Seth Forshee <seth.forshee@xxxxxxxxxxxxx> writes: > On Thu, Jul 16, 2015 at 12:44:49AM -0500, Eric W. Biederman wrote: >> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: >> >> > On Wed, Jul 15, 2015 at 10:04 PM, Eric W. Biederman >> > <ebiederm@xxxxxxxxxxxx> wrote: >> >> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: >> >> >> >>> >> >>> So here's the semantic question: >> >>> >> >>> Suppose an unprivileged user (uid 1000) creates a user namespace and a >> >>> mount namespace. They stick a file (owned by uid 1000 as seen by >> >>> init_user_ns) in there and mark it setuid root and give it fcaps. >> >> >> >> To make this make sense I have to ask, is this file on a filesystem >> >> where uid 1000 as seen by the init_user_ns stored as uid 1000 on >> >> the filesystem? Or is this uid 0 as seen by the filesystem? >> >> >> >> I assume this is uid 0 on the filesystem in question or else your >> >> unprivileged user would not have sufficient privileges over the >> >> filesystem to setup fcaps. >> > >> > I was thinking uid 0 as seen by the filesystem. But even if it were >> > uid 1000, the unprivileged user can still set whatever mode and xattrs >> > they want -- they control the backing store. >> >> Yes. And that is what I was really asking. Are we taking about a >> filesystem where the user controls the backing store? >> >> >>> Then global root gets an fd to this filesystem. If they execve the >> >>> file directly, then, with my patch 4, it won't act as setuid 1000 and >> >>> the fcaps will be ignored. Even with my patch 4, though, if they bind >> >>> mount the fs and execve the file from their bind mount, it will act as >> >>> setuid 1000. Maybe this is odd. However, with Seth's patch 3, the >> >>> fcaps will (correctly) not be honored. >> >> >> >> With patch 3 you can also think of it as fcaps being honored and you >> >> get all the caps in the appropriate user namespace, but since you are >> >> not in that user namespace and so don't have a place to store them >> >> in struct cred you don't get the file caps. >> >> >> >> From the philosophy of interpreting the file as defined by the >> >> filesystem in principle we could extend struct cred so you actually >> >> get the creds just in uid 1000s user namespace, but that is very >> >> unlikely to be worth it. >> > >> > I agree. >> > >> >> >> >>> I tend to thing that, if we're not honoring the fcaps, we shouldn't be >> >>> honoring the setuid bit either. After all, it's really not a trusted >> >>> file, even though the only user who could have messed with it really >> >>> is the apparent owner. >> >> >> >> For the file caps we can't honor them because you don't have the bits >> >> in struct cred. >> >> >> >> For setuid we can honor it, and setuid is something that the user >> >> namespace allows. >> >> >> > >> > We certainly *can* honor it. But why should we? I'd be more >> > comfortable with this if the contents of an untrusted filesystem were >> > really treated as just data. >> >> In these weird bleed through situtations I don't know that we should. >> But extending nosuid protections in this way is a bit like yama >> a bit gratuitious stomping don't care cases in the semantics to >> make bugs harder to exploit. >> >> >>> And, if we're going to say we don't trust the file and shouldn't honor >> >>> setuid or fcaps, then merging all the functionality into mnt_may_suid >> >>> could make sense. Yes, these two things do different things, but they >> >>> could hook in to the same place. >> >> >> >> There are really two separate questions: >> >> - Do we trust this filesystem? >> >> - Do you have the bits to implement this concept? >> >> >> >> Even if in this specific context the two questions wind up looking >> >> exactly the same. I think it makes a lot of sense to ask the two >> >> questions separately. As future maintenance changes may cause the >> >> implementation of the questions to diverge. >> >> >> > >> > Agreed. >> > >> > Unless someone thinks of an argument to the contrary, I'd say "no, we >> > don't trust this filesystem". I could be convinced otherwise. >> >> But this is context dependent. From the perspective of the container >> we really do want to trust the filesystem. As the container root set it >> up, and if he isn't being hostile likely has a use for setfcaps files >> and setuid files and all of the rest. >> >> Perhaps I should phrase it as: >> - In this context do we trust the code? AKA mnt_may_suid? >> - What do these bits mean in this context? (Usually something more complicated). >> >> Which says to me we want both patches 3 and 4 (even if 4 uses s_user_ns) >> because 3 is different than 4. > > So what I'll do is: > > - Add a s_user_ns check to mnt_may_suid > - Keep the (now redundant) s_user_ns check in get_file_caps > > I'm on the fence about having both the mnt and user ns checks in > mnt_may_suid - it might be overkill, but it still adds the protection > against clearing MNT_NOSUID in a bind mount. So I guess I'll keep the > mnt ns check. That sounds like a plan. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
