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    Subject[RFC][PATCH 5/7][v4] Protect cinit from blocked fatal signals

    From: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
    Subject: [RFC][PATCH 5/7][v4] Protect cinit from blocked fatal signals

    Normally SIG_DFL signals to global and container-init are dropped early.
    But if a signal is blocked when it is posted, we cannot drop the signal
    since the receiver may install a handler before unblocking the signal.
    Once this signal is queued however, the receiver container-init has
    no way of knowing if the signal was sent from an ancestor or descendant
    namespace. This patch ensures that contianer-init drops all SIG_DFL
    signals in get_signal_to_deliver() except SIGKILL/SIGSTOP.

    If SIGSTOP/SIGKILL originate from a descendant of container-init they
    are never queued (i.e dropped in sig_ignored() in an earler patch).

    If SIGSTOP/SIGKILL originate from parent namespace, the signal is queued
    and container-init processes the signal.

    See comments in patch below for details.

    Changelog[v2]:
    - Rename sig_unkillable() to unkillable_by_sig()
    - Remove SIGNAL_UNKILLABLE_FROM_NS flag and simplify (Oleg Nesterov)
    - Set SIGNAL_UNKILLABLE for container-init in this patch.

    Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
    ---
    kernel/fork.c | 2 ++
    kernel/signal.c | 41 +++++++++++++++++++++++++++++++++++++++--
    2 files changed, 41 insertions(+), 2 deletions(-)

    diff --git a/kernel/fork.c b/kernel/fork.c
    index dba2d3f..d3e93ef 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -812,6 +812,8 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
    atomic_set(&sig->live, 1);
    init_waitqueue_head(&sig->wait_chldexit);
    sig->flags = 0;
    + if (clone_flags & CLONE_NEWPID)
    + sig->flags |= SIGNAL_UNKILLABLE;
    sig->group_exit_code = 0;
    sig->group_exit_task = NULL;
    sig->group_stop_count = 0;
    diff --git a/kernel/signal.c b/kernel/signal.c
    index 5c4374f..660fadd 100644
    --- a/kernel/signal.c
    +++ b/kernel/signal.c
    @@ -1818,6 +1818,41 @@ static int ptrace_signal(int signr, siginfo_t *info,
    return signr;
    }

    +/*
    + * Return 1 if the process owning @signal should NOT terminate as a result of
    + * the signal @signr. Return 0 otherwise.
    + *
    + * Specifically if process owning @signal is
    + * - neither global nor a container-init, return 0
    + * - the global-init, return 1.
    + * - container-init, return 0 if signal is SIGKILL or SIGSTOP. Return
    + * 1 otherwise.
    + *
    + * sig_ignored() drops any unblocked fatal signals to global/container-init
    + * from within the same namespace. This of course includes SIGKILL/SIGSTOP
    + * which can never be blocked. sig_ignored() does not drop the SIGKILL/
    + * SIGSTOP if the are from an ancestor namespace.
    + *
    + * So, @signal is for a container-init and if @signr is either SIGKILL or
    + * SIGSTOP, it must have come from an ancestor namespace. So container-init
    + * should be killable (return 0).
    + *
    + * If @signal refers to a container-init and @signr is neither SIGKILL nor
    + * SIGSTOP, it was queued because it was blocked when it was posted. The
    + * signal may have come from same container - hence it should not be
    + * killable (return 1).
    + *
    + * Note:
    + * This means that SIGKILL is the only sure way to terminate a
    + * container-init even from ancestor namespace.
    + */
    +static int unkillable_by_sig(struct signal_struct *signal, int signr)
    +{
    + if ((signal->flags & SIGNAL_UNKILLABLE) && !sig_kernel_only(signr))
    + return 1;
    + return 0;
    +}
    +
    int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka,
    struct pt_regs *regs, void *cookie)
    {
    @@ -1909,9 +1944,11 @@ relock:

    /*
    * Global init gets no signals it doesn't want.
    + * Container-init gets no signals it doesn't want from same
    + * container.
    */
    - if (unlikely(signal->flags & SIGNAL_UNKILLABLE) &&
    - !signal_group_exit(signal))
    + if (unkillable_by_sig(signal, signr) &&
    + !signal_group_exit(signal))
    continue;

    if (sig_kernel_stop(signr)) {
    --
    1.5.2.5


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