Re: [PATCH v1 2/2] x86/KASLR: Increase BRK pages for KASLR memory randomization

[Thomas Garnier <thgarnie@google.com>]

On Mon, Aug 8, 2016 at 10:16 PM, Mika Penttilä
<mika.penttila@nextfour.com> wrote:
> On 08/08/2016 09:40 PM, 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.
>>
>> This bug was found while doing extensive testing of KASLR memory
>> randomization on different type of hardware.
>>
>> Signed-off-by: Thomas Garnier <thgarnie@google.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.
>> +      * We may need twice more pages when KASLR memoy randomization is
>> +      * enabled.
>> +      */
>> +     if (IS_ENABLED(CONFIG_RANDOMIZE_MEMORY))
>> +             tables *= 2;
>> +
>>       base = __pa(extend_brk(tables, PAGE_SIZE));
>>
>>       pgt_buf_start = base >> PAGE_SHIFT;
>>
>
> You should increase the reserve also :
> RESERVE_BRK(early_pgt_alloc, INIT_PGT_BUF_SIZE);

Yes, that's right. I will resend v2 changing INIT_PGD_BUF_SIZE.

>
>
> --Mika
>