From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mimi Zohar Subject: Re: [PATCH RFC] user-namespaced file capabilities - now with more magic Date: Thu, 19 May 2016 16:53:56 -0400 Message-ID: <1463691236.2465.74.camel@linux.vnet.ibm.com> References: <20160502035452.GA31837@mail.hallyn.com> <87h9egp2oq.fsf@x220.int.ebiederm.org> <20160503051921.GA31551@mail.hallyn.com> <87bn4nhejj.fsf@x220.int.ebiederm.org> <20160507231012.GA11076@pc.thejh.net> <20160511210221.GA24015@mail.hallyn.com> <20160516211523.GA5282@mail.hallyn.com> <20160516214804.GA5926@mail.hallyn.com> <20160518215752.GA9187@mail.hallyn.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20160518215752.GA9187-7LNsyQBKDXoIagZqoN9o3w@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "Serge E. Hallyn" Cc: LKML , Jann Horn , "Eric W. Biederman" , Seth Forshee , LSM , "Andrew G. Morgan" , Kees Cook , Michael Kerrisk-manpages , "Serge E. Hallyn" , Linux API , Andy Lutomirski , Linux Containers List-Id: linux-api@vger.kernel.org On Wed, 2016-05-18 at 16:57 -0500, Serge E. Hallyn wrote: > This patch introduces a new security.nscapability xattr. It > is mostly like security.capability, but also lists a 'rootid'. > This is the uid_t (in init_user_ns) of the root id (uid 0 in a > namespace) in whose namespaces the file capabilities may take > effect. > > A privileged (cap_setfcap) process in the initial user ns may > set and read this xattr directly. However, its real intent is > to be used as a transparent fallback in user namespaces. > > Root in a user ns cannot be trusted to write security.capability > xattrs, because any user on the host could map his own uid to root > in a namespace, write the xattr, and execute the file with privilege > on the host. > > With this patch, when root in a user ns asks to write security.capability, > the kernel will transparently write a security.nscapability xattr > instead, filling in the kuid of the calling user's root uid. Subsequently, > any task executing the file which has the noted k_uid as its root uid, > or which is in a descendent user_ns of such a user_ns, will run the > file with capabilities. > > When reading the security.capability xattr from a non-init user_ns, a valid > security.nscapability will be shown if it exists. Such a task is not > allowed to read security.nscapability. This could be accomodated, however Add the word "directly" as "to read security.nscapability directly". > it requires the kernel to convert the kuid_t to a valid uid in the reader's > user_ns. So for now it's simply not supported. I really like the idea that the kernel transparently replaces nscapability for capability. > Only a single security.nscapability xattr may be written. This patch > could be expanded to allow a list of capabilities and rootids, however > I do not believe that to be a worthwhile use case. Ok > This allows a simple setxattr to work, allows tar/untar to > work, and allows us to tar in one namespace and untar in > another while preserving the capability, without risking > leaking privilege into a parent namespace. > > Note - listxattr is not being handled here. So results of that can be > inconsistent with get/setxattr. Fixing that will require yet more > deceit in fs/xattr.c. > > Note2 - it may be less sneaky to hide all the magic behind the > security.nscapability xattr. So userspace would need to know to > use that xattr name when needed, but with the same format as > security.capability. The kuid_t rootid would be filled in by the > kernel. That's a middle ground between my last patch and this one. The less userspace needs to differentiate between running in a namespace and not, the better. Note3 - capability is currently protected by EVM, when enabled. Should ns_capability also be a protected xattr? > Signed-off-by: Serge Hallyn > --- > fs/xattr.c | 18 ++- > include/linux/capability.h | 8 +- > include/uapi/linux/capability.h | 19 +++ > include/uapi/linux/xattr.h | 3 + > security/commoncap.c | 253 ++++++++++++++++++++++++++++++++++++++-- > 5 files changed, 291 insertions(+), 10 deletions(-) > > diff --git a/fs/xattr.c b/fs/xattr.c > index 4861322..5c0e7ae 100644 > --- a/fs/xattr.c > +++ b/fs/xattr.c > @@ -94,11 +94,26 @@ int __vfs_setxattr_noperm(struct dentry *dentry, const char *name, > { > struct inode *inode = dentry->d_inode; > int error = -EOPNOTSUPP; > + void *wvalue = NULL; > + size_t wsize = 0; > int issec = !strncmp(name, XATTR_SECURITY_PREFIX, > XATTR_SECURITY_PREFIX_LEN); > > - if (issec) > + if (issec) { > inode->i_flags &= ~S_NOSEC; > + /* if root in a non-init user_ns tries to set > + * security.capability, write a security.nscapability > + * in its place */ > + if (!strcmp(name, "security.capability") && > + current_user_ns() != &init_user_ns) { > + cap_setxattr_make_nscap(dentry, value, size, &wvalue, &wsize); > + if (!wvalue) > + return -EPERM; > + value = wvalue; > + size = wsize; > + name = "security.nscapability"; > + } The call to capable_wrt_inode_uidgid() is hidden behind cap_setxattr_make_nscap(). Does it make sense to call it here instead, before the security.capability test? This would lay the foundation for doing something similar for IMA. (Will continue reviewing ...) Mimi