On Thu, Sep 23, 2021 at 04:31:05PM -0700, Kees Cook wrote: > The /proc/$pid/wchan file has been broken by default on x86_64 for 4 > years now[1]. As this remains a potential leak of either kernel > addresses (when symbolization fails) or limited observation of kernel > function progress, just remove the contents for good. > > Unconditionally set the contents to "0" and also mark the wchan > field in /proc/$pid/stat with 0. > > This leaves kernel/sched/fair.c as the only user of get_wchan(). But > again, since this was broken for 4 years, was this profiling logic > actually doing anything useful? > > [1] https://lore.kernel.org/lkml/20210922001537.4ktg3r2ky3b3r6yp@treble/ > > Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> > Cc: Vito Caputo <vcaputo@xxxxxxxxxxx> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> <snip> Please don't deliberately break WCHANs wholesale. This is a very useful tool for sysadmins to get a vague sense of where processes are spending time in the kernel on production systems without affecting performance or having to restart things under instrumentation. I don't see how providing the symbol name of a given task's kernel function, especially if shallow near the user->kernel entrypoint, is a worrisome information leak. Just make sure it's not failing open with addresses like my original report documented seems to happen spuriously as-is w/kallsyms. When I worked full-time as a sysadmin WCHAN's were regularly the first thing to look at in `ps -o stat,wchan | grep D` when things were falling over. e.g.: ``` root@shells:/root# ps -o stat,wchan | grep D D io_schedule ``` Furthermore this is a well documented on dead trees and understood *nix/posix system observation technique. Even the POSIX ps(1) man page documents it: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ps.html Frankly I'm a bit mortified that I have to write this email. Today I'm hoping to test Josh's patch @ https://lore.kernel.org/all/20210831083625.59554-1-zhengqi.arch@xxxxxxxxxxxxx/ Thanks, Vito Caputo
