6.6-stable review patch. If anyone has any objections, please let me know.
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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 @@ -224,8 +224,15 @@ 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));