linux-kernel - Re: [PATCHv11.1 11/19] x86/tdx: Convert shared memory back to private on kexec

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Date: Wed, 5 Jun 2024 15:21:42 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...el.com>, adrian.hunter@...el.com, 
	ardb@...nel.org, ashish.kalra@....com, bhe@...hat.com, 
	dave.hansen@...ux.intel.com, elena.reshetova@...el.com, haiyangz@...rosoft.com, hpa@...or.com, 
	jun.nakajima@...el.com, kai.huang@...el.com, kexec@...ts.infradead.org, 
	kys@...rosoft.com, linux-acpi@...r.kernel.org, linux-coco@...ts.linux.dev, 
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org, ltao@...hat.com, mingo@...hat.com, 
	peterz@...radead.org, rafael@...nel.org, rick.p.edgecombe@...el.com, 
	sathyanarayanan.kuppuswamy@...ux.intel.com, seanjc@...gle.com, tglx@...utronix.de, thomas.lendacky@....com, 
	x86@...nel.org
Subject: Re: [PATCHv11.1 11/19] x86/tdx: Convert shared memory back to
 private on kexec

On Tue, Jun 04, 2024 at 08:05:54PM +0200, Borislav Petkov wrote:
> On Tue, Jun 04, 2024 at 07:14:00PM +0300, Kirill A. Shutemov wrote:
> > 			/*
> > 			 * If tdx_enc_status_changed() fails, it leaves memory
> > 			 * in an unknown state. If the memory remains shared,
> > 			 * it can result in an unrecoverable guest shutdown on
> > 			 * the first accessed through a private mapping.
> 
> "access"

Okay.

> So this sentence above can go too, right?

I don't think so.

> Because that comment is in tdx_kexec_finish() and we're basically going
> off to kexec. So can a guest even access it through a private mapping?
> We're shutting down so nothing is running anymore...

This kernel can't. But the next kernel can.

If a page can be accessed via private mapping is determined by the
presence in Secure EPT. This state persist across kexec.

> > 			 * The kdump kernel boot is not impacted as it uses
> > 			 * a pre-reserved memory range that is always private.
> > 			 * However, gathering crash information could lead to
> > 			 * a crash if it accesses unconverted memory through
> > 			 * a private mapping.
> 
> When does the kexec kernel even get such a private mapping? It is not
> even up yet...

Crash kernel provides access to this memory via /proc/vmcore. Crash kernel
will assume all memory there is private.

> > 			 * pr_err() may assist in understanding such crashes.
> 
> "Print error info in order to leave bread crumbs for debugging." is what
> I'd say.

Okay.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov