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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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