[RFC PATCH 4/4] allow killing tasks in your own or child userns

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

Changelog:
	Dec 8: Fixed bug in my check_kill_permission pointed out by
	       Eric Biederman.

To test:
	1. Test killing tasks as usual.  No change.
	2. Clone a new user namespace without a new pidns.
	   a. You CAN kill -CONT tasks in your thread group but outside
	      your user ns.
	   b. You can NOT otherwise kill tasks outside your user_ns.
	   c. Inside your new userns, signal semantics are as normal
	      with respect to userids, CAP_KILL, and thread groups.

Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
---
 kernel/signal.c |   27 ++++++++++++++++++++++-----
 1 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 4e3cff1..677025c 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -635,6 +635,27 @@ static inline bool si_fromuser(const struct siginfo *info)
 		(!is_si_special(info) && SI_FROMUSER(info));
 }
 
+static inline int kill_ok_by_cred(struct cred *cred, struct cred *tcred)
+{
+	if (cred->user->user_ns != tcred->user->user_ns) {
+		/* userids are not equivalent - either you have the
+		   capability to the target user ns or you don't */
+		if (ns_capable(tcred->user->user_ns, CAP_KILL))
+			return 1;
+		return 0;
+	}
+
+	/* same user namespace - usual credentials checks apply */
+	if ((cred->euid ^ tcred->suid) &&
+	    (cred->euid ^ tcred->uid) &&
+	    (cred->uid  ^ tcred->suid) &&
+	    (cred->uid  ^ tcred->uid) &&
+	    !ns_capable(tcred->user->user_ns, CAP_KILL))
+		return 0;
+
+	return 1;
+}
+
 /*
  * Bad permissions for sending the signal
  * - the caller must hold the RCU read lock
@@ -659,11 +680,7 @@ static int check_kill_permission(int sig, struct siginfo *info,
 	cred = current_cred();
 	tcred = __task_cred(t);
 	if (!same_thread_group(current, t) &&
-	    (cred->euid ^ tcred->suid) &&
-	    (cred->euid ^ tcred->uid) &&
-	    (cred->uid  ^ tcred->suid) &&
-	    (cred->uid  ^ tcred->uid) &&
-	    !capable(CAP_KILL)) {
+	    !kill_ok_by_cred(cred, tcred)) {
 		switch (sig) {
 		case SIGCONT:
 			sid = task_session(t);
-- 
1.7.2.3