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

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Date:   Wed, 20 Dec 2017 09:32:01 +0100
From:   "Dr. Manfred Spraul" <manfred@...orfullife.com>
To:     Michal Hocko <mhocko@...nel.org>, linux-api@...r.kernel.org
Cc:     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
Subject: Re: shmctl(SHM_STAT) vs. /proc/sysvipc/shm permissions discrepancies

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.

--
     Manfred