linux-kernel - Re: [PATCH v4 1/3] x86: Introduce a new constant KERNEL_MAPPING

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Date:   Fri, 3 Mar 2017 15:28:30 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Baoquan He <bhe@...hat.com>
Cc:     tglx@...utronix.de, hpa@...or.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        keescook@...omium.org, yinghai@...nel.org, anderson@...hat.com,
        luto@...nel.org, thgarnie@...gle.com, kuleshovmail@...il.com
Subject: Re: [PATCH v4 1/3] x86: Introduce a new constant KERNEL_MAPPING_SIZE

On Fri, Mar 03, 2017 at 09:11:52PM +0800, Baoquan He wrote:
> And another meaning of defining kernel iamge size and mapping size
> differently is we can randomize the limited kernel image in the mapping
> area. If they are the same or kernel image can be very large, the
> position will be fixed or very few, kernel text KASLR will be
> meaningless.

This is simply not true:

@@ -408,9 +408,9 @@ static unsigned long find_random_virt_addr(unsigned long minimum,
        /*
         * There are how many CONFIG_PHYSICAL_ALIGN-sized slots
         * that can hold image_size within the range of minimum to
-        * KERNEL_IMAGE_SIZE?
+        * KERNEL_MAPPING_SIZE?
         */
-       slots = (KERNEL_IMAGE_SIZE - minimum - image_size) /
+       slots = (KERNEL_MAPPING_SIZE - minimum - image_size) /
                 CONFIG_PHYSICAL_ALIGN + 1;

*With* kaslr, KERNEL_IMAGE_SIZE = 1G and KERNEL_MAPPING_SIZE = 1G.
Before your patch KERNEL_IMAGE_SIZE = 1G too with kaslr enabled.

-- 
Regards/Gruss,
    Boris.

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