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SubjectRe: [PATCH] POSIX message queues
Am Freitag den, 30. August 2002, um 11:48, schrieb Amos Waterland:

> On Thu, Aug 29, 2002 at 11:53:50PM +0200, wrote:
>> some comments as asked for:
>> I know that it's nowhere stated, but POSIX mqueues are perfectly
>> designed to be implemented in userspace with locking facilities
>> provided by the system.
> I am not sure if this is correct. You can achieve proper locking in
> userspace, but I do not think you achieve proper security.

Well, I can't think of efficient inter process locks without
kernel/scheduler help.
Do you want to use a spinlock, with lowering priority or even sleep, use
a pipe/fifo/flock or waiting in sigsuspend? This all is implemented in
kernel space.
How would you implement entirely userspace locking?
With futexes (fast "userspace" locks) only the uncontented case is
in userspace - if there's contention the process waits inside the
kernel - or
does get a notification from the kernel (AWAIT or FD)

> I assume you are proposing an implementation based on shared memory:
> which means that at least some pages of the shared memory must be
> writable. If the processes cooperate and only write to the shared pages
> through library routines which use sychronization, things are ok, but a
> malicious process could forge messages or perform DOS attacks etc. by
> bypassing the mq_*() functions and using write().

yes, of course that could be compromised by a process with the same uid.
This process could simply kill the other process too.
The shm_open() employs proper file system permission on the object.

> The mq_maxmsg and mq_msgsize members of the mq_attr structure required
> if O_CREAT is passed to mq_open() ensure that an implementation can
> prevent the kernel memory DoS you mention: a malicious application can
> only fill up the MQ memory.

And how many mqueues am I allowed to create?
You would need an extra resource limit for that.

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