Date: Sat, 3 Mar 2018 09:53:25 +0000 From: Justin Cormack <justin@specialbusservice.com> To: Mariusz Zaborski <oshogbo@freebsd.org> Cc: "<cl-capsicum-discuss@lists.cam.ac.uk>" <cl-capsicum-discuss@lists.cam.ac.uk>, freebsd-hackers@freebsd.org Subject: Re: [capsicum] unlinkfd Message-ID: <CAK4o1Wyk54chHobhUkb2PBUtaWOF2rDv6tkX_bFGY6D331xUqw@mail.gmail.com> In-Reply-To: <20180302183514.GA99279@x-wing> References: <20180302183514.GA99279@x-wing>
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I think it would make sense to have an unlinkfd() that unlinks the file fro= m everywhere, so it does not need a name to be specified. This might be hard to implement. For temporary files, I really like Linux memfd_create(2) that opens an anon= ymous 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. On 2 March 2018 at 18:35, Mariusz Zaborski <oshogbo@freebsd.org> wrote: > Hello, > > Today I would like to propose a new syscall called unlinkfd(2) which came= up > during a discussion with Ed Maste. > > 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 c= heck > 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 mome= nt ago. > > Another reason of implementing unlinkfd(2) came to us when we were trying > to sandbox some applications like: uudecode/b64decode or bspatch. It occu= red > 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 directo= ry and > remove a single file. > > It looks even more bizarre if we would think about a program which operat= es on > multiple files. If we would analyze a situation with two totally differen= t > directories like `/tmp` and `/home/oshogbo` we would end up with pre open= ing > 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 impract= ical! > > I think that opening directories also presents some wider attack vector b= ecause > 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 di= rectory. > > 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 t= hat > will allow us to remove files. Another idea was that we should perhaps re= design > programs to create some subdirs work on the subdirs and then remove all f= iles in > this subdir. I don=E2=80=99t feel that creating a Casper service is a goo= d idea because > we still have exactly the same issue of race condition. In my opinion cre= ating > subdirs is also a problem for us. > > 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. > > 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 director= y but > with the risk of RC. > > In conclusion, I think we need syscall like unlinkfd(2), which turn out t= aht 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 no= r > 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 ar= e using > a fd so there is no Ambient Authority because we are proving that we alre= ady > have access to that file. Thanks to that the syscall can be safely used w= ith > 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 so= mething with that idea! > If you are intereted in patch you can find it here: > https://reviews.freebsd.org/D14567 > > 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
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