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    Subject[PATCH 4/6] wait_for_helper: SIGCHLD from user-space can lead to use-after-free
    1. wait_for_helper() calls allow_signal(SIGCHLD) to ensure the child
    can't autoreap itself.

    However, this means that a spurious SIGCHILD from user-space can
    set TIF_SIGPENDING and:

    - kernel_thread() or sys_wait4() can fail due to signal_pending()

    - worse, wait4() can fail before ____call_usermodehelper() execs
    or exits. In this case the caller may kfree(subprocess_info)
    while the child still uses this memory.

    Change the code to use SIG_DFL instead of magic "(void __user *)2"
    set by allow_signal(). This means that SIGCHLD won't be delivered,
    yet the child won't autoreap itsefl.

    The problem is minor, only root can send a signal to this kthread.

    2. If sys_wait4(&ret) fails it doesn't populate "ret", in this case
    wait_for_helper() reports a random value from uninitialized var.

    With this patch sys_wait4() should never fail, but still it makes
    sense to initialize ret = -ECHILD so that the caller can notice
    the problem.

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Acked-by: Neil Horman <nhorman@tuxdriver.com>
    ---

    kernel/kmod.c | 10 +++++-----
    1 file changed, 5 insertions(+), 5 deletions(-)

    --- 34-rc1/kernel/kmod.c~4_SIGCHLD_RACE 2010-03-15 20:17:47.000000000 +0100
    +++ 34-rc1/kernel/kmod.c 2010-03-15 20:28:31.000000000 +0100
    @@ -175,16 +175,16 @@ static int wait_for_helper(void *data)
    struct subprocess_info *sub_info = data;
    pid_t pid;

    - /* Install a handler: if SIGCLD isn't handled sys_wait4 won't
    - * populate the status, but will return -ECHILD. */
    - allow_signal(SIGCHLD);
    + /* If SIGCLD is ignored sys_wait4 won't populate the status. */
    + spin_lock_irq(&current->sighand->siglock);
    + current->sighand->action[SIGCHLD-1].sa.sa_handler = SIG_DFL;
    + spin_unlock_irq(&current->sighand->siglock);

    pid = kernel_thread(____call_usermodehelper, sub_info, SIGCHLD);
    if (pid < 0) {
    sub_info->retval = pid;
    } else {
    - int ret;
    -
    + int ret = -ECHILD;
    /*
    * Normally it is bogus to call wait4() from in-kernel because
    * wait4() wants to write the exit code to a userspace address.


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