linux-kernel - Re: [PATCH v3 02/24] x86/virt/tdx: Add SEAMCALL wrapper tdh_mem_page

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

Message-ID: <aWn4P2zx1u+27ZPp@yzhao56-desk.sh.intel.com>
Date: Fri, 16 Jan 2026 16:35:11 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
CC: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Du, Fan"
	<fan.du@...el.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>, "Gao, Chao"
	<chao.gao@...el.com>, "Hansen, Dave" <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>, "michael.roth@....com"
	<michael.roth@....com>, "Weiny, Ira" <ira.weiny@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "nik.borisov@...e.com"
	<nik.borisov@...e.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>, "Peng,
 Chao P" <chao.p.peng@...el.com>, "francescolavra.fl@...il.com"
	<francescolavra.fl@...il.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"Annapurve, Vishal" <vannapurve@...gle.com>, "Edgecombe, Rick P"
	<rick.p.edgecombe@...el.com>, "Miao, Jun" <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 02/24] x86/virt/tdx: Add SEAMCALL wrapper
 tdh_mem_page_demote()

Hi Kai,
Thanks for reviewing!

On Fri, Jan 16, 2026 at 09:00:29AM +0800, Huang, Kai wrote:
> 
> > 
> > Enable tdh_mem_page_demote() only on TDX modules that support feature
> > TDX_FEATURES0.ENHANCE_DEMOTE_INTERRUPTIBILITY, which does not return error
> > TDX_INTERRUPTED_RESTARTABLE on basic TDX (i.e., without TD partition) [2].
> > 
> > This is because error TDX_INTERRUPTED_RESTARTABLE is difficult to handle.
> > The TDX module provides no guaranteed maximum retry count to ensure forward
> > progress of the demotion. Interrupt storms could then result in a DoS if
> > host simply retries endlessly for TDX_INTERRUPTED_RESTARTABLE. Disabling
> > interrupts before invoking the SEAMCALL also doesn't work because NMIs can
> > also trigger TDX_INTERRUPTED_RESTARTABLE. Therefore, the tradeoff for basic
> > TDX is to disable the TDX_INTERRUPTED_RESTARTABLE error given the
> > reasonable execution time for demotion. [1]
> > 
> 
> [...]
> 
> > v3:
> > - Use a var name that clearly tell that the page is used as a page table
> >   page. (Binbin).
> > - Check if TDX module supports feature ENHANCE_DEMOTE_INTERRUPTIBILITY.
> >   (Kai).
> > 
> [...]
> 
> > +u64 tdh_mem_page_demote(struct tdx_td *td, u64 gpa, int level, struct page *new_sept_page,
> > +			u64 *ext_err1, u64 *ext_err2)
> > +{
> > +	struct tdx_module_args args = {
> > +		.rcx = gpa | level,
> > +		.rdx = tdx_tdr_pa(td),
> > +		.r8 = page_to_phys(new_sept_page),
> > +	};
> > +	u64 ret;
> > +
> > +	if (!tdx_supports_demote_nointerrupt(&tdx_sysinfo))
> > +		return TDX_SW_ERROR;
> > 
> 
> For the record, while I replied my suggestion [*] to this patch in v2, it
> was basically because the discussion was already in that patch -- I didn't
> mean to do this check inside tdh_mem_page_demote(), but do this check in
> KVM page fault patch and return 4K as maximum mapping level.
> 
> The precise words were:
> 
>   So if the decision is to not use 2M page when TDH_MEM_PAGE_DEMOTE can 
>   return TDX_INTERRUPTED_RESTARTABLE, maybe we can just check this 
>   enumeration in fault handler and always make mapping level as 4K?
Right. I followed it in the last patch (patch 24).

> Looking at this series, this is eventually done in your last patch.  But I
> don't quite understand what's the additional value of doing such check and
> return TDX_SW_ERROR in this SEAMCALL wrapper.
> 
> Currently in this series, it doesn't matter whether this wrapper returns
> TDX_SW_ERROR or the real TDX_INTERRUPTED_RESTARTABLE -- KVM terminates the
> TD anyway (see your patch 8) because this is unexpected as checked in your
> last patch.
> 
> IMHO we should get rid of this check in this low level wrapper.
You are right, the wrapper shouldn't hit this error after the last patch.

However, I found it's better to introduce the feature bit
TDX_FEATURES0_ENHANCE_DEMOTE_INTERRUPTIBILITY and the helper
tdx_supports_demote_nointerrupt() together with the demote SEAMCALL wrapper.
This way, people can understand how the TDX_INTERRUPTED_RESTARTABLE error is
handled for this SEAMCALL. Invoking the helper in this patch also gives the
helper a user :)

What do you think about changing it to a WARN_ON_ONCE()? i.e.,
WARN_ON_ONCE(!tdx_supports_demote_nointerrupt(&tdx_sysinfo));


> [*]:
> https://lore.kernel.org/all/fbf04b09f13bc2ce004ac97ee9c1f2c965f44fdf.camel@intel.com/#t