netdev - Re: [kernel-hardening] [PATCH v4] scripts: add leaking

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Date:   Thu, 9 Nov 2017 10:13:26 +0530
From:   Kaiwan N Billimoria <kaiwan.billimoria@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Laura Abbott <labbott@...hat.com>,
        "Tobin C. Harding" <me@...in.cc>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "Theodore Ts'o" <tytso@....edu>, Kees Cook <keescook@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tycho Andersen <tycho@...ker.com>,
        "Roberts, William C" <william.c.roberts@...el.com>,
        Tejun Heo <tj@...nel.org>,
        Jordan Glover <Golden_Miller83@...tonmail.ch>,
        Greg KH <gregkh@...uxfoundation.org>,
        Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>,
        Ian Campbell <ijc@...lion.org.uk>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <wilal.deacon@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Chris Fries <cfries@...gle.com>,
        Dave Weinstein <olorin@...gle.com>,
        Daniel Micay <danielmicay@...il.com>,
        Djalal Harouni <tixxdz@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [kernel-hardening] [PATCH v4] scripts: add leaking_addresses.pl

> But I don't know if there is anything else than the profiling code
> that _really_ wants access to /proc/kallsyms in user space as a
> regular user.

Am unsure about this, but kprobes? (/jprobes/kretprobes), and by
extension, wrappers over this infra (like SystemTap)?
I (hazily) recollect a script I once wrote (years back though) that
collects kernel virtual addresses off of kallsyms for the purpose of
passing them to a 'helper' kernel module that uses kprobes. I realize
that 'modern' kprobes exposes APIs that just require the symbolic name
& that they're anyway at kernel privilege... but the point is, a
usermode script was picking up and passing the kernel addresses.

Also, what about kernel addresses exposed via System.map?
Oh, just checked, it's root rw only.. pl ignore.

> That said, that patch also fixes the /proc/kallsyms root check, in
> that now you can do:
>
>     sudo head < /proc/kallsyms
>
> and it still shows all zeroes - because the file was *opened* as a
> normal user. That's how UNIX file access security works, and how it is
> fundamentally supposed to work (ie passing a file descriptor to a sui
> program doesn't magically make it gain privileges).

Indeed.