shmctl(SHM_STAT) vs. /proc/sysvipc/shm permissions discrepancies

From: Michal Hocko
Date: Tue Dec 19 2017 - 04:48:59 EST

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.

I am not a security expert to judge whether this leak can have some
interesting consequences but I am really interested whether this is
something we want to keep that way. Do we want to filter and dump only
shmids the caller has access to? This would break the delegation AFAICS.
Do we want to make the file root only? That would probably break an
existing userspace as well.

Or should we simply allow SHM_STAT for processes without a read permission
because the same information can be read by other means already?

Any other ideas?
Michal Hocko