linux-kernel - Re: [kernel-hardening] [RFC V2 4/6] lib: vsprintf: default kptr

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Date:   Wed, 4 Oct 2017 21:13:39 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        "Tobin C. Harding" <me@...in.cc>,
        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>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        William Roberts <william.c.roberts@...el.com>,
        Chris Fries <cfries@...gle.com>,
        Dave Weinstein <olorin@...gle.com>
Subject: Re: [kernel-hardening] [RFC V2 4/6] lib: vsprintf: default
 kptr_restrict to the maximum value

On Wed, Oct 4, 2017 at 7:28 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Oct 4, 2017 at 10:08 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> So I honestly doubt the value of kptr_restrict. Any *sane* policy
>> pretty much has to be in the caller, and by thinking about what you
>> print out. IOW, things like proc_pid_wchan().
>
> Looking at /proc/kallsyms is actually a prime example of this.
>
> IOW, the old "open /proc/kallsyms as a normal user, then make it stdin
> for some suid-root program that can be fooled to output it probably
> works on it.

Actually, /proc/kallsyms uses %pK, which hacks around this issue
by checking for `euid != uid` in addition to the capability check - so this
isn't exploitable through a typical setuid program.