linux-kernel - Re: [PATCH] prctl: add PR_[GS]ET

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Date:   Tue, 31 Jul 2018 18:26:02 +0200
From:   Jann Horn <jannh@...gle.com>
To:     j@...ron.ch
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux API <linux-api@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] prctl: add PR_[GS]ET_KILLABLE

On Mon, Jul 30, 2018 at 10:01 AM Jürg Billeter <j@...ron.ch> wrote:
>
> PR_SET_KILLABLE clears the SIGNAL_UNKILLABLE flag. This allows
> CLONE_NEWPID tasks to restore normal signal behavior, opting out of the
> special signal protection for init processes.
>
> This is required for job control in a shell that uses CLONE_NEWPID for
> child processes.
>
> This prctl does not allow setting the SIGNAL_UNKILLABLE flag, only
> clearing.
>
> Signed-off-by: Jürg Billeter <j@...ron.ch>
> ---
[...]
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 38509dc1f77b..264de630d548 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
[...]
> +       case PR_SET_KILLABLE:
> +               if (arg2 != 1 || arg3 || arg4 || arg5)
> +                       return -EINVAL;
> +               me->signal->flags &= ~SIGNAL_UNKILLABLE;
> +               break;

I don't have an opinion on this patchset otherwise, but should this
prctl maybe block PR_SET_KILLABLE if you're actually the real init
process? This seems like it could potentially lead to weird things.

This code in kernel/fork.c seems to rely on the fact that global init
is SIGNAL_UNKILLABLE, and probably also leads to weirdness if
container init is non-SIGNAL_UNKILLABLE:

        /*
         * Siblings of global init remain as zombies on exit since they are
         * not reaped by their parent (swapper). To solve this and to avoid
         * multi-rooted process trees, prevent global and container-inits
         * from creating siblings.
         */
        if ((clone_flags & CLONE_PARENT) &&
                                current->signal->flags & SIGNAL_UNKILLABLE)
                return ERR_PTR(-EINVAL);