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

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Date:   Tue, 10 Apr 2018 12:33:35 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     Michal Hocko <mhocko@...nel.org>, adobriyan@...il.com,
        willy@...radead.org, mguzik@...hat.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v3 PATCH] mm: introduce arg_lock to protect arg_start|end and
 env_start|end in mm_struct



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 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.

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