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Date: Mon, 08 Aug 2016 21:14:32 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Borislav Petkov <bp@...e.de>, Thomas Garnier <thgarnie@...gle.com>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
Joerg Roedel <jroedel@...e.de>, Dave Young <dyoung@...hat.com>,
"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
Lv Zheng <lv.zheng@...el.com>, Baoquan He <bhe@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Mark Salter <msalter@...hat.com>,
Aleksey Makarov <aleksey.makarov@...aro.org>,
Kees Cook <keescook@...omium.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Christian Borntraeger <borntraeger@...ibm.com>,
Fabian Frederick <fabf@...net.be>, Toshi Kani <toshi.kani@...com>,
Dan Williams <dan.j.williams@...el.com>, x86@...nel.org,
linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v1 2/2] x86/KASLR: Increase BRK pages for KASLR memory randomization
On August 8, 2016 9:01:42 PM PDT, Borislav Petkov <bp@...e.de> wrote:
>On Mon, Aug 08, 2016 at 11:40:07AM -0700, Thomas Garnier wrote:
>> Default implementation expects 6 pages maximum are needed for low
>page
>> allocations. If KASLR memory randomization is enabled, the worse case
>> of e820 layout would require 12 pages (no large pages). It is due to
>the
>> PUD level randomization and the variable e820 memory layout.
>
>Can't this number of required pages be computed based on the
>randomization offset or somesuch instead of maxing it out by default on
>every machine which enables CONFIG_RANDOMIZE_MEMORY?
>
>> This bug was found while doing extensive testing of KASLR memory
>> randomization on different type of hardware.
>>
>> Signed-off-by: Thomas Garnier <thgarnie@...gle.com>
>> ---
>> Based on next-20160805
>> ---
>> arch/x86/mm/init.c | 8 ++++++++
>> 1 file changed, 8 insertions(+)
>>
>> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
>> index 6209289..3a27e6a 100644
>> --- a/arch/x86/mm/init.c
>> +++ b/arch/x86/mm/init.c
>> @@ -130,6 +130,14 @@ void __init early_alloc_pgt_buf(void)
>> unsigned long tables = INIT_PGT_BUF_SIZE;
>> phys_addr_t base;
>>
>> + /*
>> + * Depending on the machine e860 memory layout and the PUD
>alignement.
>
> e820 alignment
>
>> + * We may need twice more pages when KASLR memoy randomization is
>
> memory
>
>> + * enabled.
>> + */
>> + if (IS_ENABLED(CONFIG_RANDOMIZE_MEMORY))
>> + tables *= 2;
>> +
>> base = __pa(extend_brk(tables, PAGE_SIZE));
>>
>> pgt_buf_start = base >> PAGE_SHIFT;
>> --
>> 2.8.0.rc3.226.g39d4020
>>
Upping the brk by a few pages is not a big deal. Unused brk gets reclaimed at init release time.
--
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