Re: [PATCH] Gerd Hoffman's move-vsyscall-into-user-address-range patch

[Zachary Amsden <zach@vmware.com>]

Chris Wright wrote:
> * Zachary Amsden (zach@vmware.com) 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.
>   

Now that I see it, it doesn't look bad at all.  I had imagined a host of 
holy horrors unfolding from it, but clearly that is not the case.  I 
think there is still the sysexit path that needs some change, but in 
total, there should be almost zero cycle impact.  I envisioned trying to 
get the thread info for the return address would be awkward, but you've 
already switched the stack at this point, so it is really almost free.

Zach

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