|From:||Eric Paris <eparis-AT-redhat.com>|
|To:||Kees Cook <kees.cook-AT-canonical.com>|
|Subject:||Re: [PATCH v2] fs: block cross-uid sticky symlinks|
|Date:||Tue, 01 Jun 2010 11:34:37 -0400|
|Cc:||Christoph Hellwig <hch-AT-infradead.org>, James Morris <jmorris-AT-namei.org>, linux-kernel-AT-vger.kernel.org, linux-security-module-AT-vger.kernel.org, linux-fsdevel-AT-vger.kernel.org, linux-doc-AT-vger.kernel.org, Randy Dunlap <rdunlap-AT-xenotime.net>, Andrew Morton <akpm-AT-linux-foundation.org>, Jiri Kosina <jkosina-AT-suse.cz>, Dave Young <hidave.darkstar-AT-gmail.com>, Martin Schwidefsky <schwidefsky-AT-de.ibm.com>, David Howells <dhowells-AT-redhat.com>, Ingo Molnar <mingo-AT-elte.hu>, Peter Zijlstra <a.p.zijlstra-AT-chello.nl>, "Eric W. Biederman" <ebiederm-AT-xmission.com>, Tim Gardner <tim.gardner-AT-canonical.com>, "Serge E. Hallyn" <serue-AT-us.ibm.com>|
On Tue, 2010-06-01 at 07:52 -0700, Kees Cook wrote: > On Tue, Jun 01, 2010 at 07:55:02AM -0400, Eric Paris wrote: > > On Tue, 2010-06-01 at 03:55 -0400, Christoph Hellwig wrote: > > > On Mon, May 31, 2010 at 08:24:23PM -0700, Kees Cook wrote: > > > > > > My rationale is that if it's in commoncaps, it's effective for everyone, so > > > > it might as well be in core VFS. If the VFS objections really do boil down > > > > to "not in fs/" then I'm curious if doing this in commoncaps is acceptable. > > > > > > If you think the objection is about having things in fs/ you're smoking > > > some really bad stuff. > > > > Sounds to me like we should probably follow the same path as > > mmap_min_addr. We should add these hooks right in the VFS where they > > belong (much like mmap_min_addr hooks into the vm) and control them 2 > > ways. > > > > 1) a Kconfig so distros can choose to turn it on or off by default > > 2) a /proc interface so root can turn it off > > > > Nothing about that precludes additional similar checks inside an LSM > > (like CONFIG_LSM_MMAP_MIN_ADDR) which can be more finely controlled. So > > maybe we want to follow up with the core VFS check with new checks in > > SELinux (and maybe apparmour). This allows the user to disable the > > general check and still be provided with some modicum of protection. > > You might ask why not ONLY do the check in SELinux and drop the generic > > check, but we have seen with mmap_min_addr that the SELinux unconfined > > user can do damn well anything it wants to, so having a non-LSM version > > of appropriate security checks is highly regarded. > > Would a CONFIG for this be overkill? mmap_min_addr is a little different > in that there was desire to control a bottom limit on it, etc. Given this > is either "on" or "off", I think just a sysctl is needed? Seems like one of Alan's main arguments is that you should not turn it on 'by default.' I assume most distros will want it on by default. Alan made the same argument against mmap_min_addr (known to break dos emu) but I think most major distros have it on by default these days even if it does break those weird obscure use cases. I guess distros can do it through sysctl but Fedora, at least, likes to keep those default if possible, which is why I suggested the CONFIG. In any case, putting this right square in the VFS where it happens makes the most sense to me. I'd also like to point out that I don't buy the argument that per user /tmp/ is a 'better' solution for the general case. Any application that would be broken by this change will also be broken by per user /tmp. Now, if we used filesystem namespaces regularly for years and users, administrators, and developers dealt with them often I agree that would probably be the preferred solution. It would solve this issue, but in introduces a whole host of other problems that are even more obvious and even likely to bite people. I probably would move the security hook down into __do_follow_link and put this check down there as well, but I think you still have a problem with d_parent. I don't see what keeps d_parent from being freed while you are using it.... -Eric -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to firstname.lastname@example.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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