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.
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