Re: [Xen-devel] Hypercall privilege check

["Steven Y. Ko" <sko@xxxxxxxxxxx>]

> > But I guess my question was not clear enough, because I was actually
> > asking about Xen and how it enforces hypercalls to be made only from
> > ring 1. I just assumed that Xen checks if int 0x82 is executed with
> > the right privilege. But when I tried to find where Xen actually does
> > it, I couldn't find it. I hope my question is more clear this time :)
>
> Ah, I see what you mean.
>
> I think the hardware actually enforces this...  lemme see...
>
> Yes, here we are: in xen/arch/x86/x86_32/traps.c::244 we have a:
>
>     /* The hypercall entry vector is only accessible from ring 1. */
>     _set_gate(idt_table+HYPERCALL_VECTOR, 14, 1, &hypercall);
>
> So int 0x82 is only permitted to be executed from ring 1.  And the x86
> interrupt gate things enforce it in hardware, I assume.  x86 scares me, and
> so I'm being a bit vague here!
Great! Exactly what I was looking for. Thanks.

> I imagine you'd get a GPF if you tried to hypercall from userspace, but I'm
> not so familiar with lowlevel x86 goop (at least, not when my manual's in
> another building!) so I can't be certain.
>
> Trivia: As a result of hypercalls being only allowed from ring 1, the Minix
> port (for Xen 2.0, I think) which uses ring 1, 2 and 3 has portions of the
> kernel in ring 2 calling portions in ring 1 to actually perform hypercalls.
This is interesting. I think I should look at Minix port. Thanks a lot.

- Steve

> Cheers,
> Mark
>
> > - Steve
> >
> > On 6/28/07, Mark Williamson <mark.williamson@xxxxxxxxxxxx> wrote:
> > > > I'm wondering where Xen actually checks whether a hypercall is made
> > > > from ring 1 or ring 3. Could anyone point me out to the code? Any
> > > > pointer would be much appreciated. Thanks!
> > >
> > > Hypercalls are made from ring 1.  The userspace tools do make hypercalls
> > > sometimes, but they do it by calling a special kernel driver, which then
> > > does the actual hypercall.  *actually* what that does is to jump into the
> > > hypercall transfer page at the right location.  This page contains the
> > > actual implementation of the hypercall (using this indirection allows
> > > future versions of Xen to supply different implementations of the calls,
> > > if appropriate).
> > >
> > > Take a look at:
> > > drivers/xen/privcmd/privcmd.c  in the XenLinux tree.  This is used by the
> > > dom0 tools to do control plane operations.
> > >
> > > Also, take a look at: include/asm-i386/mach-xen/asm/hypercall.h which
> > > contains implementations of some other hypercalls.
> > >
> > > HTH,
> > > Cheers,
> > > mark
> > >
> > > --
> > > Dave: Just a question. What use is a unicyle with no seat?  And no
> > > pedals! Mark: To answer a question with a question: What use is a
> > > skateboard? Dave: Skateboards have wheels.
> > > Mark: My wheel has a wheel!
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
>
>
>
> --
> Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
> Mark: To answer a question with a question: What use is a skateboard?
> Dave: Skateboards have wheels.
> Mark: My wheel has a wheel!
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