* Zachary Amsden (zach@xxxxxxxxxx) wrote:
Let's dive into it. How do you get the randomization without sacrificing syscall performance? Do you randomize on boot, dynamically, or on a per-process level?
The latter, on exec.
Because I can see some issues with per-process randomization that will certainly cost some amount of cycles on the system call path. Marginal perhaps, but that is exactly where you don't want to shed cycles unnecessarily, and the complexity of the whole thing will go up quite a bit I think.
The crux is here:
+ OFFSET(TI_sysenter_return, thread_info, sysenter_return);
- pushl $SYSENTER_RETURN
+ * Push current_thread_info()->sysenter_return to the stack.
+ * A tiny bit of offset fixup is necessary - 4*4 means the 4 words
+ * pushed above; +8 corresponds to copy_thread's esp0 setting.
+ pushl (TI_sysenter_return-THREAD_SIZE+8+4*4)(%esp)
and in binfmt_elf during exec thread_info->sysenter_return is setup
based on the randomized mapping it does for vdso
+ ti->sysenter_return = &SYSENTER_RETURN_OFFSET + addr;
I think it's not so bad, but I can't say I've benchmarked the cost.