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    Subject[RFC][PATCH 6/7][v4] SI_USER: Masquerade si_pid when crossing pid ns boundary

    From: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
    Subject: [RFC][PATCH 6/7][v4] SI_USER: Masquerade si_pid when crossing pid ns boundary

    When sending a signal to a descendant namespace, we must set ->si_pid to
    0 since the sender does not have a valid pid in the descendant namespace.
    But we cannot do this in sys_kill() since a) we do not yet know the pid
    namespace of receiver b) the pid number in sys_kill() may correspond to
    a process group and processes in the process group may be in different
    namespaces. So masquerade si_pid at the lower level, send_signal()
    function.

    Note:
    - If rt_sigqueueinfo() sets si_code to SI_USER when sending a
    signal across a pid namespace boundary, the value in ->si_pid
    will be cleared to 0.

    Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
    ---
    kernel/signal.c | 32 +++++++++++++++++++++++++++++++-
    1 files changed, 31 insertions(+), 1 deletions(-)

    diff --git a/kernel/signal.c b/kernel/signal.c
    index 660fadd..5a6aea8 100644
    --- a/kernel/signal.c
    +++ b/kernel/signal.c
    @@ -885,6 +885,34 @@ static inline int siginfo_from_ancestor_ns(struct task_struct *t,
    return 0;
    }

    +/*
    + * masquerade_si_pid():
    + *
    + * Called when signal is crossing pid namespace boundary. Compute
    + * pid of sender in receiver's pid namespace.
    + *
    + * We only care about SI_USER here. Other si_codes are addressed
    + * either at the 'origin' of signal or set to a default value in
    + * send_signal(). But we can't address SI_USER at 'origin', i.e .
    + * in sys_kill(), since we may have a process group, rather than
    + * a specific process there. And different processes in same pgrp
    + * can be in different namespaces.
    + */
    +static void masquerade_si_pid(struct task_struct *t, siginfo_t *info)
    +{
    + if (is_si_special(info) || SI_FROMKERNEL(info))
    + return;
    +
    + /*
    + * When crossing pid namespace boundary, SI_USER signal can only
    + * go from ancestor to descendant ns but not the other way. So,
    + * just ->si_pid to 0 since, the sender will not have a pid in
    + * the receiver's namespace.
    + */
    + if (info->si_code == SI_USER)
    + info->si_pid = 0;
    +}
    +
    static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
    int group)
    {
    @@ -946,6 +974,8 @@ static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
    break;
    default:
    copy_siginfo(&q->info, info);
    + if (from_ancestor_ns)
    + masquerade_si_pid(t, &q->info);
    break;
    }
    } else if (!is_si_special(info)) {
    @@ -2343,7 +2373,7 @@ sys_kill(pid_t pid, int sig)
    info.si_signo = sig;
    info.si_errno = 0;
    info.si_code = SI_USER;
    - info.si_pid = task_tgid_vnr(current);
    + info.si_pid = 0; /* masquerade in send_signal() */
    info.si_uid = current_uid();

    return kill_something_info(sig, &info, pid);
    --
    1.5.2.5


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