Re: [PATCH] prctl: add PR_[GS]ET_PDEATHSIG_PROC

[Oleg Nesterov <oleg@redhat.com>]

On 09/09, Jürg Billeter wrote:
>
> PR_SET_PDEATHSIG_PROC sets a process-based death signal.

I think the patch is technically correct,

> Unlike
> PR_SET_PDEATHSIG, this is inherited across fork to allow killing a whole
> subtree without race conditions.

but I am still not sure this is right... at least I can't understand the
"without race conditions" above.

IOW, the child can do prctl(PR_SET_PDEATHSIG_PROC, SIGKILL) right after fork(),
why this is not enough to kill a whole subtree without race conditions?

OTOH. If you want to kill a whole sub-tree then perhaps the exiting process
should simply send the ->pdeath_signal_proc to the whole sub-tree? Not that
I really think this makes more sense, but if we add the new API we should
discuss everything we can.

Say, CLONE_PARENT. Should it succeed if ->pdeath_signal_proc != 0 ?

Anyway, I think this patch needs more reviewers. Let me add Linus and
Michael. Again, I am not worried about correctness, the patch is simple,
but the new API always needs a thorough discussion.

Oleg.

> This can be used for sandboxing when combined with a seccomp filter.
> 
> There have been previous attempts to support this by changing the
> behavior of PR_SET_PDEATHSIG. However, that would break existing
> applications. See https://marc.info/?l=linux-kernel&m=117621804801689
> and https://bugzilla.kernel.org/show_bug.cgi?id=43300
> 
> Signed-off-by: Jürg Billeter <j@bitron.ch>
> ---
>  fs/exec.c                    |  1 +
>  include/linux/sched/signal.h |  3 +++
>  include/uapi/linux/prctl.h   |  4 ++++
>  kernel/cred.c                |  1 +
>  kernel/exit.c                |  4 ++++
>  kernel/fork.c                |  2 ++
>  kernel/sys.c                 | 11 +++++++++++
>  security/apparmor/lsm.c      |  1 +
>  security/selinux/hooks.c     |  1 +
>  9 files changed, 28 insertions(+)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 01a9fb9d8ac3..bb389c3c596d 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1353,6 +1353,7 @@ void setup_new_exec(struct linux_binprm * bprm)
>  	if (bprm->secureexec) {
>  		/* Make sure parent cannot signal privileged process. */
>  		current->pdeath_signal = 0;
> +		current->signal->pdeath_signal_proc = 0;
>  
>  		/*
>  		 * For secureexec, reset the stack limit to sane default to
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index 2a0dd40b15db..c5c137e5ef39 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -103,6 +103,9 @@ struct signal_struct {
>  	int			group_stop_count;
>  	unsigned int		flags; /* see SIGNAL_* flags below */
>  
> +	/* The signal sent when the parent dies: */
> +	int			pdeath_signal_proc;
> +
>  	/*
>  	 * PR_SET_CHILD_SUBREAPER marks a process, like a service
>  	 * manager, to re-parent orphan (double-forking) child processes
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index a8d0759a9e40..04508e81d4f2 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -197,4 +197,8 @@ struct prctl_mm_map {
>  # define PR_CAP_AMBIENT_LOWER		3
>  # define PR_CAP_AMBIENT_CLEAR_ALL	4
>  
> +/* Process-based variant of PDEATHSIG */
> +#define PR_SET_PDEATHSIG_PROC		48
> +#define PR_GET_PDEATHSIG_PROC		49
> +
>  #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/cred.c b/kernel/cred.c
> index ecf03657e71c..0192a94670e1 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -448,6 +448,7 @@ int commit_creds(struct cred *new)
>  		if (task->mm)
>  			set_dumpable(task->mm, suid_dumpable);
>  		task->pdeath_signal = 0;
> +		task->signal->pdeath_signal_proc = 0;
>  		smp_wmb();
>  	}
>  
> diff --git a/kernel/exit.c b/kernel/exit.c
> index a35d8a17e01f..1be0616239e0 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -635,6 +635,10 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
>  	if (unlikely(p->exit_state == EXIT_DEAD))
>  		return;
>  
> +	if (p->signal->pdeath_signal_proc)
> +		group_send_sig_info(p->signal->pdeath_signal_proc,
> +				    SEND_SIG_NOINFO, p);
> +
>  	/* We don't want people slaying init. */
>  	p->exit_signal = SIGCHLD;
>  
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 24a4c0be80d5..f6482392ece9 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1412,6 +1412,8 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
>  
>  	mutex_init(&sig->cred_guard_mutex);
>  
> +	sig->pdeath_signal_proc = current->signal->pdeath_signal_proc;
> +
>  	return 0;
>  }
>  
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 2855ee73acd0..c47e92fa5370 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2210,6 +2210,17 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>  	case PR_GET_PDEATHSIG:
>  		error = put_user(me->pdeath_signal, (int __user *)arg2);
>  		break;
> +	case PR_SET_PDEATHSIG_PROC:
> +		if (!valid_signal(arg2)) {
> +			error = -EINVAL;
> +			break;
> +		}
> +		me->signal->pdeath_signal_proc = arg2;
> +		break;
> +	case PR_GET_PDEATHSIG_PROC:
> +		error = put_user(me->signal->pdeath_signal_proc,
> +				 (int __user *)arg2);
> +		break;
>  	case PR_GET_DUMPABLE:
>  		error = get_dumpable(me->mm);
>  		break;
> diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
> index 7a82c0f61452..c8bd6b1331c1 100644
> --- a/security/apparmor/lsm.c
> +++ b/security/apparmor/lsm.c
> @@ -628,6 +628,7 @@ static void apparmor_bprm_committing_creds(struct linux_binprm *bprm)
>  	aa_inherit_files(bprm->cred, current->files);
>  
>  	current->pdeath_signal = 0;
> +	current->signal->pdeath_signal_proc = 0;
>  
>  	/* reset soft limits and set hard limits for the new label */
>  	__aa_transition_rlimits(label, new_ctx->label);
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index ad3b0f53ede0..574d6238f8de 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2527,6 +2527,7 @@ static void selinux_bprm_committing_creds(struct linux_binprm *bprm)
>  
>  	/* Always clear parent death signal on SID transitions. */
>  	current->pdeath_signal = 0;
> +	current->signal->pdeath_signal_proc = 0;
>  
>  	/* Check whether the new SID can inherit resource limits from the old
>  	 * SID.  If not, reset all soft limits to the lower of the current
> -- 
> 2.14.1
>