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

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Date: Fri, 17 May 2024 16:44:45 +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 16:39, Kirill A. Shutemov wrote:
> 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.

Just another data point: Before using this machine I was testing on
another one with older firmware. That one really didn't support NOM_RBP_MOD
and I needed to build the kernel with CONFIG_FRAME_POINTER enabled to get
past the check you are mentioning above.


Juergen

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