Re: [PATCH 3/4] x86,module: Detect VMX vs SLD conflicts

Kees Cook
2020-04-07T23:15:16Z

On Tue, Apr 07, 2020 at 09:25:56PM +0000, David Laight wrote:
> From: Peter Zijlstra
> > Sent: 07 April 2020 12:03
> >
> > It turns out that with Split-Lock-Detect enabled (default) any VMX
> > hypervisor needs at least a little modification in order to not blindly
> > inject the #AC into the guest without the guest being ready for it.
> >
> > Since there is no telling which module implements a hypervisor, scan
> > all out-of-tree modules' text and look for VMX instructions and refuse
> > to load it when SLD is enabled (default) and the module isn't marked
> > 'sld_safe'.
> ...
> > + while (text < text_end) {
> > + kernel_insn_init(&insn, text, text_end - text);
> > + insn_get_length(&insn);
> > +
> > + if (WARN_ON_ONCE(!insn_complete(&insn))) {
> > + pr_err("Module text malformed: %s\n", mod->name);
> > + return -ENOEXEC;
> > + }
> > +
> > + if (!allow_vmx && insn_is_vmx(&insn)) {
> > + pr_err("Module has VMX instructions and is not marked 'sld_safe', boot with:
> > 'split_lock_detect=off': %s\n", mod->name);
> > + return -ENOEXEC;
> > + }
> > +
> > + text += insn.length;
> > + }
>
> There is a slight flaw in the above.
> A malicious module can hide the required instruction by jumping into the
> middle of a long instruction.
>
> Even checking branch targets hit instruction barriers isn't enough,
> an indirect jump could be used.

If I understand the goals here, it's to provide feedback for good actors
doing things that they don't realize aren't safe. Trying to stop a
malicious module from doing malicious things is basically impossible:
it can just load a data blob and self-modify, etc. :)

Though, Peter, this does get me thinking: if this is meant to be helpful
for module authors tripping over things they shouldn't be touching,
perhaps every test needs to include explicit recommendations? It's
_kind_ of doing this already. Maybe the above should be:

pr_err("%s: contains VMX instructions but is not marked 'sld_safe'. Please see <url> or boot with: 'split_lock_detect=off' to ignore.\n", mod->name);

?

--
Kees Cook