linux-kernel - Re: [PATCH] x86/tdx: Generate SIGBUS on userspace MMIO

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Date: Tue, 11 Jun 2024 09:16:13 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>
Cc: linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
 cho@...rosoft.com, decui@...rosoft.com, John.Starks@...rosoft.com
Subject: Re: [PATCH] x86/tdx: Generate SIGBUS on userspace MMIO

On 6/10/24 06:55, Dave Hansen wrote:
>> Enlightened userspace may choose to handle MMIO on their own if the
>> kernel does not emulate it.
>>
>> Handle the EPT_VIOLATION exit reason for userspace and deliver SIGBUS
>> instead of SIGSEGV. SIGBUS is more appropriate for the MMIO situation.
> Is any userspace _actually_ doing this?  Sure, SIGBUS is more
> appropriate but in practice unprepared userspace crashes either way.

I also can't help but wonder if there's a better way to do this.

Just thinking out loud.... Ideally, we'd reject creating a potentially
troublesome VMA at mmap() time.  That's way better than, for instance,
panic()'ing at some random place in the middle of program execution.

But I guess that's likely not possible because someone could be doing a
VM_MIXEDMAP VMA that only has normal private pages and never _actually_
needs or has a shared page mapped.

I'd still love to know what actual kernel drivers and actual userspace
would be involved in this whole dance.  It's still way too theoretical
for me.