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

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Message-ID: <CA+55aFyZHKjQuOAtEiStSWOXqHc5ip-1yDm4Qj6adVhKXTHWyg@mail.gmail.com>
Date:   Tue, 6 Sep 2016 12:48:17 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     Kees Cook <keescook@...omium.org>, Jiri Olsa <jolsa@...hat.com>,
        Jiri Olsa <jolsa@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] fs/proc/kcore.c: Omit kernel text area for hardened
 usercopy feature

On Tue, Sep 6, 2016 at 12:41 PM, Andi Kleen <andi@...stfloor.org> wrote:
>
> I suspect it's more than just /proc/kcore, there could be also
> legitimate cases to read kernel text from /dev/mem or /dev/kmem

Yes, that's probably true. Although I suspect that we should just say
that user-copy hardening is incompatible with /dev/kmem and
!STRICT_DEVMEM.

At least Fedora seems to have

  CONFIG_DEVMEM=y
  # CONFIG_DEVKMEM is not set
  CONFIG_STRICT_DEVMEM=y

which should mean that you already should not be able to access normal
RAM using /dev/[k]mem - ie it's purely for legacy X server kind of
situations.

So we could just make  HARDENED_USERCOPY force those settings. It's
not like you should ever have anything else in any situation where you
care about security *anyway*, so...

                 Linus