- Disallow bogus layer combinations.
syzbot has started to produce repros that create bogus layer combinations. So far it has only been able to reproduce a WARN_ON, which has already been fixed in stable, by acf3062a7e1c ("ovl: relax WARN_ON()..."), but other real bugs could be lurking if those setups are allowed. We decided to detect and error on these setups on mount, to stop syzbot (and attackers) from trying to attack overlayfs this way. To stop syzbot from mutating this class of repros on stable kernel you MAY apply these 3 patches, but in any case, I would wait a while to see if more bugs are reported on master. Although this solves a problem dating before 4.19, I have no plans of backporting these patches further back.
146d62e5a586 ovl: detect overlapping layers 9179c21dc6ed ovl: don't fail with disconnected lower NFS 1dac6f5b0ed2 ovl: fix bogus -Wmaybe-unitialized warning
I've queued these 3 for 4.19.
FYI, an overlayfs regression has been reported: https://github.com/containers/libpod/issues/3540
Caused by commit "ovl: detect overlapping layers"
I am working on a fix. In retrospect, given my lengthy disclaimer above, it seems that this patch should not have been applied to stable (yet). I believe that this patch belongs to a class of fixed that should soak in master for a while before being considered for stable. On my part, I will not propose these sort of fixed in the future, with or without a disclaimer until they have soaked in master.
Thanks, Amir.