Re: [PATCH] ptrace: being capable wrt a process requires mapped uids/gids

["Serge E. Hallyn" <serge.hallyn@ubuntu.com>]

On Sat, Dec 26, 2015 at 10:27:33PM +0100, Jann Horn wrote:
> On Sat, Dec 26, 2015 at 03:17:29PM -0600, Serge E. Hallyn wrote:
> > On Sat, Dec 26, 2015 at 03:52:31AM +0100, Jann Horn wrote:
> > > ptrace_has_cap() checks whether the current process should be
> > > treated as having a certain capability for ptrace checks
> > > against another process. Until now, this was equivalent to
> > > has_ns_capability(current, target_ns, CAP_SYS_PTRACE).
> > > 
> > > However, if a root-owned process wants to enter a user
> > > namespace for some reason without knowing who owns it and
> > > therefore can't change to the namespace owner's uid and gid
> > > before entering, as soon as it has entered the namespace,
> > > the namespace owner can attach to it via ptrace and thereby
> > > gain access to its uid and gid.
> > > 
> > > While it is possible for the entering process to switch to
> > > the uid of a claimed namespace owner before entering,
> > > causing the attempt to enter to fail if the claimed uid is
> > > wrong, this doesn't solve the problem of determining an
> > > appropriate gid.
> > > 
> > > With this change, the entering process can first enter the
> > > namespace and then safely inspect the namespace's
> > > properties, e.g. through /proc/self/{uid_map,gid_map},
> > > assuming that the namespace owner doesn't have access to
> > > uid 0.
> > > 
> > > Changed in v2: The caller needs to be capable in the
> > > namespace into which tcred's uids/gids can be mapped.
> > > 
> > > Signed-off-by: Jann Horn <jann@thejh.net>
> > > ---
> > >  kernel/ptrace.c | 33 ++++++++++++++++++++++++++++-----
> > >  1 file changed, 28 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> > > index b760bae..260a08d 100644
> > > --- a/kernel/ptrace.c
> > > +++ b/kernel/ptrace.c
> > > @@ -20,6 +20,7 @@
> > >  #include <linux/uio.h>
> > >  #include <linux/audit.h>
> > >  #include <linux/pid_namespace.h>
> > > +#include <linux/user_namespace.h>
> > >  #include <linux/syscalls.h>
> > >  #include <linux/uaccess.h>
> > >  #include <linux/regset.h>
> > > @@ -207,12 +208,34 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
> > >  	return ret;
> > >  }
> > >  
> > > -static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
> > > +static bool ptrace_has_cap(const struct cred *tcred, unsigned int mode)
> > >  {
> > > +	struct user_namespace *tns = tcred->user_ns;
> > > +
> > > +	/* When a root-owned process enters a user namespace created by a
> > > +	 * malicious user, the user shouldn't be able to execute code under
> > > +	 * uid 0 by attaching to the root-owned process via ptrace.
> > > +	 * Therefore, similar to the capable_wrt_inode_uidgid() check,
> > > +	 * verify that all the uids and gids of the target process are
> > > +	 * mapped into a namespace below the current one in which the caller
> > > +	 * is capable.
> > > +	 * No fsuid/fsgid check because __ptrace_may_access doesn't do it
> > > +	 * either.
> > > +	 */
> > > +	while (
> > > +	    !kuid_has_mapping(tns, tcred->euid) ||
> > > +	    !kuid_has_mapping(tns, tcred->suid) ||
> > > +	    !kuid_has_mapping(tns, tcred->uid)  ||
> > > +	    !kgid_has_mapping(tns, tcred->egid) ||
> > > +	    !kgid_has_mapping(tns, tcred->sgid) ||
> > > +	    !kgid_has_mapping(tns, tcred->gid)) {
> > > +		tns = tns->parent;
> > 
> > Sorry, i can't quite remember - is there a way for a task in init_user_ns to have
> > INVALID_UID | INVALID_GID ?  I.e. any point in breaking here if tns == &init_user_n?
> 
> I assumed that there isn't because the comment above the definition of from_kuid()
> says so. Checking... the syscalls for setting uid/gid seem to enforce that uid/gid
> aren't -1, and setuid/setgid executables require the uid/gid to be mapped. So it
> seems to be true.

Yeah, I knew I'd read it somewhere but couldn't find the comment.  Thanks.