From 635bc4def026a24e071436f4f356ea08c0eed6ff Mon Sep 17 00:00:00 2001 From: Amir Goldstein Date: Sun, 7 Dec 2025 11:44:55 +0100 Subject: [PATCH] fsnotify: do not generate ACCESS/MODIFY events on child for special files 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 CC: stable@vger.kernel.org Signed-off-by: Amir Goldstein Signed-off-by: Jan Kara --- fs/notify/fsnotify.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c index 46bfc543f946..63dd44931989 100644 --- a/fs/notify/fsnotify.c +++ b/fs/notify/fsnotify.c @@ -270,8 +270,15 @@ int __fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data, /* * 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)); -- 2.47.3