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

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Message-ID: <20240604180554.GIZl9XgscEI3PUvR-W@fat_crate.local>
Date: Tue, 4 Jun 2024 20:05:54 +0200
From: Borislav Petkov <bp@...en8.de>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
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 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"

So this sentence above can go too, right?

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...

> 			 * 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...

> 			 * pr_err() may assist in understanding such crashes.

"Print error info in order to leave bread crumbs for debugging." is what
I'd say.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette