Re: [PATCH 6/9] virtio_pci: harden MSI-X interrupts

From: Thomas Gleixner <tglx@linutronix.de>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: "Kuppuswamy,
	Sathyanarayanan" <sathyanarayanan.kuppuswamy@linux.intel.com>,
	Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
	"kaplan, david" <david.kaplan@amd.com>,
	Peter Zijlstra <peterz@infradead.org>,
	virtualization <virtualization@lists.linux-foundation.org>,
	James E J Bottomley <James.Bottomley@hansenpartnership.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Peter H Anvin <hpa@zytor.com>, Andi Kleen <ak@linux.intel.com>,
	Jonathan Corbet <corbet@lwn.net>, Helge Deller <deller@gmx.de>,
	X86 ML <x86@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	"Hetzelt, Felicitas" <f.hetzelt@tu-berlin.de>,
	Tony Luck <tony.luck@intel.com>, Borislav Petkov <bp@alien8.de>,
	Andy Lutomirski <luto@kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Richard Henderson <rth@twiddle.net>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Sean Christopherson <seanjc@google.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	pbonzini <pbonzini@redhat.com>,
	"David S . Miller" <davem@davemloft.net>,
	Kirill Shutemov <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH 6/9] virtio_pci: harden MSI-X interrupts
Date: Tue, 14 Sep 2021 00:31:36 +0200	[thread overview]
Message-ID: <87sfy8ds53.ffs@tglx> (raw)
In-Reply-To: <20210913164934-mutt-send-email-mst@kernel.org>

On Mon, Sep 13 2021 at 16:54, Michael S. Tsirkin wrote:

> On Mon, Sep 13, 2021 at 09:38:30PM +0200, Thomas Gleixner wrote:
>> On Mon, Sep 13 2021 at 15:07, Jason Wang wrote:
>> > On Mon, Sep 13, 2021 at 2:50 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>> >> > But doen't "irq is disabled" basically mean "we told the hypervisor
>> >> > to disable the irq"?  What extractly prevents hypervisor from
>> >> > sending the irq even if guest thinks it disabled it?
>> >>
>> >> More generally, can't we for example blow away the
>> >> indir_desc array that we use to keep the ctx pointers?
>> >> Won't that be enough?
>> >
>> > I'm not sure how it is related to the indirect descriptor but an
>> > example is that all the current driver will assume:
>> >
>> > 1) the interrupt won't be raised before virtio_device_ready()
>> > 2) the interrupt won't be raised after reset()
>> 
>> If that assumption exists, then you better keep the interrupt line
>> disabled until virtio_device_ready() has completed
>
> started not completed. device is allowed to send
> config interrupts right after DRIVER_OK status is set by
> virtio_device_ready.

Whatever:

 * Define the exact point from which on the driver is able to handle the
   interrupt and put the enable after that point

 * Define the exact point from which on the driver is unable to handle
   the interrupt and put the disable before that point

The above is blury.

>> and disable it again
>> before reset() is invoked. That's a question of general robustness and
>> not really a question of trusted hypervisors and encrypted guests.
>
> We can do this for some MSIX interrupts, sure. Not for shared interrupts though.

See my reply to the next patch. The problem is the same:

 * Define the exact point from which on the driver is able to handle the
   interrupt and allow the handler to proceed after that point

 * Define the exact point from which on the driver is unable to handle
   the interrupt and ensure that the handler denies to proceed before
   that point

Same story just a different mechanism.

Thanks,

        tglx
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