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Tsirkin" To: m'te'a physical Cc: qemu-devel@nongnu.org Subject: Re: [PATCH] virtio: validate split queue head count before popping Message-ID: <20260710022025-mutt-send-email-mst@kernel.org> References: <20260709100529.41267-1-physicalmtea@gmail.com> <20260709084105-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Fri, Jul 10, 2026 at 09:34:54AM +0800, m'te'a physical wrote: > I think virtio_queue_empty_rcu() is no longer needed after this change. > > For this caller, the pieces it covered are either already handled elsewhere or > kept explicitly in the split pop path. > > The disabled-device check is already done by virtqueue_pop() before it calls > virtqueue_split_pop(). > > The empty-queue check is covered by virtqueue_num_heads(): rc == 0 is the > normal empty case, and rc < 0 is the invalid avail index distance case that > this > patch wants to reject. > > The read barrier is also covered by virtqueue_num_heads(), since it does > smp_rmb() when it returns a positive head count, before the caller reads the > avail ring entry with virtqueue_get_head(). > > The only old check that I think should stay explicit in virtqueue_split_pop() > is > !vq->vring.avail. virtqueue_num_heads() does not check that, and the old > behavior for an unconfigured split queue was to treat it as empty. So v2 keeps > that guard and replaces the rest of virtio_queue_empty_rcu() with > virtqueue_num_heads(): Yea this is our hacky way to check the mr cache is valid. >     RCU_READ_LOCK_GUARD(); >     if (unlikely(!vq->vring.avail)) { >         goto done; >     } > >     rc = virtqueue_num_heads(vq, vq->last_avail_idx); >     if (rc <= 0) { >         goto done; >     } > > With that, virtio_queue_empty_rcu() has no remaining users and can be removed. > > If this looks right, I can send a v2 patch with this change. > > Michael S. Tsirkin 于2026年7月9日周四 20:43写道： > > On Thu, Jul 09, 2026 at 06:05:29PM +0800, Jia Jia wrote: > > virtqueue_split_pop() reads the next split avail ring entry after > > virtio_queue_empty_rcu() reports work. Call virtqueue_num_heads() before > > consuming that entry, so an avail index distance larger than the queue > > size is rejected. > > > > virtqueue_num_heads() also keeps the read barrier needed before the avail > > ring entry is read. > > > > This prevents an invalid split queue state from being expanded into > > repeated device command processing. > > > > Link: https://gitlab.com/qemu-project/qemu/-/issues/3930 > > Signed-off-by: Jia Jia > > --- > > Tested with qemu-system-x86_64 11.0.50, built from origin/master > > f893c46c3931 plus this patch, configured with: > >   --target-list=x86_64-softmmu --enable-kvm --disable-tcg > > > > The original virtio-iommu live-vring qtest reproducer used for the > > report completed successfully on the patched build without host OOM, > > confirming that this reproducer is fixed by the patch. > > > >  hw/virtio/virtio.c | 8 +++++--- > >  1 file changed, 5 insertions(+), 3 deletions(-) > > > > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c > > index f4d86a3655..252bff8641 100644 > > --- a/hw/virtio/virtio.c > > +++ b/hw/virtio/virtio.c > > @@ -1751,9 +1751,11 @@ static void *virtqueue_split_pop(VirtQueue *vq, > size_t sz) > >      if (virtio_queue_empty_rcu(vq)) { > >          goto done; > >      } > > -    /* Needed after virtio_queue_empty(), see comment in > > -     * virtqueue_num_heads(). */ > > -    smp_rmb(); > > + > > +    rc = virtqueue_num_heads(vq, vq->last_avail_idx); > > +    if (rc <= 0) { > > +        goto done; > > +    } > > > Do we still need virtio_queue_empty_rcu then? Why? > > >      /* When we start there are none of either input nor output. */ > >      out_num = in_num = elem_entries = 0; > > -- > > 2.34.1 > >