linux-kernel - Re: [PATCH] x86/tdx: Preserve shared bit on mprotect()

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Date: Fri, 12 Apr 2024 20:48:56 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com"
	<mingo@...hat.com>, "Hansen, Dave" <dave.hansen@...el.com>, "bp@...en8.de"
	<bp@...en8.de>
CC: "Cui, Dexuan" <decui@...rosoft.com>, "Reshetova, Elena"
	<elena.reshetova@...el.com>, "x86@...nel.org" <x86@...nel.org>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "hpa@...or.com" <hpa@...or.com>,
	"sathyanarayanan.kuppuswamy@...ux.intel.com"
	<sathyanarayanan.kuppuswamy@...ux.intel.com>, "thomas.lendacky@....com"
	<thomas.lendacky@....com>, "cho@...rosoft.com" <cho@...rosoft.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/tdx: Preserve shared bit on mprotect()

On Fri, 2024-04-12 at 22:12 +0300, Kirill A. Shutemov wrote:
> The TDX guest platform takes one bit from the physical address to
> indicate if the page is shared (accessible by VMM). This bit is not part
> of the physical_mask and is not preserved during mprotect(). As a
> result, the 'shared' bit is lost during mprotect() on shared mappings.
> 
> _COMMON_PAGE_CHG_MASK specifies which PTE bits need to be preserved
> during modification. AMD includes 'sme_me_mask' in the define to
> preserve the 'encrypt' bit.
> 
> To cover both Intel and AMD cases, include 'cc_mask' in
> _COMMON_PAGE_CHG_MASK instead of 'sme_me_mask'.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Fixes: 41394e33f3a0 ("x86/tdx: Extend the confidential computing API to
> support TDX guests")
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Chris Oo <cho@...rosoft.com>
> Cc: Dexuan Cui <decui@...rosoft.com>

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>

So does this mean there is shared memory mapped to userspace? Or is this a
theoretical correctness thing?