linux-kernel - Re: shmctl(SHM_STAT) vs. /proc/sysvipc/shm permissions discrepancies

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Message-ID: <CAKgNAkhkkx3znnfUN3rsY+SL7k5R+W0ui8__y1-WMLG=PFrCuQ@mail.gmail.com>
Date:   Wed, 20 Dec 2017 09:44:47 +0100
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     "Dr. Manfred Spraul" <manfred@...orfullife.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Waychison <mikew@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: shmctl(SHM_STAT) vs. /proc/sysvipc/shm permissions discrepancies

Hi Manfred,

On 20 December 2017 at 09:32, Dr. Manfred Spraul
<manfred@...orfullife.com> wrote:
> Hi Michal,
>
> On 12/19/2017 10:48 AM, Michal Hocko wrote:
>>
>> Hi,
>> we have been contacted by our partner about the following permission
>> discrepancy
>> 1. Create a shared memory segment with permissions 600 with user A using
>>     shmget(key, 1024, 0600 | IPC_CREAT)
>> 2. ipcs -m should return an output as follows:
>>
>> ------ Shared Memory Segments --------
>> key        shmid      owner      perms      bytes      nattch     status
>> 0x58b74326 759562241  A          600        1024       0
>>
>> 3. Try to read the metadata with shmctl(0, SHM_STAT,...) as user B.
>> 4. shmctl will return -EACCES
>>
>> The supper set information provided by shmctl can be retrieved by
>> reading /proc/sysvipc/shm which does not require read permissions
>> because it is 444.
>>
>> It seems that the discrepancy is there since ae7817745eef ("[PATCH] ipc:
>> add generic struct ipc_ids seq_file iteration") when the proc interface
>> has been introduced. The changelog is really modest on information or
>> intention but I suspect this just got overlooked during review. SHM_STAT
>> has always been about read permission and it is explicitly documented
>> that way.
>
> Are you sure that this patch changed the behavior?
> The proc interface is much older.

Yes, I think that's correct. The /proc/sysvipc interface appeared in
2.3.x, and AFAIK the behavior was already different from *_STAT back
then.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/