On Tue, Dec 08, 2020 at 06:40:38PM +0100, to...@tuxteam.de wrote: > I lost track a bit about your other hypotheses, but this mode changing > bit when O_CREAT is given somehow rings a bell.
I googled "linux o_creat sticky" and got this: https://patchwork.kernel.org/project/kernel-hardening/patch/20180416175918.GA13494@beast/ Disallows open of FIFOs or regular files not owned by the user in world writable sticky directories, unless the owner is the same as that of the directory or the file is opened without the O_CREAT flag. The purpose is to make data spoofing attacks harder. This protection can be turned on and off separately for FIFOs and regular files via sysctl, just like the symlinks/hardlinks protection. This patch is based on Openwall's "HARDEN_FIFO" feature by Solar Designer. I don't know whether that's actually applied to the bullseye kernels, but it's definitely something to look into.
