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

[Jann Horn <jann@thejh.net>]

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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.


> > +	}
> > +
> >  	if (mode & PTRACE_MODE_NOAUDIT)
> > -		return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
> > +		return has_ns_capability_noaudit(current, tns, CAP_SYS_PTRACE);
> >  	else
> > -		return has_ns_capability(current, ns, CAP_SYS_PTRACE);
> > +		return has_ns_capability(current, tns, CAP_SYS_PTRACE);
> >  }
> >  
> >  /* Returns 0 on success, -errno on denial. */
> > @@ -241,7 +264,7 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> >  	    gid_eq(cred->gid, tcred->sgid) &&
> >  	    gid_eq(cred->gid, tcred->gid))
> >  		goto ok;
> > -	if (ptrace_has_cap(tcred->user_ns, mode))
> > +	if (ptrace_has_cap(tcred, mode))
> >  		goto ok;
> >  	rcu_read_unlock();
> >  	return -EPERM;
> > @@ -252,7 +275,7 @@ ok:
> >  		dumpable = get_dumpable(task->mm);
> >  	rcu_read_lock();
> >  	if (dumpable != SUID_DUMP_USER &&
> > -	    !ptrace_has_cap(__task_cred(task)->user_ns, mode)) {
> > +	    !ptrace_has_cap(__task_cred(task), mode)) {
> >  		rcu_read_unlock();
> >  		return -EPERM;
> >  	}
> > -- 
> > 2.1.4
> > 
> > --
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