Re: [PATCH RESEND V2 1/2] x86/mce: Fix missing address mask in recovery for errors in TDX/SEAM non-root mode

[Adrian Hunter <adrian.hunter@intel.com>]

On 21/08/2025 16:25, Borislav Petkov wrote:
> On Thu, Aug 21, 2025 at 10:24:22AM +0300, Adrian Hunter wrote:
>> Something like below would work, but doesn't answer Dave's question
>> of why not do it in mce_read_aux()

Thanks for looking at this.

> So, let me see what I understand from all this bla: you want to zap the KeyID
> from mci_addr because it is completely useless there. So zap it.

Not exactly.  I just want to fix the bug whereby the mce handler fails
to mark the affected page as poisoned because it does not remove the KeyID
from the address before looking-up the page.

> You can't make any other changes to mci_addr because that goes to luserspace.
> 
> So far so good.
> 
> Now, all that other bla leads me to believe that there might be some need to
> dump the raw mci_addr value after all.
> 
> If so, your patch is not needed.
> 
> Which makes me think, all yall folks need to make up your mind here.
> 
> And you need to get rid of all that extraneous information in your commit
> message:
> 
> "Investigation of user space expectations has concluded it..."
> 
> No investigation needed - this is exported to userspace so you can't touch it.

It is a bit of a grey area.  No one expects to find non-address bits in
struct mce addr, and the kernel already strips low-order bits, and in the
case of SMCA, high-order bits too, refer smca_extract_err_addr().

> The one and only question you need to answer is, do you really need KeyID in
> it or not. And whatever you do, once you do it, we're stuck with it because it
> goes out to userspace.

No, the KeyID is almost useless.  It is just a dynamically allocated
number. It might indicate which TDX VM encountered the error, except the
MCE is fatal to the VM, so a fatal error is reported to the VMM anyway.

However, it is allowed to extend struct mce, so adding KeyID or
raw MCI ADDR later is quite possible.

> Especially if you want this backported to stable.

The bug exists in stable.  Ideally it would get fixed there too somehow.