From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Serge E. Hallyn" Subject: Re: [PATCH RFC] user-namespaced file capabilities - now with more magic Date: Fri, 20 May 2016 14:26:07 -0500 Message-ID: <20160520192607.GA11601@mail.hallyn.com> References: <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> <1463691236.2465.74.camel@linux.vnet.ibm.com> <20160520034048.GA31216@mail.hallyn.com> <1463743150.2465.100.camel@linux.vnet.ibm.com> <87mvnklh20.fsf@x220.int.ebiederm.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <87mvnklh20.fsf-JOvCrm2gF+uungPnsOpG7nhyD016LWXt@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: "Eric W. Biederman" Cc: Kees Cook , Linux API , Linux Containers , "Serge E. Hallyn" , LKML , Andy Lutomirski , Seth Forshee , LSM , Michael Kerrisk-manpages , Jann Horn , Mimi Zohar , "Andrew G. Morgan" List-Id: linux-api@vger.kernel.org Quoting Eric W. Biederman (ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org): > Mimi Zohar writes: > > > On Thu, 2016-05-19 at 22:40 -0500, Serge E. Hallyn wrote: > >> Quoting Mimi Zohar (zohar-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org): > >> > On Wed, 2016-05-18 at 16:57 -0500, Serge E. Hallyn wrote: > > > >> > > 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. > >> > >> Might make sense to move that. Though looking at it with fresh eyes I wonder > >> whether adding less code here at __vfs_setxattr_noperm(), i.e. > >> > >> if (!cap_setxattr_makenscap(dentry, &value, &size, &name)) > >> return -EPERM; > >> > >> would be cleaner. > > > > Yes, it would be cleaner, but I'm suggesting you do all the hard work > > making it generic. Then the rest of us can follow your lead. Its more > > likely that you'll get it right. At a high level, it might look like: > > > > /* Permit root in a non-init user_ns to modify the security > > * namespace xattr equivalents (eg. nscapability, ns_ima, etc). > > */ > > if ((current_user_ns() != &init_user_ns) && > > capable_wrt_inode_uidgid(inode, CAP_SETFCAP)) { > > > > if security..capability > > call capability /* set nscapability? */ > > > > else if security.ima > > call ima /* set ns_ima? */ > > } > > Hmm. I am confused about this part of the strategy. > > I don't understand the capability vs nscapability distinction. It seems > to add complexity without benefit. ... Well, yes, we could simply make a new version of security.capability xattr, and make rootid == 0 mean it was written by the init_user_ns. Is that what you mean? -serge