linux-kernel - Re: [PATCH] fs/proc/kcore.c: Omit kernel text area for hardened usercopy feature

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Message-ID: <20160902151713.GM5871@two.firstfloor.org>
Date:   Fri, 2 Sep 2016 08:17:13 -0700
From:   Andi Kleen <andi@...stfloor.org>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Andi Kleen <andi@...stfloor.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] fs/proc/kcore.c: Omit kernel text area for hardened
 usercopy feature

On Fri, Sep 02, 2016 at 02:25:45PM +0200, Jiri Olsa wrote:
> One of the bullets for hardened usercopy feature is:
>   - object must not overlap with kernel text
> 
> which is what we expose via /proc/kcore. We can hit
> this check and crash the system very easily just by
> reading the text area in kcore file:
> 
>   usercopy: kernel memory exposure attempt detected from ffffffff8179a01f (<kernel text>) (4065 bytes)
>   kernel BUG at mm/usercopy.c:75!
> 
> Omitting kernel text area from kcore when there's
> hardened usercopy feature is enabled.

That will completely break PT decoding, which relies on looking
at the kernel text in /proc/kcore.

Need a different fix here, perhaps some special copy function
that is not hardened.

-Andi