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From:   Kees Cook <keescook@google.com>
Date:   Sat, 10 Feb 2018 11:14:18 -0800
Message-ID: <CAGXu5jK=46Oz63Q5rPbDuGzGxDPFpx5asE3-RLunuvQkEsF4cA@mail.gmail.com>
Subject: Re: [PATCH 4.9 46/92] x86/alternative: Print unadorned pointers
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        "# 3.4.x" <stable@vger.kernel.org>, Borislav Petkov <bp@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Rik van Riel <riel@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Jiri Kosina <jikos@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Hansen <dave.hansen@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Paul Turner <pjt@google.com>,
        David Woodhouse <dwmw@amazon.co.uk>
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On Fri, Feb 9, 2018 at 11:23 PM, Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> On Fri, Feb 09, 2018 at 02:01:32PM -0800, Kees Cook wrote:
>> On Fri, Feb 9, 2018 at 5:39 AM, Greg Kroah-Hartman
>> <gregkh@linuxfoundation.org> wrote:
>> > 4.9-stable review patch.  If anyone has any objections, please let me know.
>> >
>> > ------------------
>> >
>> > From: Borislav Petkov <bp@suse.de>
>> >
>> > (cherry picked from commit 0e6c16c652cadaffd25a6bb326ec10da5bcec6b4)
>> >
>> > After commit ad67b74d2469 ("printk: hash addresses printed with %p")
>> > pointers are being hashed when printed. However, this makes the alternative
>> > debug output completely useless. Switch to %px in order to see the
>> > unadorned kernel pointers.
>>
>> This missed a "Fixes:" tag so probably missed automated checking on
>> how far back to port this. It shouldn't go back beyond 4.15 (where
>> ad67b74d2469 first appeared).
>
> Good point.  Should we instead be using %pK for this change instead?  Or
> should we just backport commit ad67b74d2469 to 4.14?  :)

ad67b74d2469 is pretty disruptive, I can't recommend putting it in
4.14. I mean, I think it'd be great, but I suspect other people would
find it very surprising. :)

I would just revert this commit from kernels earlier than 4.15; this
%p usage is in debugging, so %pK doesn't make much sense here.

-Kees

-- 
Kees Cook
Pixel Security