On Thu, 21 Dec 2017, Michael Kerrisk (man-pages) wrote:

Hi Michal,

On 21 December 2017 at 09:02, Michal Hocko <mho...@kernel.org> wrote:
On Wed 20-12-17 17:17:46, Michael Kerrisk wrote:
Hello Michal,

On 20 December 2017 at 10:20, Michal Hocko <mho...@kernel.org> 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.

So I took a crack at this, and my only doubt is whether or not the lsm
security hooks should be considered or not. Specifically, should the
SHM_STAT_ALL case consider security_shm_shmctl()?

While the relevant persmission checks that allow for the discripancies
between 0444 procfs and a 0600 via creating the ipc object are done in
ipcperms() returning -1, is there a scenario where some lsm policy could
change the /proc/sysvipc/ interface? If not, I think we can avoid it, but
I'm not a security person.

Thanks,
Davidlohr


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
interfaces.)

Cheers,

Michael



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

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