Received-SPF: pass (google.com: domain of linux-kernel+bounces-211296-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Date: Wed, 12 Jun 2024 12:50:43 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>, linux-coco@lists.linux.dev, 
	linux-kernel@vger.kernel.org, cho@microsoft.com, decui@microsoft.com, 
	John.Starks@microsoft.com
Subject: Re: [PATCH] x86/tdx: Generate SIGBUS on userspace MMIO
Message-ID: <mpbfcnfnqthq32rsn57ge2ypkhzpa3kqxpr2u3rqodxur66voy@zsavzwkxqkhz>
References: <20240528100919.520881-1-kirill.shutemov@linux.intel.com>
 <4df2ebee-40c0-4ea3-8909-13b90f049ff1@intel.com>
 <fa788a95-1814-4782-8a36-f9dce7a04b66@intel.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <fa788a95-1814-4782-8a36-f9dce7a04b66@intel.com>

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