On Thu, Dec 02, 2021 at 01:59:55PM +0100, Christian Brauner wrote: > On Wed, Dec 01, 2021 at 02:29:09PM -0500, James Bottomley wrote: > > On Wed, 2021-12-01 at 12:35 -0500, Stefan Berger wrote: > > > On 12/1/21 11:58, James Bottomley wrote: > > > > On Tue, 2021-11-30 at 11:06 -0500, Stefan Berger wrote: > > > > > From: Denis Semakin <denis.semakin@xxxxxxxxxx> > > > > > > > > > > Use integrity_admin_ns_capable() to check corresponding > > > > > capability to allow read/write IMA policy without CAP_SYS_ADMIN > > > > > but with CAP_INTEGRITY_ADMIN. > > > > > > > > > > Signed-off-by: Denis Semakin <denis.semakin@xxxxxxxxxx> > > > > > --- > > > > > security/integrity/ima/ima_fs.c | 2 +- > > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > > > diff --git a/security/integrity/ima/ima_fs.c > > > > > b/security/integrity/ima/ima_fs.c > > > > > index fd2798f2d224..6766bb8262f2 100644 > > > > > --- a/security/integrity/ima/ima_fs.c > > > > > +++ b/security/integrity/ima/ima_fs.c > > > > > @@ -393,7 +393,7 @@ static int ima_open_policy(struct inode > > > > > *inode, > > > > > struct file *filp) > > > > > #else > > > > > if ((filp->f_flags & O_ACCMODE) != O_RDONLY) > > > > > return -EACCES; > > > > > - if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN)) > > > > > + if (!integrity_admin_ns_capable(ns->user_ns)) > > > > so this one is basically replacing what you did in RFC 16/20, which > > > > seems a little redundant. > > > > > > > > The question I'd like to ask is: is there still a reason for > > > > needing CAP_INTEGRITY_ADMIN? My thinking is that now IMA is pretty > > > > much tied to requiring a user (and a mount, because of > > > > securityfs_ns) namespace, there might not be a pressing need for an > > > > admin capability separated from CAP_SYS_ADMIN because the owner of > > > > the user namespace passes the ns_capable(..., CAP_SYS_ADMIN) > > > > check. The rationale in > > > > > > Casey suggested using CAP_MAC_ADMIN, which I think would also work. > > > > > > CAP_MAC_ADMIN (since Linux 2.6.25) > > > Allow MAC configuration or state changes. Implemented > > > for > > > the Smack Linux Security Module (LSM). > > > > > > > > > Down the road I think we should cover setting file extended > > > attributes with the same capability as well for when a user signs > > > files or installs packages with file signatures. A container runtime > > > could hold CAP_SYS_ADMIN while setting up a container and mounting > > > filesystems and drop it for the first process started there. Since we > > > are using the user namespace to spawn an IMA namespace, we would then > > > require CAP_SYSTEM_ADMIN to be left available so that the user can do > > > IMA related stuff in the container (set or append to the policy, > > > write file signatures). I am not sure whether that should be the case > > > or rather give the user something finer grained, such as > > > CAP_MAC_ADMIN. So, it's about granularity... > > > > It's possible ... any orchestration system that doesn't enter a user > > namespace has to strictly regulate capabilities. I'm probably biased > > because I always use a user_ns so I never really had to mess with > > capabilities. > > > > > > https://kernsec.org/wiki/index.php/IMA_Namespacing_design_considerations > > > > > > > > Is effectively "because CAP_SYS_ADMIN is too powerful" but that's > > > > no longer true of the user namespace owner. It only passes the > > > > ns_capable() check not the capable() one, so while it does get > > > > CAP_SYS_ADMIN, it can only use it in a few situations which > > > > represent quite a power reduction already. > > > > > > At least docker containers drop CAP_SYS_ADMIN. > > > > Well docker doesn't use the user_ns. But even given that, > > CAP_SYS_ADMIN is always dropped for most container systems. What > > happens when you enter a user namespace is the ns_capable( ..., > > CAP_SYS_ADMIN) check returns true if you're the owner of the user_ns, > > in the same way it would for root. So effectively entering a user > > namespace without CAP_SYS_ADMIN but mapping the owner id to 0 (what > > unshare -r --user does) gives you back a form of CAP_SYS_ADMIN that > > responds only in the places in the kernel that have a ns_capable() > > check instead of a capable() one (most of the places you list below). > > This is the principle of how unprivileged containers actually work ... > > and the source of some of our security problems if you get back an > > ability to do something you shouldn't be allowed to do as an > > unprivileged user. > > > > > I am not sure what the decision was based on but probably they don't > > > want to give the user what is not absolutely necessary, but usage of > > > user namespaces (with IMA namespaces) would kind of force it to be > > > available then to do IMA-related stuff ... > > > > > > Following this man page here > > > https://man7.org/linux/man-pages/man7/user_namespaces.7.html > > > > > > CAP_SYS_ADMIN in a user namespace is about > > > > > > - bind-mounting filesystems > > > > > > - mounting /proc filesystems > > > > > > - creating nested user namespaces > > > > > > - configuring UTS namespace > > > > > > - configuring whether setgroups() can be used > > > > > > - usage of setns() > > > > > > > > > Do we want to add '- only way of *setting up* IMA related stuff' to > > > this list? > > > > I don't see why not, but other container people should weigh in > > because, as I said, I mostly use the user namespace and unprivileged > > containers and don't bother with capabilities. > > There are very few scenarios where dropping capabilities in an > unprivileged container makes sense. In a lot of other scenarios it is > just a misunderstanding of the meaning of capabilities and their > relationship to user namespaces. Usually, granting a full set of > capabilities to the payload of an unprivigileged container is the right > thing to do. All things that are properly namespaced will check > capabilities in the relevant user namespace. Those that aren't will > check them against the initial user namespaces. > > But I do think the question of whether or not ima should go into > cap_sys_admin is more a question of capability semantics then it is in > how exactly ima is namespaced. We do have agreed before that overloading > cap_sys_admin further isn't ideal. Often we end up rectifying that > mistake later. For example, how we moved stuff like criu, bpf, and perf > to their own capability. Now we're left with stuff like: > > static inline bool perfmon_capable(void) > { > return capable(CAP_PERFMON) || capable(CAP_SYS_ADMIN); > } > > static inline bool bpf_capable(void) > { > return capable(CAP_BPF) || capable(CAP_SYS_ADMIN); > } > > static inline bool checkpoint_restore_ns_capable(struct user_namespace *ns) > { > return ns_capable(ns, CAP_CHECKPOINT_RESTORE) || > ns_capable(ns, CAP_SYS_ADMIN); > } > > for the sake of adhering to legacy behavior. I think we can skip over > that mistake and introduce cap_sys_integrity. (Or under CAP_MAC_ADMIN as suggested elsewhere in the thread as I saw just now.)
