kvm.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: "Woodhouse, David" <dwmw@amazon.co.uk>
To: "pbonzini@redhat.com" <pbonzini@redhat.com>,
	"butterflyhuangxx@gmail.com" <butterflyhuangxx@gmail.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: There is a null-ptr-deref bug in kvm_dirty_ring_get in virt/kvm/dirty_ring.c
Date: Wed, 17 Nov 2021 09:46:18 +0000	[thread overview]
Message-ID: <4b739ed0ce31e459eb8af9f5b0e2b1516d8e4517.camel@amazon.co.uk> (raw)
In-Reply-To: <9eb83cdd-9314-0d1f-0d4b-0cf4432e1e84@redhat.com>

On Thu, 2021-10-21 at 22:08 +0200, Paolo Bonzini wrote:
> On 18/10/21 19:14, butt3rflyh4ck wrote:
> > {
> > struct kvm_vcpu *vcpu = kvm_get_running_vcpu();  //-------> invoke
> > kvm_get_running_vcpu() to get a vcpu.
> > 
> > WARN_ON_ONCE(vcpu->kvm != kvm); [1]
> > 
> > return &vcpu->dirty_ring;
> > }
> > ```
> > but we had not called KVM_CREATE_VCPU ioctl to create a kvm_vcpu so
> > vcpu is NULL.
> 
> It's not just because there was no call to KVM_CREATE_VCPU; in general
> kvm->dirty_ring_size only works if all writes are associated to a
> specific vCPU, which is not the case for the one of
> kvm_xen_shared_info_init.
> 
> David, what do you think?  Making dirty-page ring buffer incompatible
> with Xen is ugly and I'd rather avoid it; taking the mutex for vcpu 0 is
> not an option because, as the reporter said, you might not have even
> created a vCPU yet when you call KVM_XEN_HVM_SET_ATTR.  The remaining
> option would be just "do not mark the page as dirty if the ring buffer
> is active".  This is feasible because userspace itself has passed the
> shared info gfn; but again, it's ugly...

I think I am coming to quite like that 'remaining option' as long as we
rephrase it as follows:

 KVM does not mark the shared_info page as dirty, and userspace is
 expected to *assume* that it is dirty at all times. It's used for
 delivering event channel interrupts and the overhead of marking it
 dirty each time is just pointless.

I've merged the patch I sent yesterday into my pfncache series because
I realised we needed the same there, but I'll look at making the Xen
code *not* mark the shinfo page dirty when it writes the clock data
there.



Amazon Development Centre (London) Ltd. Registered in England and Wales with registration number 04543232 with its registered office at 1 Principal Place, Worship Street, London EC2A 2FA, United Kingdom.



  parent reply	other threads:[~2021-11-17  9:46 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-18 17:14 butt3rflyh4ck
2021-10-21 20:08 ` Paolo Bonzini
2021-10-28  7:42   ` butt3rflyh4ck
2021-11-08  5:11   ` butt3rflyh4ck
2021-11-16 15:41   ` butt3rflyh4ck
2021-11-16 16:22   ` [EXTERNAL] " David Woodhouse
2021-11-16 17:07     ` David Woodhouse
2021-11-17  9:46   ` Woodhouse, David [this message]
2021-11-17 16:49     ` Paolo Bonzini
2021-11-17 18:10       ` Woodhouse, David
2021-11-20 10:16   ` KVM: Warn if mark_page_dirty() is called without an active vCPU David Woodhouse
2021-11-22 17:01     ` Sean Christopherson
2021-11-22 17:52       ` David Woodhouse
2021-11-22 18:49         ` Sean Christopherson
2022-01-13 12:06     ` Christian Borntraeger
2022-01-13 12:14       ` Paolo Bonzini
2022-01-13 12:29         ` [PATCH] KVM: avoid warning on s390 in mark_page_dirty Christian Borntraeger
2022-01-13 12:31           ` David Woodhouse
2022-01-18  8:37           ` Christian Borntraeger
2022-01-18  8:44             ` Paolo Bonzini
2022-01-18  8:53               ` Christian Borntraeger
2022-01-18 11:44                 ` Paolo Bonzini
2022-01-13 12:30         ` KVM: Warn if mark_page_dirty() is called without an active vCPU David Woodhouse
2022-01-13 12:51           ` Christian Borntraeger
2022-01-13 13:22             ` David Woodhouse
2022-01-13 15:09               ` Christian Borntraeger
2022-01-13 14:36           ` Paolo Bonzini

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=4b739ed0ce31e459eb8af9f5b0e2b1516d8e4517.camel@amazon.co.uk \
    --to=dwmw@amazon.co.uk \
    --cc=butterflyhuangxx@gmail.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=pbonzini@redhat.com \
    --subject='Re: There is a null-ptr-deref bug in kvm_dirty_ring_get in virt/kvm/dirty_ring.c' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).