Re: [PATCH v3 08/11] mips: Properly account for stack randomization and stack guard gap

[Alex Ghiti <alex@ghiti.fr>]

On 4/18/19 1:30 AM, Kees Cook wrote:
> On Wed, Apr 17, 2019 at 12:31 AM Alexandre Ghiti <alex@ghiti.fr> wrote:
>> This commit takes care of stack randomization and stack guard gap when
>> computing mmap base address and checks if the task asked for randomization.
>> This fixes the problem uncovered and not fixed for mips here:
>> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1429066.html
> same URL change here please...
>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> Acked-by: Kees Cook <keescook@chromium.org>


Thanks !


>
> -Kees
>
>> ---
>>   arch/mips/mm/mmap.c | 14 ++++++++++++--
>>   1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
>> index 2f616ebeb7e0..3ff82c6f7e24 100644
>> --- a/arch/mips/mm/mmap.c
>> +++ b/arch/mips/mm/mmap.c
>> @@ -21,8 +21,9 @@ unsigned long shm_align_mask = PAGE_SIZE - 1; /* Sane caches */
>>   EXPORT_SYMBOL(shm_align_mask);
>>
>>   /* gap between mmap and stack */
>> -#define MIN_GAP (128*1024*1024UL)
>> -#define MAX_GAP ((TASK_SIZE)/6*5)
>> +#define MIN_GAP                (128*1024*1024UL)
>> +#define MAX_GAP                ((TASK_SIZE)/6*5)
>> +#define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))
>>
>>   static int mmap_is_legacy(struct rlimit *rlim_stack)
>>   {
>> @@ -38,6 +39,15 @@ static int mmap_is_legacy(struct rlimit *rlim_stack)
>>   static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
>>   {
>>          unsigned long gap = rlim_stack->rlim_cur;
>> +       unsigned long pad = stack_guard_gap;
>> +
>> +       /* Account for stack randomization if necessary */
>> +       if (current->flags & PF_RANDOMIZE)
>> +               pad += (STACK_RND_MASK << PAGE_SHIFT);
>> +
>> +       /* Values close to RLIM_INFINITY can overflow. */
>> +       if (gap + pad > gap)
>> +               gap += pad;
>>
>>          if (gap < MIN_GAP)
>>                  gap = MIN_GAP;
>> --
>> 2.20.1
>>