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

[Mika PenttilÃ]

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@xxxxxxxxxx>
> ---
> 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);


--Mika