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Re: [PATCH v7 3/9] seccomp: introduce writer locking

To: Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v7 3/9] seccomp: introduce writer locking
From: Oleg Nesterov <oleg@redhat.com>
Date: Tue, 24 Jun 2014 18:52:16 +0200
Cc: linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>, Alexei Starovoitov <ast@plumgrid.com>, "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Daniel Borkmann <dborkman@redhat.com>, Will Drewry <wad@chromium.org>, Julien Tinnes <jln@chromium.org>, David Drysdale <drysdale@google.com>, linux-api@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-mips@linux-mips.org, linux-arch@vger.kernel.org, linux-security-module@vger.kernel.org
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Kees,

I am still trying to force myself to read and try to understand what
this series does ;) Just a minor nit so far.

On 06/23, Kees Cook wrote:
>
> @@ -1142,6 +1168,7 @@ static struct task_struct *copy_process(unsigned long 
> clone_flags,
>  {
>       int retval;
>       struct task_struct *p;
> +     unsigned long irqflags;
>  
>       if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
>               return ERR_PTR(-EINVAL);
> @@ -1196,7 +1223,6 @@ static struct task_struct *copy_process(unsigned long 
> clone_flags,
>               goto fork_out;
>  
>       ftrace_graph_init_task(p);
> -     get_seccomp_filter(p);
>  
>       rt_mutex_init_task(p);
>  
> @@ -1434,7 +1460,13 @@ static struct task_struct *copy_process(unsigned long 
> clone_flags,
>               p->parent_exec_id = current->self_exec_id;
>       }
>  
> -     spin_lock(&current->sighand->siglock);
> +     spin_lock_irqsave(&current->sighand->siglock, irqflags);
> +
> +     /*
> +      * Copy seccomp details explicitly here, in case they were changed
> +      * before holding tasklist_lock.
> +      */
> +     copy_seccomp(p);
>  
>       /*
>        * Process group and session signals need to be delivered to just the
> @@ -1446,7 +1478,7 @@ static struct task_struct *copy_process(unsigned long 
> clone_flags,
>       */
>       recalc_sigpending();
>       if (signal_pending(current)) {
> -             spin_unlock(&current->sighand->siglock);
> +             spin_unlock_irqrestore(&current->sighand->siglock, irqflags);
>               write_unlock_irq(&tasklist_lock);
>               retval = -ERESTARTNOINTR;
>               goto bad_fork_free_pid;
> @@ -1486,7 +1518,7 @@ static struct task_struct *copy_process(unsigned long 
> clone_flags,
>       }
>  
>       total_forks++;
> -     spin_unlock(&current->sighand->siglock);
> +     spin_unlock_irqrestore(&current->sighand->siglock, irqflags);
>       write_unlock_irq(&tasklist_lock);
>       proc_fork_connector(p);
>       cgroup_post_fork(p);

It seems that the only change copy_process() needs is copy_seccomp() under the 
locks.
Everythinh else (irqflags games) looks obviously unneeded?

> @@ -524,6 +528,9 @@ static long seccomp_set_mode(unsigned long seccomp_mode, 
> char __user *filter)
>       }
>  #endif
>
> +     if (unlikely(!lock_task_sighand(current, &irqflags)))
> +             goto out_free;
> +

Unless this task is exiting (namely, it has already called exit_notify()),
lock_task_sighand(current) must not fail. Looks like you can simly do
spin_lock_irq(->siglock).

Oleg.


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