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Subject[PATCH] signals: check_kill_permission: check session under tasklist_lock
(on top of signals-cleanup-security_task_kill-usage-implementation.patch)

This wasn't documented, but as Atsushi Tsuji <a-tsuji@bk.jp.nec.com> pointed
out check_kill_permission() needs tasklist_lock for task_session_nr().
I missed this fact when removed tasklist from the callers.

Change check_kill_permission() to take tasklist_lock for the SIGCONT case.
Re-order security checks so that we take tasklist_lock only if/when it is
actually needed. This is a minimal fix for now, tasklist will be removed
later.

Also change the code to use task_session() instead of task_session_nr().

Also, remove the SIGCONT check from cap_task_kill(), it is bogus (and the
whole function is bogus. Serge, Eric, why it is still alive?).

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 25/kernel/signal.c~CKP_TAKE_TASKLIST 2008-03-18 14:47:00.000000000 +0300
+++ 25/kernel/signal.c 2008-03-18 17:25:19.000000000 +0300
@@ -533,6 +533,7 @@ static int rm_from_queue(unsigned long m
static int check_kill_permission(int sig, struct siginfo *info,
struct task_struct *t)
{
+ struct pid *sid;
int error;

if (!valid_signal(sig))
@@ -545,11 +546,24 @@ static int check_kill_permission(int sig
if (error)
return error;

- if (((sig != SIGCONT) || (task_session_nr(current) != task_session_nr(t)))
- && (current->euid ^ t->suid) && (current->euid ^ t->uid)
- && (current->uid ^ t->suid) && (current->uid ^ t->uid)
- && !capable(CAP_KILL))
- return -EPERM;
+ if ((current->euid ^ t->suid) && (current->euid ^ t->uid) &&
+ (current->uid ^ t->suid) && (current->uid ^ t->uid) &&
+ !capable(CAP_KILL)) {
+ switch (sig) {
+ case SIGCONT:
+ read_lock(&tasklist_lock);
+ sid = task_session(t);
+ read_unlock(&tasklist_lock);
+ /*
+ * We don't return the error if sid == NULL. The
+ * task was unhashed, the caller must notice this.
+ */
+ if (!sid || sid == task_session(current))
+ break;
+ default:
+ return -EPERM;
+ }
+ }

return security_task_kill(t, info, sig, 0);
}
--- 25/security/commoncap.c~CKP_TAKE_TASKLIST 2008-03-18 17:07:02.000000000 +0300
+++ 25/security/commoncap.c 2008-03-18 17:21:10.000000000 +0300
@@ -552,10 +552,6 @@ int cap_task_kill(struct task_struct *p,
if (p->uid == current->uid)
return 0;

- /* sigcont is permitted within same session */
- if (sig == SIGCONT && (task_session_nr(current) == task_session_nr(p)))
- return 0;
-
if (secid)
/*
* Signal sent as a particular user.


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