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Message-ID: <CAGXu5jJgFujkiqBrb6k-VX8WHz8P3A__McKNOtRgGd-USuEyeQ@mail.gmail.com>
Date: Tue, 24 May 2016 10:09:16 -0700
From: Kees Cook <keescook@...omium.org>
To: Emese Revfy <re.emese@...il.com>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>,
Brad Spengler <spender@...ecurity.net>, Michal Marek <mmarek@...e.com>,
LKML <linux-kernel@...r.kernel.org>,
Masahiro Yamada <yamada.masahiro@...ionext.com>, linux-kbuild <linux-kbuild@...r.kernel.org>,
"Theodore Ts'o" <tytso@....edu>, Andrew Morton <akpm@...ux-foundation.org>, Linux-MM <linux-mm@...ck.org>,
Jens Axboe <axboe@...nel.dk>, Al Viro <viro@...iv.linux.org.uk>,
Paul McKenney <paulmck@...ux.vnet.ibm.com>, Ingo Molnar <mingo@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>, bart.vanassche@...disk.com,
"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v1 3/3] Add the extra_latent_entropy kernel parameter
On Mon, May 23, 2016 at 3:17 PM, Emese Revfy <re.emese@...il.com> wrote:
> When extra_latent_entropy is passed on the kernel command line,
> entropy will be extracted from up to the first 4GB of RAM while the
> runtime memory allocator is being initialized.
>
> Based on work created by the PaX Team.
>
> Signed-off-by: Emese Revfy <re.emese@...il.com>
> ---
> Documentation/kernel-parameters.txt | 5 +++++
> arch/Kconfig | 5 +++++
> mm/page_alloc.c | 23 +++++++++++++++++++++++
> 3 files changed, 33 insertions(+)
>
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 5349363..6c2496e 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -2862,6 +2862,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> the specified number of seconds. This is to be used if
> your oopses keep scrolling off the screen.
>
> + extra_latent_entropy
> + Enable a very simple form of latent entropy extraction
> + from the first 4GB of memory as the bootmem allocator
> + passes the memory pages to the buddy allocator.
> +
> pcbit= [HW,ISDN]
>
> pcd. [PARIDE]
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 74489df..327d1e4 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -403,6 +403,11 @@ config GCC_PLUGIN_LATENT_ENTROPY
> there is little 'natural' source of entropy normally. The cost
> is some slowdown of the boot process and fork and irq processing.
>
> + When extra_latent_entropy is passed on the kernel command line,
> + entropy will be extracted from up to the first 4GB of RAM while the
> + runtime memory allocator is being initialized. This costs even more
> + slowdown of the boot process.
> +
> Note that entropy extracted this way is not known to be cryptographically
> secure!
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ffc4f4a..c79407b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -63,6 +63,7 @@
> #include <linux/sched/rt.h>
> #include <linux/page_owner.h>
> #include <linux/kthread.h>
> +#include <linux/random.h>
>
> #include <asm/sections.h>
> #include <asm/tlbflush.h>
> @@ -1235,6 +1236,15 @@ static void __free_pages_ok(struct page *page, unsigned int order)
> }
>
> #ifdef CONFIG_GCC_PLUGIN_LATENT_ENTROPY
> +bool __meminitdata extra_latent_entropy;
> +
> +static int __init setup_extra_latent_entropy(char *str)
> +{
> + extra_latent_entropy = true;
> + return 0;
> +}
> +early_param("extra_latent_entropy", setup_extra_latent_entropy);
> +
> volatile u64 latent_entropy __latent_entropy;
> EXPORT_SYMBOL(latent_entropy);
> #endif
> @@ -1254,6 +1264,19 @@ static void __init __free_pages_boot_core(struct page *page, unsigned int order)
> __ClearPageReserved(p);
> set_page_count(p, 0);
>
> +#ifdef CONFIG_GCC_PLUGIN_LATENT_ENTROPY
> + if (extra_latent_entropy && !PageHighMem(page) && page_to_pfn(page) < 0x100000) {
> + u64 hash = 0;
> + size_t index, end = PAGE_SIZE * nr_pages / sizeof hash;
> + const u64 *data = lowmem_page_address(page);
> +
> + for (index = 0; index < end; index++)
> + hash ^= hash + data[index];
> + latent_entropy ^= hash;
> + add_device_randomness((const void *)&latent_entropy, sizeof(latent_entropy));
> + }
> +#endif
> +
We try to minimize #ifdefs in the .c code, so in this case, I think I
would define "extra_latent_entropy" during an #else above so this "if"
can be culled by the compiler automatically:
#else
# define extra_latent_entropy false
#endif
Others may have better suggestions to avoid the second #ifdef, but
this seems the cleanest way to me to tie this to the earlier #ifdef.
-Kees
> page_zone(page)->managed_pages += nr_pages;
> set_page_refcounted(page);
> __free_pages(page, order);
> --
> 2.8.1
>
--
Kees Cook
Chrome OS & Brillo Security
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