From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: shmctl(SHM_STAT) vs. /proc/sysvipc/shm permissions discrepancies Date: Wed, 20 Dec 2017 10:20:25 +0100 Message-ID: <20171220092025.GD4831@dhcp22.suse.cz> References: <20171219094848.GE2787@dhcp22.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org To: "Michael Kerrisk (man-pages)" Cc: Linux API , Manfred Spraul , Andrew Morton , Al Viro , Kees Cook , Linus Torvalds , Mike Waychison , LKML , "linux-mm@kvack.org" List-Id: linux-api@vger.kernel.org On Tue 19-12-17 17:45:40, Michael Kerrisk wrote: > Hello Michal, > > On 19 December 2017 at 10:48, Michal Hocko 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. Thanks for the clarification. > > 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. OK, I somehow suspected somebody will rely on this. > > 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. Hmm, do you mean those would iterate shmid space to find their own? That would be certainly odd. > 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. So they would like to use the syscall but the interacting parties do not have compatible permissions. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org