linux-kernel - Re: [PATCH v3 1/3] x86/64: Make kernel text mapping always take one whole page table in early boot code

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Date:   Thu, 5 Jan 2017 21:52:43 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Baoquan He <bhe@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Yinghai Lu <yinghai@...nel.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Alexander Kuleshov <kuleshovmail@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Dave Anderson <anderson@...hat.com>,
        Dave Young <dyoung@...hat.com>
Subject: Re: [PATCH v3 1/3] x86/64: Make kernel text mapping always take one
 whole page table in early boot code

On Thu, Jan 05, 2017 at 11:35:57AM -0800, Kees Cook wrote:
> The only reason I had it as an option was for kernel module space. It
> wasn't clear to me at the time if enough space remained for modules in
> all use-cases. It seems like probably there is, so I have no objection
> to making the mapping 1G unconditionally.

Oh someone will crawl out of the woodwork handwaiving that 1G of modules
is not enough. But then that someone would have to choose between kaslr
and >1G modules.

Realistically, on a typical bigger machine, the modules take up
something like <10M:

$ lsmod | awk '{ sum +=$2 } END { print sum }'
7188480

so I'm not really worried if we reduce it by default to 1G. Besides, the
reduction has been there for a while now - since CONFIG_RANDOMIZE_BASE -
so we probably would've heard complaints already...

Btw, we should probably document the reduction in the va map document
too:

---
From: Borislav Petkov <bp@...e.de>
Date: Thu, 5 Jan 2017 21:47:18 +0100
Subject: [PATCH] x86/mm: Document modules space reduction

KASLR reduces module mapping space to 1G, document that.

Signed-off-by: Borislav Petkov <bp@...e.de>
---
 Documentation/x86/x86_64/mm.txt | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/x86/x86_64/mm.txt b/Documentation/x86/x86_64/mm.txt
index 5724092db811..a737dfbc198b 100644
--- a/Documentation/x86/x86_64/mm.txt
+++ b/Documentation/x86/x86_64/mm.txt
@@ -39,6 +39,8 @@ memory window (this size is arbitrary, it can be raised later if needed).
 The mappings are not part of any other kernel PGD and are only available
 during EFI runtime calls.
 
+CONFIG_RANDOMIZE_BASE (KASLR) reduces module mapping space from 1.5G to 1G.
+
 Note that if CONFIG_RANDOMIZE_MEMORY is enabled, the direct mapping of all
 physical memory, vmalloc/ioremap space and virtual memory map are randomized.
 Their order is preserved but their base will be offset early at boot time.
-- 
2.11.0

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--