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

Jürgen Groß
2024-05-17T15:27:42Z

On 17.05.24 17:16, Dave Hansen wrote:
> On 5/17/24 07:44, Juergen Gross wrote:
>> 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.
>
> For all intents and purposes, the modules that intentionally clobber RBP
> don't support Linux. If buggy modules are accidentally clobbering RBP,
> we can debate how much the kernel should bend over to accommodate them,
> but my preference would be to ignore them.
>
> I'd much rather put a deny list in the kernel than try to tolerate RBP
> clobbering universally.

Would you be fine with adding a new X86_FEATURE (or BUG?) allowing to switch
RBP save/restore via ALTERNATIVE, controlled by a command line option?

Or maybe by adding a new CONFIG_TDX_MODULE_CAN_CLOBBER_RBP (probably using
a shorter name) option?

TBH I'm slightly puzzled that the firmware I'm using could make it outside
Intel. I'm fearing this might happen again.

Juergen