Date: Sat, 3 Mar 2018 10:41:07 +0000 From: "Robert N. M. Watson" <robert.watson@cl.cam.ac.uk> To: Justin Cormack <justin@specialbusservice.com> Cc: Mariusz Zaborski <oshogbo@freebsd.org>, "<cl-capsicum-discuss@lists.cam.ac.uk>" <cl-capsicum-discuss@lists.cam.ac.uk>, freebsd-hackers@freebsd.org Subject: Re: [capsicum] unlinkfd Message-ID: <17DE0BFF-42A2-4CD7-B09C-ABA2606C4041@cl.cam.ac.uk> In-Reply-To: <CAK4o1Wyk54chHobhUkb2PBUtaWOF2rDv6tkX_bFGY6D331xUqw@mail.gmail.com> References: <20180302183514.GA99279@x-wing> <CAK4o1Wyk54chHobhUkb2PBUtaWOF2rDv6tkX_bFGY6D331xUqw@mail.gmail.com>
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FWIW, this is part of why we introduced anonymous POSIX shared memory = objects with Capsicum in FreeBSD -- we allow shm_open(2) to be passed a = SHM_ANON special name, which causes the creation of a swap-backed, = mappable file-like object that can have I/O, memory mapping, etc, = performed on it .. but never has any persistent state across reboots = even in the event of a crash. With Capsicum you can then refine a file descriptor to the otherwise = writable object to be read-only for the purposes of delegation. There is = not, however, a mechanism to "freeze" the state of the object causing = other outstanding writable descriptors to become read-only -- certainly = something could be added, but some care regarding VM semantics would be = required -- in particular, so that faults could not be experienced as a = result of an memory store performed before the "freeze" but issued to = VFS only later. I certainly have no objection to an unlinkat(2) system call -- it's = unfortunate that a full suite of the at(2) APIs wasn't introduced in the = first place. It would be worth checking that no one else (e.g., Solaris, = Mac OS X, Linux) hasn't already added an unlinkat(2) that we can match = API semantics for. I think I take the view that for truly anonymous = objects, shm_open(2) without a name (or the Linux equiv) is the right = thing -- and hence unlinkat(2) is for more conventional use cases where = the final pathname element is known. On directories: There, I find myself falling back on a Casper-like = service, since GC'ing a single anonymous memory object is = straightforward, but GC'ing a directory hierarchy is a more messy = business. Robert > On 3 Mar 2018, at 09:53, Justin Cormack <justin@specialbusservice.com> = wrote: >=20 > I think it would make sense to have an unlinkfd() that unlinks the = file from > everywhere, so it does not need a name to be specified. This might be > hard to implement. >=20 > For temporary files, I really like Linux memfd_create(2) that opens an = anonymous > file without a name. This semantics is really useful. (Linux memfd = also has > additional options for sealing the file fo make it immutable which are = very > useful for safely passing files between processes.) Having a way to = make > unnamed temporary files solves a lot of deletion issues as the file > never needs to > be unlinked. >=20 >=20 > On 2 March 2018 at 18:35, Mariusz Zaborski <oshogbo@freebsd.org> = wrote: >> Hello, >>=20 >> Today I would like to propose a new syscall called unlinkfd(2) which = came up >> during a discussion with Ed Maste. >>=20 >> Currently in UNIX we can=E2=80=99t remove files safely. If we will = try to do so we >> always end up in a race condition. For example when we open a file, = and check >> it with fstat, etc. then we want to unlink(2) it=E2=80=A6 but the = file we are trying to >> unlink could be a different one than the one we were fstating just a = moment ago. >>=20 >> Another reason of implementing unlinkfd(2) came to us when we were = trying >> to sandbox some applications like: uudecode/b64decode or bspatch. It = occured >> to us that we don=E2=80=99t have a good way of removing single files. = Of course we can >> try to determine in which directory we are in, and then open this = directory and >> remove a single file. >>=20 >> It looks even more bizarre if we would think about a program which = operates on >> multiple files. If we would analyze a situation with two totally = different >> directories like `/tmp` and `/home/oshogbo` we would end up with pre = opening >> a root directory or keeping as many directories as we are working on = open. >> All of that effort only to remove two files. This make it totally = impractical! >>=20 >> I think that opening directories also presents some wider attack = vector because >> we are keeping a single descriptor to a directory only to remove one = file. >> Unfortunately this means that an attacker can remove all files in = that directory. >>=20 >> I proposed this as well on the last Capsicum call. There was a = suggestion that >> instead of doing a single syscall maybe we should have a Casper = service that >> will allow us to remove files. Another idea was that we should = perhaps redesign >> programs to create some subdirs work on the subdirs and then remove = all files in >> this subdir. I don=E2=80=99t feel that creating a Casper service is a = good idea because >> we still have exactly the same issue of race condition. In my opinion = creating >> subdirs is also a problem for us. >>=20 >> First we would need to redesign some of our tools and I think we = should >> simplyfiy capsicumizition of the process instead of making it harder. >>=20 >> Secondly we can create a temporary subdirectory but what will remove = it? >> We are going back to having a fd to directory in which we just = created a subdir. >> Another way would be to have Casper service which would remove a = directory but >> with the risk of RC. >>=20 >> In conclusion, I think we need syscall like unlinkfd(2), which turn = out taht it >> is easy to implement. The only downside of this implementation is = that we not >> only need to provide a fd but also a path file. This is because = inodes nor >> vnodes don=E2=80=99t contain filenames. We are comparing vnodes of = the fd and the given >> path, if they are exactly the same we remove a file. In the syscall = we are using >> a fd so there is no Ambient Authority because we are proving that we = already >> have access to that file. Thanks to that the syscall can be safely = used with >> Caspsicum. I have already discussed this with some people and they = said >> `Hey I already had that idea a while ago=E2=80=A6` so let=E2=80=99s = do something with that idea! >> If you are intereted in patch you can find it here: >> https://reviews.freebsd.org/D14567 >>=20 >> Thanks, >> -- >> Mariusz Zaborski >> oshogbo//vx | http://oshogbo.vexillium.org >> FreeBSD commiter | https://freebsd.org >> Software developer | http://wheelsystems.com >> If it's not broken, let's fix it till it is!!1 >=20
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