f4f356ea08c0eed6ff upstream.

inotify/fanotify do not allow users with no read access to a file to
subscribe to events (e.g. IN_ACCESS/IN_MODIFY), but they do allow the
same user to subscribe for watching events on children when the user
has access to the parent directory (e.g. /dev).

Users with no read access to a file but with read access to its parent
directory can still stat the file and see if it was accessed/modified
via atime/mtime change.

The same is not true for special files (e.g. /dev/null). Users will not
generally observe atime/mtime changes when other users read/write to
special files, only when someone sets atime/mtime via utimensat().

Align fsnotify events with this stat behavior and do not generate
ACCESS/MODIFY events to parent watchers on read/write of special files.
The events are still generated to parent watchers on utimensat(). This
closes some side-channels that could be possibly used for information
exfiltration [1].

[1] https://snee.la/pdf/pubs/file-notification-attacks.pdf

Reported-by: Sudheendra Raghav Neela <sneela@tugraz.at>
CC: stable@vger.kernel.org
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/notify/fsnotify.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -247,8 +247,15 @@ int __fsnotify_parent(struct dentry *den
/*
* Include parent/name in notification either if some notification
* groups require parent info or the parent is interested in this event.
+ * The parent interest in ACCESS/MODIFY events does not apply to special
+ * files, where read/write are not on the filesystem of the parent and
+ * events can provide an undesirable side-channel for information
+ * exfiltration.
*/
- parent_interested = mask & p_mask & ALL_FSNOTIFY_EVENTS;
+ parent_interested = mask & p_mask & ALL_FSNOTIFY_EVENTS &&
+ !(data_type == FSNOTIFY_EVENT_PATH &&
+ d_is_special(dentry) &&
+ (mask & (FS_ACCESS | FS_MODIFY)));
if (parent_needed || parent_interested) {
/* When notifying parent, child should be passed as data */
WARN_ON_ONCE(inode != fsnotify_data_inode(data, data_type));