On Tue, Apr 10, 2018 at 11:28:13AM -0700, Yang Shi wrote:
> > 
> > At the first glance, it looks feasible to me. Will look into deeper
> > later.
> 
> A further look told me this might be *not* feasible.
> 
> It looks the new lock will not break check_data_rlimit since in my patch
> both start_brk and brk is protected by mmap_sem. The code flow might look
> like below:
> 
> CPU A                             CPU B
> --------                       --------
> prctl                               sys_brk
>                                       down_write
> check_data_rlimit           check_data_rlimit (need mm->start_brk)
>                                       set brk
> down_write                    up_write
> set start_brk
> set brk
> up_write
> 
> If CPU A gets the mmap_sem first, it will set start_brk and brk, then CPU B
> will check with the new start_brk. And, prctl doesn't care if sys_brk is run
> before it since it gets the new start_brk and brk from parameter.
> 
> If we protect start_brk and brk with the new lock, sys_brk might get old
> start_brk, then sys_brk might break rlimit check silently, is that right?
> 
> So, it looks using new lock in prctl and keeping mmap_sem in brk path has
> race condition.

I fear so. The check_data_rlimit implies that all elements involved into
validation (brk, start_brk, start_data, end_data) are not changed unpredicably
until written back into mm. In turn if we guard start_brk,brk only (as
it is done in the patch) the check_data_rlimit may pass on wrong data
I think. And as you mentioned the race above exact the example of such
situation. I think for prctl case we can simply left use of mmap_sem
as it were before the patch, after all this syscall is really in cold
path all the time.

        Cyrill

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