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

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Message-ID: <c0067319-2653-4cbd-8fee-1ccf21b1e646@suse.com>
Date: Fri, 17 May 2024 17:27:39 +0200
From: Jürgen Groß <jgross@...e.com>
To: Dave Hansen <dave.hansen@...el.com>,
 "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 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