Re: Xen Security Advisory 466 v3 (CVE-2024-53241) - Xen hypercall page unsafe against speculative attacks

[Jürgen Groß <jgross@xxxxxxxx>]

On 23.12.24 15:24, David Woodhouse wrote:
> On Tue, 2024-12-17 at 12:18 +0000, Xen.org security team wrote:
> >               Xen Security Advisory CVE-2024-53241 / XSA-466
> >                                  version 3
> >
> >           Xen hypercall page unsafe against speculative attacks
> >
> > UPDATES IN VERSION 3
> > ====================
> >
> > Update of patch 5, public release.
> Can't we even use the hypercall page early in boot? Surely we have to
> know whether we're running on an Intel or AMD CPU before we get to the
> point where we can enable any of the new control-flow integrity
> support? Do we need to jump through those hoops do do that early
> detection and setup?
The downside of this approach would be to have another variant to do
hypercalls. So you'd have to replace the variant being able to use AMD
or INTEL specific instructions with a function doing the hypercall via
the hypercall page.

I'm planning to send patches for Xen and the kernel to add CPUID feature
bits indicating which instruction to use. This will make life much easier.

> Enabling the hypercall page is also one of the two points where Xen
> will 'latch' that the guest is 64-bit, which affects the layout of the
> shared_info, vcpu_info and runstate structures.
>
> The other such latching point is when the guest sets
> HVM_PARAM_CALLBACK_IRQ, and I *think* that should work in all
> implementations of the Xen ABI (including QEMU/KVM and EC2). But would
> want to test.
>
> But perhaps it wouldn't hurt for maximal compatibility for Linux to set
> the hypercall page *anyway*, even if Linux doesn't then use it — or
> only uses it during early boot?
I'm seeing potential problems with that approach when someone is using
an out-of-tree module doing hypercalls.

With having the hypercall page present such a module would add a way to do
speculative attacks, while deleting the hypercall page would result in a
failure trying to load such a module.


Juergen

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