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

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Message-ID: <20160905162739.GO5871@two.firstfloor.org>
Date:   Mon, 5 Sep 2016 09:27:39 -0700
From:   Andi Kleen <andi@...stfloor.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Andi Kleen <andi@...stfloor.org>, Jiri Olsa <jolsa@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        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 Mon, Sep 05, 2016 at 10:47:22AM +0200, Jiri Olsa wrote:
> On Fri, Sep 02, 2016 at 08:17:13AM -0700, Andi Kleen wrote:
> > 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.
> 
> how about something like this

Looks good to me, but you would need the *_nocheck variant for non x86
architectures too of course.

-Andi