Re: [Xen-devel] Re: System Call Interception

[Mathieu Ropert <mro@xxxxxxxxxx>]

Anthony Liguori wrote:
> Neha Sood wrote:
> > Hi there,
> >
> > I have a question related to intercepting guest OS system calls in 
> > Xen to provide logging mechanism. As a part of my project, i have to 
> > log all the system calls issued by the guest OS in the Xen hypervisor 
> > for secure logging. I am new to Xen and have been reading the source 
> > code. I have read about Fast Handler for system call. What is the 
> > fast handler and how the system call works in Xen. Could you please 
> > provide me some starting point how to do that?
> On i386 at least, syscalls are delivered directly to the guest (they 
> aren't intercepted by Xen).
> I'm not sure if the same is true on x86-64 but at any rate, the first 
> thing to do would be to make sure Xen intercepts syscalls.  You will 
> have to find some mechanism to "log" these events which will likely 
> involved a ring queue and some sort of daemon in dom0.
All syscalls go through Xen on x86-64 (as guest runs in ring3), so you 
can insert some logs in the handling code (xen/arch/x86/x86_64/entry.S).
But as stated before, would be much easier to do it in the guest kernel 
code for most OS. Now if you talk about logging calls from a 
non-modified guest (using HVM),  it should be theorically possible but i 
don't where you should insert your code (probably in the interrupt 
handling code somewhere in xen/arch/x86/hvm, can't be more precise 
though, don't know this part very well).
About the "fast" system call, it's a new feature added on x86-64 (and i 
think on the lastest x86 CPUs) to handle what it names says "fast system 
calls". It's used as an alternative to the software interrupts 
(generated by the "int" instruction) to handle system call faster than 
the usual way. They are entered via the "syscall" instruction and exited 
with "sysret" (some processors also implements the "sysenter" and 
"sysexit" instructions, check Intel/AMD manuals for more detail).
Regards,

Mathieu
> However, as Mats suggests, are you sure this is really what you want 
> to do?  The audit infrastructure in Linux is designed just to do this 
> sort of thing...
> Regards,
>
> Anthony Liguori
>
> > Will it be a very difficult to log guest OS system calls in Xen ? If 
> > no, how to start with and what are files have to be changed.
> > I would really appreciate your help.
> >
> > Thanks,
> > Neha
> >
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