userns: allow killing tasks in your own or child userns

Changelog:
	Dec  8: Fixed bug in my check_kill_permission pointed out by
	        Eric Biederman.
	Dec 13: Apply Eric's suggestion to pass target task into kill_ok_by_cred()
	        for clarity
	Dec 31: address comment by Eric Biederman:
		don't need cred/tcred in check_kill_permission.
	Jan  1: use const cred struct.
	Jan 11: Per Bastian Blank's advice, clean up kill_ok_by_cred().
	Feb 16: kill_ok_by_cred: fix bad parentheses
	Feb 23: per akpm, let compiler inline kill_ok_by_cred

Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Daniel Lezcano <daniel.lezcano@free.fr>
Acked-by: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 22 insertions, 8 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 31751868de88..324eff5468ad 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -636,13 +636,33 @@ static inline bool si_fromuser(const struct siginfo *info)
 }
 
 /*
+ * called with RCU read lock from check_kill_permission()
+ */
+static int kill_ok_by_cred(struct task_struct *t)
+{
+        const struct cred *cred = current_cred();
+        const struct cred *tcred = __task_cred(t);
+
+        if (cred->user->user_ns == tcred->user->user_ns &&
+            (cred->euid == tcred->suid ||
+             cred->euid == tcred->uid ||
+             cred->uid  == tcred->suid ||
+             cred->uid  == tcred->uid))
+                return 1;
+
+        if (ns_capable(tcred->user->user_ns, CAP_KILL))
+                return 1;
+
+        return 0;
+}
+
+/*
  * Bad permissions for sending the signal
  * - the caller must hold the RCU read lock
  */
 static int check_kill_permission(int sig, struct siginfo *info,
                                  struct task_struct *t)
 {
-        const struct cred *cred, *tcred;
         struct pid *sid;
         int error;
 
@@ -656,14 +676,8 @@ static int check_kill_permission(int sig, struct siginfo *info,
         if (error)
                 return error;
 
-        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(t)) {
                 switch (sig) {
                 case SIGCONT:
                         sid = task_session(t);