Re: [PATCH] x86/kvm/tdx: Save %rbp in TDX_MODULE_CALL

Juergen Gross
2024-05-17T14:08:06Z

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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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