linux-kernel - Re: [PATCH v3 00/24] KVM: TDX huge page support for private memory

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <aYoS45AZNY0rUJQD@google.com>
Date: Mon, 9 Feb 2026 09:01:23 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@...el.com>, Fan Du <fan.du@...el.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, Kai Huang <kai.huang@...el.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, Dave Hansen <dave.hansen@...el.com>, 
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "vbabka@...e.cz" <vbabka@...e.cz>, 
	"tabba@...gle.com" <tabba@...gle.com>, "david@...nel.org" <david@...nel.org>, "kas@...nel.org" <kas@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Ira Weiny <ira.weiny@...el.com>, 
	"francescolavra.fl@...il.com" <francescolavra.fl@...il.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "nik.borisov@...e.com" <nik.borisov@...e.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Chao P Peng <chao.p.peng@...el.com>, "michael.roth@....com" <michael.roth@....com>, 
	Vishal Annapurve <vannapurve@...gle.com>, "sagis@...gle.com" <sagis@...gle.com>, Chao Gao <chao.gao@...el.com>, 
	Jun Miao <jun.miao@...el.com>, "jgross@...e.com" <jgross@...e.com>, 
	"pgonda@...gle.com" <pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 00/24] KVM: TDX huge page support for private memory

On Tue, Feb 03, 2026, Yan Zhao wrote:
> On Fri, Jan 30, 2026 at 07:32:48AM -0800, Sean Christopherson wrote:
> > On Mon, Jan 19, 2026, Yan Zhao wrote:
> > > On Sat, Jan 17, 2026 at 12:58:02AM +0800, Edgecombe, Rick P wrote:
> > > > On Fri, 2026-01-16 at 08:31 -0800, Sean Christopherson wrote:
> > > IIUC, this concern should be gone as Dave has agreed to use "pfn" as the
> > > SEAMCALL parameter [1]?
> > > Then should we invoke "KVM_MMU_WARN_ON(!tdx_is_convertible_pfn(pfn));" in KVM
> > > for every pfn of a huge mapping? Or should we keep the sanity check inside the
> > > SEAMCALL wrappers?
> > 
> > I don't have a strong preference.  But if it goes in KVM, definitely guard it with
> > KVM_MMU_WARN_ON().
> Thank you for your insights, Sean!
> 
> > > BTW, I have another question about the SEAMCALL wrapper implementation, as Kai
> > > also pointed out in [2]: since the SEAMCALL wrappers now serve as APIs available
> > > to callers besides KVM, should the SEAMCALL wrappers return TDX_OPERAND_INVALID
> > > or WARN_ON() (or WARN_ON_ONCE()) on sanity check failure?
> > 
> > Why not both?  But maybe TDX_SW_ERROR instead of TDX_OPERAND_INVALID?
> Hmm, I previously returned TDX_OPERAND_INVALID for non-aligned base PFN.
> TDX_SW_ERROR is also ok if we want to indicate that passing an invalid PFN is a
> software error.
> (I had tdh_mem_page_demote() return TDX_SW_ERROR when an incompatible TDX module
> is used, i.e., when !tdx_supports_demote_nointerrupt()).
> 
> > If an API has a defined contract and/or set of expectations, and those expectations
> > aren't met by the caller, then a WARN is justified.  But the failure still needs
> > to be communicated to the caller.
> Ok.
> 
> The reason for 'not both' is that there's already TDX_BUG_ON_2() in KVM after
> the SEAMCALL wrapper returns a non-BUSY error. I'm not sure if having double
> WARN_ON_ONCE() calls is good, so I intended to let the caller decide whether to
> warn.

Two WARNs isn't the end of the world.  It might even be helpful in some cases,
e.g. to more precisely document what went wrong.

> > > By returning TDX_OPERAND_INVALID, the caller can check the return code, adjust
> > > the input or trigger WARN_ON() by itself;
> > > By triggering WARN_ON() directly in the SEAMCALL wrapper, we need to document
> > > this requirement for the SEAMCALL wrappers and have the caller invoke the API
> > > correctly.
> > 
> > Document what exactly?  Most of this should be common sense.  E.g. we don't generally
> > document that pointers must be non-NULL, because that goes without saying 99.9%
> > of the time.
> Document the SEAMCALL wrapper's expectations. e.g., for demote, a PFN must be
> 2MB-aligned, or the caller must not invoke tdh_mem_page_demote() if a TDX module
> does not support feature ENHANCED_DEMOTE_INTERRUPTIBILITY...

FWIW, for me, all of those are self-explanatory and/or effectively covered by the
TDX specs.

> > IMO, that holds true here as well.  E.g. trying to map memory into a TDX guest
> > that isn't convertible is obviously a bug, I don't see any value in formally
> > documenting that requirement.
> Do we need a comment for documentation above the tdh_mem_page_demote() API?

I wouldn't bother, but I truly don't care if the TDX subsystem wants to document
everything in gory detail.