Re: [PATCH] proc: Disable /proc/$pid/wchan

[Vito Caputo]

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