Re: [v3 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Yang Shi]

On 4/10/18 12:17 PM, Cyrill Gorcunov wrote:
> 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 AÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ CPU B
> > --------ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ --------
> > prctlÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ sys_brk
> >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ down_write
> > check_data_rlimitÂÂÂÂÂÂÂÂÂÂ check_data_rlimit (need mm->start_brk)
> >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ set brk
> > down_writeÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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.

The race condition is just valid when protecting start_brk, brk, 
start_data and end_data with the new lock, but keep using mmap_sem in 
brk path.

So, we should just need make a little tweak to have mmap_sem protect 
start_brk, brk, start_data and end_data, then have the new lock protect 
others so that we still can remove mmap_sem in proc as the patch is 
aimed to do.

Yang

> 	Cyrill