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

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Date: Wed, 12 Jun 2024 12:50:43 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: 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>, 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 Tue, Jun 11, 2024 at 09:16:13AM -0700, Dave Hansen wrote:
> 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.

I am not sure I follow.

panic() is only for catastrophic cases: VMM pulled memory from under us or
we mapped unaccepted memory into userspace. It should never happen.

We have the same check is_private_gpa() in virt_exception_kernel().

-- 
  Kiryl Shutsemau / Kirill A. Shutemov