Messages in this thread | | | Date | Mon, 27 Jun 2022 10:51:48 -0500 | Subject | Re: [PATCH 0/2] Introduce security_create_user_ns() | From | Frederick Lawler <> |
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On 6/27/22 7:11 AM, Christian Brauner wrote: > On Thu, Jun 23, 2022 at 11:21:37PM -0400, Paul Moore wrote: >> On Wed, Jun 22, 2022 at 10:24 AM Frederick Lawler <fred@cloudflare.com> wrote: >>> On 6/21/22 7:19 PM, Casey Schaufler wrote: >>>> On 6/21/2022 4:39 PM, Frederick Lawler wrote: >>>>> While creating a LSM BPF MAC policy to block user namespace creation, we >>>>> used the LSM cred_prepare hook because that is the closest hook to >>>>> prevent >>>>> a call to create_user_ns(). >>>>> >>>>> The calls look something like this: >>>>> >>>>> cred = prepare_creds() >>>>> security_prepare_creds() >>>>> call_int_hook(cred_prepare, ... >>>>> if (cred) >>>>> create_user_ns(cred) >>>>> >>>>> We noticed that error codes were not propagated from this hook and >>>>> introduced a patch [1] to propagate those errors. >>>>> >>>>> The discussion notes that security_prepare_creds() >>>>> is not appropriate for MAC policies, and instead the hook is >>>>> meant for LSM authors to prepare credentials for mutation. [2] >>>>> >>>>> Ultimately, we concluded that a better course of action is to introduce >>>>> a new security hook for LSM authors. [3] >>>>> >>>>> This patch set first introduces a new security_create_user_ns() function >>>>> and create_user_ns LSM hook, then marks the hook as sleepable in BPF. >>>> >>>> Why restrict this hook to user namespaces? It seems that an LSM that >>>> chooses to preform controls on user namespaces may want to do so for >>>> network namespaces as well. >>> >>> IIRC, CLONE_NEWUSER is the only namespace flag that does not require >>> CAP_SYS_ADMIN. There is a security use case to prevent this namespace >>> from being created within an unprivileged environment. I'm not opposed >>> to a more generic hook, but I don't currently have a use case to block >>> any others. We can also say the same is true for the other namespaces: >>> add this generic security function to these too. >>> >>> I'm curious what others think about this too. >> >> While user namespaces are obviously one of the more significant >> namespaces from a security perspective, I do think it seems reasonable >> that the LSMs could benefit from additional namespace creation hooks. >> However, I don't think we need to do all of them at once, starting >> with a userns hook seems okay to me. >> >> I also think that using the same LSM hook as an access control point >> for all of the different namespaces would be a mistake. At the very > > Agreed. > >> least we would need to pass a flag or some form of context to the hook >> to indicate which new namespace(s) are being requested and I fear that >> is a problem waiting to happen. That isn't to say someone couldn't >> mistakenly call the security_create_user_ns(...) from the mount >> namespace code, but I suspect that is much easier to identify as wrong >> than the equivalent security_create_ns(USER, ...). > > Yeah, I think that's a pretty unlikely scenario. > >> >> We also should acknowledge that while in most cases the current task's >> credentials are probably sufficient to make any LSM access control >> decisions around namespace creation, it's possible that for some >> namespaces we would need to pass additional, namespace specific info >> to the LSM. With a shared LSM hook this could become rather awkward. > > Agreed. > >> >>>> Also, the hook seems backwards. You should >>>> decide if the creation of the namespace is allowed before you create it. >>>> Passing the new namespace to a function that checks to see creating a >>>> namespace is allowed doesn't make a lot off sense. >>> >>> I think having more context to a security hook is a good thing. >> >> This is one of the reasons why I usually like to see at least one LSM >> implementation to go along with every new/modified hook. The >> implementation forces you to think about what information is necessary >> to perform a basic access control decision; sometimes it isn't always >> obvious until you have to write the access control :) > > I spoke to Frederick at length during LSS and as I've been given to > understand there's a eBPF program that would immediately use this new > hook. Now I don't want to get into the whole "Is the eBPF LSM hook > infrastructure an LSM" but I think we can let this count as a legitimate > first user of this hook/code. > >> >> [aside: If you would like to explore the SELinux implementation let me >> know, I'm happy to work with you on this. I suspect Casey and the >> other LSM maintainers would also be willing to do the same for their >> LSMs.] >>
I can take a shot at making a SELinux implementation, but the question becomes: is that for v2 or a later patch? I don't think the implementation for SELinux would be too complicated (i.e. make a call to avc_has_perm()?) but, testing and revisions might take a bit longer.
>> In this particular case I think the calling task's credentials are >> generally all that is needed. You mention that the newly created > > Agreed. > >> namespace would be helpful, so I'll ask: what info in the new ns do >> you believe would be helpful in making an access decision about its >> creation? >>
In the other thread [1], there was mention of xattr mapping support. As I understand Caseys response to this thread [2], that feature is no longer requested for this hook.
Users can still access the older parent ns from the passed in cred, but I was thinking of handling the transition point here. There's probably more suitable hooks for that case.
>> Once we've sorted that we can make a better decision about the hook >> placement, but right now my gut feeling is that we only need to pass >> the task's creds, and I think placing the hook right after the UID/GID >> mapping check (before the new ns allocation) would be the best spot. >
I don't specifically have a use case to pass the new user namespace for this hook at this time. I'll move the hook in v2.
> When I toyed with this I placed it directly into create_user_ns() and > only relied on the calling task's cred. I just created an eBPF program > that verifies the caller is capable(CAP_SYS_ADMIN). Since both the > chrooted and mapping check return EPERM it doesn't really matter that > much where exactly. Conceptually it makes more sense to me to place it > after the mapping check because then all the preliminaries are done. >
Agreed.
> Christian
Links: 1. https://lore.kernel.org/all/4ae12ee6-959c-51cb-9d7a-54adb3a0ea53@schaufler-ca.com/ 2. https://lore.kernel.org/all/4b62f0c5-9f3c-e0bc-d836-1b7cdea429da@schaufler-ca.com/
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