Re: [PATCH v2] virtio-pci: Fix the use of an uninitialized irqfd.

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Hi Cindy,

On 17/6/24 11:55, Cindy Lu wrote:
> The crash was reported in MAC OS and NixOS, here is the link for this bug
> https://gitlab.com/qemu-project/qemu/-/issues/2334
> https://gitlab.com/qemu-project/qemu/-/issues/2321
> 
> The root cause is the function virtio_pci_set_guest_notifiers() was not called
> in the virtio_input device.So the vector_irqfd was not initialized
> 
> So the fix is to add the check for vector_irqfd.
> 
> This fix is verified in vyatta,MacOS,NixOS,fedora system.
> 
> The bt tree for this bug is:
> Thread 6 "CPU 0/KVM" received signal SIGSEGV, Segmentation fault.
> [Switching to Thread 0x7c817be006c0 (LWP 1269146)]
> kvm_virtio_pci_vq_vector_use () at ../qemu-9.0.0/hw/virtio/virtio-pci.c:817
> 817	    if (irqfd->users == 0) {
> (gdb) thread apply all bt
> ...
> Thread 6 (Thread 0x7c817be006c0 (LWP 1269146) "CPU 0/KVM"):
> 0  kvm_virtio_pci_vq_vector_use () at ../qemu-9.0.0/hw/virtio/virtio-pci.c:817
> 1  kvm_virtio_pci_vector_use_one () at ../qemu-9.0.0/hw/virtio/virtio-pci.c:893
> 2  0x00005983657045e2 in memory_region_write_accessor () at ../qemu-9.0.0/system/memory.c:497
> 3  0x0000598365704ba6 in access_with_adjusted_size () at ../qemu-9.0.0/system/memory.c:573
> 4  0x0000598365705059 in memory_region_dispatch_write () at ../qemu-9.0.0/system/memory.c:1528
> 5  0x00005983659b8e1f in flatview_write_continue_step.isra.0 () at ../qemu-9.0.0/system/physmem.c:2713
> 6  0x000059836570ba7d in flatview_write_continue () at ../qemu-9.0.0/system/physmem.c:2743
> 7  flatview_write () at ../qemu-9.0.0/system/physmem.c:2774
> 8  0x000059836570bb76 in address_space_write () at ../qemu-9.0.0/system/physmem.c:2894
> 9  0x0000598365763afe in address_space_rw () at ../qemu-9.0.0/system/physmem.c:2904
> 10 kvm_cpu_exec () at ../qemu-9.0.0/accel/kvm/kvm-all.c:2917
> 11 0x000059836576656e in kvm_vcpu_thread_fn () at ../qemu-9.0.0/accel/kvm/kvm-accel-ops.c:50
> 12 0x0000598365926ca8 in qemu_thread_start () at ../qemu-9.0.0/util/qemu-thread-posix.c:541
> 13 0x00007c8185bcd1cf in ??? () at /usr/lib/libc.so.6
> 14 0x00007c8185c4e504 in clone () at /usr/lib/libc.so.6
> 
> Fixes: 2ce6cff94d ("virtio-pci: fix use of a released vector")
> Cc: qemu-stable@nongnu.org
> 
> Signed-off-by: Cindy Lu <lulu@redhat.com>
> ---
>   hw/virtio/virtio-pci.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index b1d02f4b3d..502aad28b2 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -1442,7 +1442,9 @@ static void virtio_pci_set_vector(VirtIODevice *vdev,
>        * Otherwise just need to set the new vector on the device.
>        */
>       if (kvm_irqfd && old_vector != VIRTIO_NO_VECTOR) {
> -        kvm_virtio_pci_vector_release_one(proxy, queue_no);
> +        if (proxy->vector_irqfd) {

Shouldn't this go into called virtio_pci_get_notifier()?

> +            kvm_virtio_pci_vector_release_one(proxy, queue_no);

We ignore this function return value, is it safe/expected?

> +        }
>       }