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Date: Fri, 17 May 2024 17:39:51 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Juergen Gross <jgross@suse.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org, 
	linux-coco@lists.linux.dev, Dave Hansen <dave.hansen@linux.intel.com>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] x86/kvm/tdx: Save %rbp in TDX_MODULE_CALL
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On Fri, May 17, 2024 at 04:08:03PM +0200, Juergen Gross wrote:
> On 17.05.24 15:55, Kirill A. Shutemov wrote:
> > On Fri, May 17, 2024 at 02:14:50PM +0200, Juergen Gross wrote:
> > > While testing TDX host support patches, a crash of the host has been
> > > observed a few instructions after doing a seamcall. Reason was a
> > > clobbered %rbp (set to 0), which occurred in spite of the TDX module
> > > offering the feature NOT to modify %rbp across TDX module calls.
> > > 
> > > In order not having to build the host kernel with CONFIG_FRAME_POINTER,
> > > save %rbp across a seamcall/tdcall.
> > 
> > There's a feature in TDX module 1.5 that prevents RBP modification across
> > TDH.VP.ENTER SEAMCALL. See NO_RBP_MOD in TDX Module 1.5 ABI spec.
> > 
> > I think it has to be enabled for all TDs and TDX modules that don't
> > support it need to be rejected.
> > 
> 
> Yes, I know. I'm using the patch series:
> 
>   [PATCH v19 000/130] KVM TDX basic feature support
> 
> which I think does exactly that (see setup_tdparams() and tdx_module_setup()).

Looks like the check is broken:

https://lore.kernel.org/all/46mh5hinsv5mup2x7jv4iu2floxmajo2igrxb3haru3cgjukbg@v44nspjozm4h/

> Nevertheless the clobbering happened, and saving/restoring %rbp made the
> issue to go away. I suspect there is a path left still clobbering %rbp.

What is your TDX module version? My guess is that NOM_RBP_MOD is not
supported by it and given that the check is broken nobody enforces it.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov