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    Subject[PATCH 4.4 11/22] kernel/signal.c: remove the no longer needed SIGNAL_UNKILLABLE check in complete_signal()
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Oleg Nesterov <oleg@redhat.com>

    commit 426915796ccaf9c2bd9bb06dc5702225957bc2e5 upstream.

    complete_signal() checks SIGNAL_UNKILLABLE before it starts to destroy
    the thread group, today this is wrong in many ways.

    If nothing else, fatal_signal_pending() should always imply that the
    whole thread group (except ->group_exit_task if it is not NULL) is
    killed, this check breaks the rule.

    After the previous changes we can rely on sig_task_ignored();
    sig_fatal(sig) && SIGNAL_UNKILLABLE can only be true if we actually want
    to kill this task and sig == SIGKILL OR it is traced and debugger can
    intercept the signal.

    This should hopefully fix the problem reported by Dmitry. This
    test-case

    static int init(void *arg)
    {
    for (;;)
    pause();
    }

    int main(void)
    {
    char stack[16 * 1024];

    for (;;) {
    int pid = clone(init, stack + sizeof(stack)/2,
    CLONE_NEWPID | SIGCHLD, NULL);
    assert(pid > 0);

    assert(ptrace(PTRACE_ATTACH, pid, 0, 0) == 0);
    assert(waitpid(-1, NULL, WSTOPPED) == pid);

    assert(ptrace(PTRACE_DETACH, pid, 0, SIGSTOP) == 0);
    assert(syscall(__NR_tkill, pid, SIGKILL) == 0);
    assert(pid == wait(NULL));
    }
    }

    triggers the WARN_ON_ONCE(!(task->jobctl & JOBCTL_STOP_PENDING)) in
    task_participate_group_stop(). do_signal_stop()->signal_group_exit()
    checks SIGNAL_GROUP_EXIT and return false, but task_set_jobctl_pending()
    checks fatal_signal_pending() and does not set JOBCTL_STOP_PENDING.

    And his should fix the minor security problem reported by Kyle,
    SECCOMP_RET_TRACE can miss fatal_signal_pending() the same way if the
    task is the root of a pid namespace.

    Link: http://lkml.kernel.org/r/20171103184246.GD21036@redhat.com
    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Reported-by: Dmitry Vyukov <dvyukov@google.com>
    Reported-by: Kyle Huey <me@kylehuey.com>
    Reviewed-by: Kees Cook <keescook@chromium.org>
    Tested-by: Kyle Huey <me@kylehuey.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/signal.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/kernel/signal.c
    +++ b/kernel/signal.c
    @@ -919,9 +919,9 @@ static void complete_signal(int sig, str
    * then start taking the whole group down immediately.
    */
    if (sig_fatal(p, sig) &&
    - !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) &&
    + !(signal->flags & SIGNAL_GROUP_EXIT) &&
    !sigismember(&t->real_blocked, sig) &&
    - (sig == SIGKILL || !t->ptrace)) {
    + (sig == SIGKILL || !p->ptrace)) {
    /*
    * This signal will be fatal to the whole group.
    */

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