fsnotify: do not generate ACCESS/MODIFY events on child for special files

Overview

In the Linux kernel, the following vulnerability has been resolved:

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

Affected

• Linux >= 72acc854427948efed7a83da27f7dc3239ac9afc < df2711544b050aba703e6da418c53c7dc5d443ca
• Linux >= 72acc854427948efed7a83da27f7dc3239ac9afc < 859bdf438f01d9aa7f84b09c1202d548c7cad9e8
• Linux >= 72acc854427948efed7a83da27f7dc3239ac9afc < 6a7d7d96eeeab7af2bd01afbb3d9878a11a13d91
• Linux >= 72acc854427948efed7a83da27f7dc3239ac9afc < e0643d46759db8b84c0504a676043e5e341b6c81
• Linux >= 72acc854427948efed7a83da27f7dc3239ac9afc < 82f7416bcbd951549e758d15fc1a96a5afc2e900
• Linux >= 72acc854427948efed7a83da27f7dc3239ac9afc < 7a93edb23bcf07a3aaf8b598edfc2faa8fbcc0b6
• Linux >= 72acc854427948efed7a83da27f7dc3239ac9afc < 635bc4def026a24e071436f4f356ea08c0eed6ff
• Linux 2.6.36

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.