diff mbox

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

Message ID 1470681607-36883-2-git-send-email-thgarnie@google.com
State New, archived
Headers show

Commit Message

Thomas Garnier Aug. 8, 2016, 6:40 p.m. UTC
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(+)

Comments

Borislav Petkov Aug. 9, 2016, 4:01 a.m. UTC | #1
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@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.

				    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
>
H. Peter Anvin Aug. 9, 2016, 4:14 a.m. UTC | #2
On August 8, 2016 9:01:42 PM PDT, Borislav Petkov <bp@suse.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@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.
>
>				    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.
Mika Penttilä Aug. 9, 2016, 5:16 a.m. UTC | #3
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);


--Mika
Thomas Garnier Aug. 9, 2016, 2:47 p.m. UTC | #4
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
>
diff mbox

Patch

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;