From: Yang Shi <yang.shi@linux.alibaba.com>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>,
adobriyan@gmail.com, willy@infradead.org, mguzik@redhat.com,
akpm@linux-foundation.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [v3 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct
Date: Tue, 10 Apr 2018 12:33:35 -0700 [thread overview]
Message-ID: <e868b50d-88a3-a649-d998-b7f2bb2c40aa@linux.alibaba.com> (raw)
In-Reply-To: <20180410191742.GE2041@uranus.lan>
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
next prev parent reply other threads:[~2018-04-10 19:33 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-09 21:52 [v3 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct Yang Shi
2018-04-10 8:48 ` Cyrill Gorcunov
2018-04-10 9:09 ` Michal Hocko
2018-04-10 9:40 ` Cyrill Gorcunov
2018-04-10 10:42 ` Michal Hocko
2018-04-10 11:02 ` Cyrill Gorcunov
2018-04-10 11:10 ` Michal Hocko
2018-04-10 12:28 ` Cyrill Gorcunov
2018-04-10 16:21 ` Yang Shi
2018-04-10 18:28 ` Yang Shi
2018-04-10 19:17 ` Cyrill Gorcunov
2018-04-10 19:33 ` Yang Shi [this message]
2018-04-10 20:06 ` Cyrill Gorcunov
2018-04-12 12:18 ` Michal Hocko
2018-04-12 16:20 ` Yang Shi
2018-04-13 6:56 ` Michal Hocko
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