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

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Message-ID: <20240605162419.GJZmCRM8V6xooyvm9H@fat_crate.local>
Date: Wed, 5 Jun 2024 18:24:19 +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 Wed, Jun 05, 2024 at 03:21:42PM +0300, Kirill A. Shutemov wrote:
> If a page can be accessed via private mapping is determined by the
> presence in Secure EPT. This state persist across kexec.

I just love it how I tickle out details each time I touch this comment
because we three can't write a single concise and self-contained
explanation. :-(

Ok, next version:

"Private mappings persist across kexec. If tdx_enc_status_changed() fails
in the first kernel, it leaves memory in an unknown state.

If that memory remains shared, accessing it in the *next* kernel through
a private mapping will result in an unrecoverable guest shutdown.

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 which is possible when accessing that memory through
/proc/vmcore, for example.

In all cases, print error info in order to leave enough bread crumbs for
debugging."

I think this is getting in the right direction as it actually makes
sense now.

-- 
Regards/Gruss,
    Boris.

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