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Message-ID: <20171220091326.GC4831@dhcp22.suse.cz> 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
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