Re: [PATCH] [RFC][WIP] namespace.c: Allow some unprivileged proc mounts when not fully visible

From: Eric W. Biederman
Date: Wed Apr 04 2018 - 20:00:24 EST

Alexey Dobriyan <adobriyan@xxxxxxxxx> writes:

>> The only option I have seen proposed that might qualify as something
>> general purpose and simple is a new filesystem that is just the process
>> directories of proc.
> While "mount -t pid" and "mount -t sysctl" are decades overdue, I don't
> think they cover everything.
> IIRC some gcc versions read /proc/meminfo on every invocation. Now
> imagine such program doesn't have a fallback if /proc/ doesn't exist
> (how many thousands such programs are there?) So user is going to ask
> for /proc with just /proc/meminfo only. At this point it is back to
> nearly full /proc.

To avoid falling susceptible to the kinds of checks in fs_fully_visible we
can only offer information about objects that root in the user namespace
has privilege over. So "mount -t pid" good. A "/proc/meminfo" bad.

Which in short means if "mount -t pid" isn't good enough. There really
isn't anything the kernel can do.