Re: shmctl(SHM_STAT) vs. /proc/sysvipc/shm permissions discrepancies
From: Michael Kerrisk (man-pages)
Date: Wed Dec 20 2017 - 03:45:17 EST
On 20 December 2017 at 09:32, Dr. Manfred Spraul
> Hi Michal,
> On 12/19/2017 10:48 AM, Michal Hocko wrote:
>> we have been contacted by our partner about the following permission
>> 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
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