Re: System call audit

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* David Woodhouse (dwmw2@infradead.org) wrote:
> On Tue, 2008-05-13 at 08:51 -0400, Mathieu Desnoyers wrote:
> > * David Woodhouse (dwmw2@infradead.org) 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. 
> 

I thought selinux did hook into syscall audit ? (sorry, I am new to the
kernel auditing field) The race I refer to is in the auditsc.c kernel
code, so syscall audit would be the one I am talking about. I refer to
selinux here just because, as of my understanding, it happens to be one
module-based callback which can hook on syscall audit.

Mathieu

-- 
Mathieu Desnoyers
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