linux-kernel - Re: [PATCH v7] seccomp, ptrace: add support for dumping seccomp filters

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Date:	Tue, 20 Oct 2015 20:00:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tycho Andersen <tycho.andersen@...onical.com>
Cc:	Kees Cook <keescook@...omium.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Will Drewry <wad@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	Pavel Emelyanov <xemul@...allels.com>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v7] seccomp, ptrace: add support for dumping seccomp
	filters

Sorry for delay...

On 10/13, Tycho Andersen wrote:
>
> --- a/include/uapi/linux/ptrace.h
> +++ b/include/uapi/linux/ptrace.h
> @@ -23,6 +23,8 @@
>
>  #define PTRACE_SYSCALL		  24
>
> +#define PTRACE_SECCOMP_GET_FILTER	40

Probably it would be better to add this at the end of other 0x42..
constants? After PTRACE_SETSIGMASK.

> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -347,6 +347,7 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
>  {
>  	struct seccomp_filter *sfilter;
>  	int ret;
> +	const bool save_orig = config_enabled(CONFIG_CHECKPOINT_RESTORE);
>  
>  	if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
>  		return ERR_PTR(-EINVAL);
> @@ -370,7 +371,7 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
>  		return ERR_PTR(-ENOMEM);
>  
>  	ret = bpf_prog_create_from_user(&sfilter->prog, fprog,
> -					seccomp_check_filter, false);
> +					seccomp_check_filter, save_orig);

Can't comment, this depends on other changes I missed... but I don't
this you need my review here ;)

> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
> +long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
> +			void __user *data)
> +{
> +	struct seccomp_filter *filter;
> +	struct sock_fprog_kern *fprog;
> +	long ret;
> +	unsigned long count = 0;
> +
> +	spin_lock_irq(&current->sighand->siglock);
> +	if (!capable(CAP_SYS_ADMIN) ||
> +	    current->seccomp.mode != SECCOMP_MODE_DISABLED) {
> +		ret = -EACCES;
> +		goto out_self;
> +	}
> +
> +	spin_lock_irq(&task->sighand->siglock);

Oh, no, you can't do this.

This is deadlockable. Suppose that this task's sub-thread traces the
caller (the current task) and does PTRACE_SECCOMP_GET_FILTER too.

In this case it can take the same 2 locks in reverse order, deadlock.

But why do you need to hold both ->siglock's at the same time?

Oleg.

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