Re: [PATCH v5 3/6] seccomp: introduce writer locking

[Peter Zijlstra <peterz@infradead.org>]

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On Thu, May 22, 2014 at 04:05:33PM -0700, Kees Cook wrote:
> Normally, task_struct.seccomp.filter is only ever read or modified by
> the task that owns it (current). This property aids in fast access
> during system call filtering as read access is lockless.
> 
> Updating the pointer from another task, however, opens up race
> conditions. To allow cross-task filter pointer updates, writes to the
> seccomp fields are now protected by a spinlock.  Read access remains
> lockless because pointer updates themselves are atomic.  However, writes
> (or cloning) often entail additional checking (like maximum instruction
> counts) which require locking to perform safely.
> 
> In the case of cloning threads, the child is invisible to the system
> until it enters the task list. To make sure a child can't be cloned
> from a thread and left in a prior state, seccomp duplication is moved
> under the tasklist_lock. Then parent and child are certain have the same
> seccomp state when they exit the lock.
> 

So I'm a complete noob on the whole seccomp thing, so maybe this is a
silly question, but.. what about object lifetimes?

Looking at put_seccomp_filter() it explicitly takes a tsk pointer,
suggesting one can call it on !current. And while it does a dec_and_test
on the object itself, run_filter() does nothing with refcounts, and
therefore can be touching dead memory.



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