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

[Michael Kerrisk (man-pages)]

Hello Michal,

On 19 December 2017 at 10:48, Michal Hocko <mhocko@xxxxxxxxxx> 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.

Yes, this was always a weirdness on Linux. Back before we got
/proc/sysvipc, it meant that ipcs(1) on Linux did not did not display
all IPC objects (unlike most other implementations, where ipcs(1)
showed everyone's objects, regardless of permissions). I remember
having an email conversation with Andries Brouwer about this, around
15 years ago. Eventually, an October 2012 series of util-linux patches
by Sami Kerola switched ipcs(1) to use /proc/sysvipc so that ipcs(1)
does now show all System V IPC objects.

> 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?

Do you mean change /proc/sysvipc/* output? I don't think that should
be changed. Modern ipcs(1) relies on it to do The Right Thing.

> 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?

The situation is certainly odd. The only risk that I see is that
modifying *_STAT behavior could lead to behavior changes in (strange?)
programs that expect SHM_STAT / MSG_STAT / SEM_STAT to return only
information about objects for which they have read permission. But, is
there a pressing reason to make the change? (Okay, I guess iterating
using *_STAT is nicer than parsing /proc/sysvipc/*.)

Cheers,

Michael


> --
> Michal Hocko
> SUSE Labs
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-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/