Re: [PATCH] lkdtm: add bad USER_DS test

[Thomas Garnier]

On Fri, Mar 24, 2017 at 1:14 AM, Heiko Carstens
<heiko.carstens@xxxxxxxxxx> wrote:
> On Thu, Mar 23, 2017 at 01:34:19PM -0700, Kees Cook wrote:
>> This adds CORRUPT_USER_DS to check that the get_fs() test on syscall return
>> still sees USER_DS during the new VERIFY_PRE_USERMODE_STATE checks.
>>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> ...
>
>> +void lkdtm_CORRUPT_USER_DS(void)
>> +{
>> +     /*
>> +      * Test that USER_DS has been set correctly on exiting a syscall.
>> +      * Since setting this higher than USER_DS (TASK_SIZE) would introduce
>> +      * an exploitable condition, we lower it instead, since that should
>> +      * not create as large a problem on an unprotected system.
>> +      */
>> +     mm_segment_t lowfs;
>> +#ifdef MAKE_MM_SEG
>> +     lowfs = MAKE_MM_SEG(TASK_SIZE - PAGE_SIZE);
>> +#else
>> +     lowfs = TASK_SIZE - PAGE_SIZE;
>> +#endif
>> +
>> +     pr_info("setting bad task size limit\n");
>> +     set_fs(lowfs);
>> +}
>
> This won't work on architectures where the set_fs() argument does not
> contain an address but an address space identifier. This is true e.g. for
> s390 and as far as I know also for sparc.
> On s390 we have complete distinct address spaces for kernel and user space
> that each start at address zero.
>

The patch that enforce USER_DS is disabled on s390 anyway. I guess, we
can just do a set_fs(KERNEL_DS) for the others.


-- 
Thomas