prlimit: do_prlimit needs to have a speculation check

do_prlimit() adds the user-controlled resource value to a pointer that
will subsequently be dereferenced.  In order to help prevent this
codepath from being used as a spectre "gadget" a barrier needs to be
added after checking the range.
Reported-by: Jordy Zomer <jordyzomer@google.com>
Tested-by: Jordy Zomer <jordyzomer@google.com>
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/sys.c

@@ -1442,6 +1442,8 @@ static int do_prlimit(struct task_struct *tsk, unsigned int resource,
 
 	if (resource >= RLIM_NLIMITS)
 		return -EINVAL;
+	resource = array_index_nospec(resource, RLIM_NLIMITS);
+
 	if (new_rlim) {
 		if (new_rlim->rlim_cur > new_rlim->rlim_max)
 			return -EINVAL;