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

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Date:   Wed, 20 Dec 2017 10:13:26 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:     "Dr. Manfred Spraul" <manfred@...orfullife.com>,
        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

On Wed 20-12-17 09:44:47, Michael Kerrisk wrote:
> 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.

I have probably misread the patch. It surely adds sysvipc_proc_fops,
maybe there was a different implementation previously. I haven't
checked.
-- 
Michal Hocko
SUSE Labs