From mboxrd@z Thu Jan 1 00:00:00 1970 From: Davidlohr Bueso Subject: Re: shmctl(SHM_STAT) vs. /proc/sysvipc/shm permissions discrepancies Date: Mon, 12 Feb 2018 09:30:37 -0800 Message-ID: <20180212173037.oruxafinai5tkv6t@linux-n805> References: <20171219094848.GE2787@dhcp22.suse.cz> <20171220092025.GD4831@dhcp22.suse.cz> <20171221080203.GZ4831@dhcp22.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "Michael Kerrisk (man-pages)" Cc: Michal Hocko , Linux API , Manfred Spraul , Andrew Morton , Al Viro , Kees Cook , Linus Torvalds , Mike Waychison , LKML , "linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org" List-Id: linux-api@vger.kernel.org On Thu, 21 Dec 2017, Michael Kerrisk (man-pages) wrote: >Hi Michal, > >On 21 December 2017 at 09:02, Michal Hocko wrote: >> On Wed 20-12-17 17:17:46, Michael Kerrisk wrote: >>> Hello Michal, >>> >>> On 20 December 2017 at 10:20, Michal Hocko 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/