pidns: fix set/getpriority and ioprio_set/get in PRIO_USER mode

setpriority(PRIO_USER, 0, x) will change the priority of tasks outside of
the current pid namespace.  This is in contrast to both the other modes of
setpriority and the example of kill(-1).  Fix this.  getpriority and
ioprio have the same failure mode, fix them too.

Eric said:

: After some more thinking about it this patch sounds justifiable.
:
: My goal with namespaces is not to build perfect isolation mechanisms
: as that can get into ill defined territory, but to build well defined
: mechanisms.  And to handle the corner cases so you can use only
: a single namespace with well defined results.
:
: In this case you have found the two interfaces I am aware of that
: identify processes by uid instead of by pid.  Which quite frankly is
: weird.  Unfortunately the weird unexpected cases are hard to handle
: in the usual way.
:
: I was hoping for a little more information.  Changes like this one we
: have to be careful of because someone might be depending on the current
: behavior.  I don't think they are and I do think this make sense as part
: of the pid namespace.

Signed-off-by: Ben Segall <bsegall@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ambrose Feinstein <ambrose@google.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

block/ioprio.c

@@ -123,7 +123,8 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
 				break;
 
 			do_each_thread(g, p) {
-				if (!uid_eq(task_uid(p), uid))
+				if (!uid_eq(task_uid(p), uid) ||
+				    !task_pid_vnr(p))
 					continue;
 				ret = set_task_ioprio(p, ioprio);
 				if (ret)
@@ -220,7 +221,8 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 				break;
 
 			do_each_thread(g, p) {
-				if (!uid_eq(task_uid(p), user->uid))
+				if (!uid_eq(task_uid(p), user->uid) ||
+				    !task_pid_vnr(p))
 					continue;
 				tmpio = get_task_ioprio(p);
 				if (tmpio < 0)

kernel/sys.c

@@ -222,7 +222,7 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
 				goto out_unlock;	/* No processes for this user */
 		}
 		do_each_thread(g, p) {
-			if (uid_eq(task_uid(p), uid))
+			if (uid_eq(task_uid(p), uid) && task_pid_vnr(p))
 				error = set_one_prio(p, niceval, error);
 		} while_each_thread(g, p);
 		if (!uid_eq(uid, cred->uid))
@@ -290,7 +290,7 @@ SYSCALL_DEFINE2(getpriority, int, which, int, who)
 				goto out_unlock;	/* No processes for this user */
 		}
 		do_each_thread(g, p) {
-			if (uid_eq(task_uid(p), uid)) {
+			if (uid_eq(task_uid(p), uid) && task_pid_vnr(p)) {
 				niceval = nice_to_rlimit(task_nice(p));
 				if (niceval > retval)
 					retval = niceval;