linux-kernel - Re: [PATCH] x86/kvm/tdx: Save %rbp in TDX_MODULE

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Message-ID: <ohvjbokpaxagc26kxmlrujab7cw3bekgi5ln7dt46cbsaxcqqh@crvqeohfazmf>
Date: Fri, 17 May 2024 16:55:49 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Juergen Gross <jgross@...e.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, 
	linux-coco@...ts.linux.dev, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/kvm/tdx: Save %rbp in TDX_MODULE_CALL

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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov