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

From: Michael Kerrisk (man-pages)
Date: Thu Dec 21 2017 - 03:56:43 EST

Hi Michal,

On 21 December 2017 at 09:02, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Wed 20-12-17 17:17:46, Michael Kerrisk wrote:
>> Hello Michal,
>> On 20 December 2017 at 10:20, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>> > On Tue 19-12-17 17:45:40, Michael Kerrisk wrote:
>> >> But, is
>> >> there a pressing reason to make the change? (Okay, I guess iterating
>> >> using *_STAT is nicer than parsing /proc/sysvipc/*.)
>> >
>> > The reporter of this issue claims that "Reading /proc/sysvipc/shm is way
>> > slower than executing the system call." I haven't checked that but I can
>> > imagine that /proc/sysvipc/shm can take quite some time when there are
>> > _many_ segments registered.
>> Yes, that makes sense.
>> > So they would like to use the syscall but
>> > the interacting parties do not have compatible permissions.
>> So, I don't think there is any security issue, since the same info is
>> available in /proc/sysvipc/*.
> Well, I am not sure this is a valid argument (maybe I just misread your
> statement).

(Or perhaps I was not clear enough; see below)

> Our security model _might_ be broken because of the sysipc
> proc interface existance already. I am not saying it is broken because
> I cannot see an attack vector based solely on the metadata information
> knowledge. An attacker still cannot see/modify the real data. But maybe
> there are some bugs lurking there and knowing the metadata might help to
> exploit them. I dunno.
> You are certainly right that modifying/adding STAT flag to comply with
> the proc interface permission model will not make the system any more
> vulnerable, though.

Yep, that was my point. Modifying _STAT behavior won't decrease security.

That said, /proc/sysvipc/* has been around for a long time now, and
nothing bad seems to have happened so far, AFAIK.

>> The only question would be whether
>> change in the *_STAT behavior might surprise some applications into
>> behaving differently. I presume the chances of that are low, but if it
>> was a concert, one could add new shmctl/msgctl/semctl *_STAT_ALL (or
>> some such) operations that have the desired behavior.
> I would lean towards _STAT_ALL because this is Linux specific behavior
> (I have looked at what BSD does here and they are checking permissions
> for STAT as well). It would also be simpler to revert if we ever find
> that this is a leak with security consequences.

Oh -- I was unaware of this BSD behavior. At least on the various UNIX
systems that I ever used SYSVIPC (including one or two ancient
commercial BSD derivatives), ipcs(1) showed all IPC objects. (On
FeeBSD, at least, it looks like ipcs(1) doesn't use the *_STAT



Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training: