Re: System call audit

[David Woodhouse]

On Tue, 2008-05-13 at 08:51 -0400, Mathieu Desnoyers wrote:
> * David Woodhouse (dwmw2@xxxxxxxxxxxxx) wrote:
> > On Mon, 2008-05-12 at 20:06 -0400, Mathieu Desnoyers wrote:
> > > Hi David,
> > > 
> > > As I am looking into the system-wide system call tracing problem, I
> > > start to wonder how auditsc deals with the fact that user-space could
> > > concurrently change the content referred to by the __user pointers.
> > 
> > In general we have to copy the content into kernel space, audit it, and
> > then act on it from there. See the explanation on the IPC audit patch at
> > http://lwn.net/Articles/125350/ for example.
> > 
> > Auditing one thing and then acting on another would be simply broken.
> > 
> > > This would be the case for execve. If we create a program with two
> > > thread; one is executing execve syscalls and the other thread would be
> > > modifying the userspace string containing the name of the program to
> > > execute.
> > 
> > I was going to suggest that that attack vector won't work, because
> > execve() kills all threads. But all you have to do to avoid that is put
> > the data in question into a shared writable mmap and modify it from
> > another _process_. And in fact I suspect there's a combination of CLONE_
> > flags which would avoid the thread-killing behaviour anyway.
> > 
> 
> Even better : if execve fails, it doesn't kill the threads. Therefore,
> all we have to do is to busy-loop doing failing execve() calls and
> atomically change the string to what we want to be executed. Can anyone
> test the sample snippet in a context where executing /bin/bash is
> disallowed on a SMP system ? I don't have a selinux setup handy. 

You were talking about audit earlier. Now you seem to be talking about
selinux. 

-- 
dwmw2

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