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

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Date: Fri, 17 May 2024 16:08:03 +0200
From: Juergen Gross <jgross@...e.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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 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()).

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

I was testing on an Emerald Rapids system:

# lscpu
Architecture:             x86_64
   CPU op-mode(s):         32-bit, 64-bit
   Address sizes:          47 bits physical, 57 bits virtual
   Byte Order:             Little Endian
CPU(s):                   256
   On-line CPU(s) list:    0-255
Vendor ID:                GenuineIntel
   BIOS Vendor ID:         Intel(R) Corporation
   Model name:             INTEL(R) XEON(R) PLATINUM 8592+
     BIOS Model name:      INTEL(R) XEON(R) PLATINUM 8592+  CPU @ 1.9GHz
     BIOS CPU family:      179
     CPU family:           6
     Model:                207
     Thread(s) per core:   2
     Core(s) per socket:   64
     Socket(s):            2
     Stepping:             2
...

BIOS version as printed during boot:

[    0.000000] DMI: Intel Corporation D50DNP/D50DNP, BIOS 
SE5C7411.86B.9535.D04.2312270518 12/27/2023

Juergen

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