Re: [PATCH 4/7] x86/kexec: Disable kexec/kdump on platforms with TDX partial write erratum

[Vishal Annapurve <vannapurve@google.com>]

On Thu, Oct 2, 2025 at 8:06 AM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 10/2/25 00:46, Juergen Gross wrote:
> > So lets compare the 2 cases with kdump enabled and disabled in your
> > scenario (crash of the host OS):
> >
> > kdump enabled: No dump can be produced due to the #MC and system is
> > rebooted.
> >
> > kdump disabled: No dump is produced and system is rebooted after crash.
> > > What is the main concern with kdump enabled? I don't see any
> > disadvantage with enabling it, just the advantage that in many cases
> > a dump will be written.
> The disadvantage is that a kernel bug from long ago results in a machine
> check. Machine checks are generally indicative of bad hardware. So the
> disadvantage is that someone mistakes the long ago kernel bug for bad
> hardware.
>
> There are two ways of looking at this:
>
> 1. A theoretically fragile kdump is better than no kdump at all. All of
>    the stars would have to align for kdump to _fail_ and we don't think
>    that's going to happen often enough to matter.
> 2. kdump happens after kernel bugs. The machine checks happen because of
>    kernel bugs. It's not a big stretch to think that, at scale, kdump is
>    going to run in to these #MCs on a regular basis.

Looking at Elena's response, I would say it's still *a* big stretch
for kdump to run into these #MCs on a regular basis as following
sequence is needed for problematic scenario:
1) Host OS bug should corrupt TDX private memory with a *partial
write*, that is part of kernel memory.
    -> i.e. PAMT tables, SEPT tables, TD VCPU/VM metadata etc.
    -> IIUC corruption of guest memory is not a concern as that
belongs to userspace.
2) TDX Module/TD shouldn't consume that poisoned memory.
    -> i.e. no walk of the metadata memory.
3) Host kernel needs to generate a bug that causes an orthogonal panic.

*partial writes* IIUC need special instructions.

>
> Does that capture the two perspectives fairly?