On Wed, Feb 05, 2020 at 01:13:37AM -0500, Michael S. Tsirkin wrote:
> On Tue, Feb 04, 2020 at 03:16:18PM +0000, Stefan Hajnoczi wrote:
> > hw/virtio.c checks vq->vring.desc != NULL to see if the vring has been
> > set up successfully.
> > 
> > When virtio_queue_set_rings() fails due to an invalid vring memory
> > address it must clear vq->vring.desc (and related fields) so we don't
> > treat this virtqueue as successfully initialized later on.
> > 
> > This bug was found by device fuzzing and can be triggered as follows:
> > 
> >   $ qemu -M pc -device virtio-blk-pci,id=drv0,drive=drive0,addr=4.0 \
> >          -drive if=none,id=drive0,file=null-co://,format=raw,auto-read-only=off \
> >          -drive if=none,id=drive1,file=null-co://,file.read-zeroes=on,format=raw \
> >          -display none \
> >          -qtest stdio
> >   endianness
> >   outl 0xcf8 0x80002020
> >   outl 0xcfc 0xe0000000
> >   outl 0xcf8 0x80002004
> >   outw 0xcfc 0x7
> >   write 0xe0000000 0x24 0x00ffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffab5cffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffab0000000001
> >   inb 0x4
> >   writew 0xe000001c 0x1
> >   write 0xe0000014 0x1 0x0d
> > 
> > The following error message is produced:
> > 
> >   qemu-system-x86_64: /home/stefanha/qemu/hw/virtio/virtio.c:286: vring_get_region_caches: Assertion `caches != NULL' failed.
> > 
> > The backtrace looks like this:
> > 
> >   #0  0x00007ffff5520625 in raise () at /lib64/libc.so.6
> >   #1  0x00007ffff55098d9 in abort () at /lib64/libc.so.6
> >   #2  0x00007ffff55097a9 in _nl_load_domain.cold () at /lib64/libc.so.6
> >   #3  0x00007ffff5518a66 in annobin_assert.c_end () at /lib64/libc.so.6
> >   #4  0x00005555559073da in vring_get_region_caches (vq=<optimized out>) at qemu/hw/virtio/virtio.c:286
> >   #5  vring_get_region_caches (vq=<optimized out>) at qemu/hw/virtio/virtio.c:283
> >   #6  0x000055555590818d in vring_used_flags_set_bit (mask=1, vq=0x5555575ceea0) at qemu/hw/virtio/virtio.c:398
> >   #7  virtio_queue_split_set_notification (enable=0, vq=0x5555575ceea0) at qemu/hw/virtio/virtio.c:398
> >   #8  virtio_queue_set_notification (vq=vq@entry=0x5555575ceea0, enable=enable@entry=0) at qemu/hw/virtio/virtio.c:451
> >   #9  0x0000555555908512 in virtio_queue_set_notification (vq=vq@entry=0x5555575ceea0, enable=enable@entry=0) at qemu/hw/virtio/virtio.c:444
> >   #10 0x00005555558c697a in virtio_blk_handle_vq (s=0x5555575c57e0, vq=0x5555575ceea0) at qemu/hw/block/virtio-blk.c:775
> >   #11 0x0000555555907836 in virtio_queue_notify_aio_vq (vq=0x5555575ceea0) at qemu/hw/virtio/virtio.c:2244
> >   #12 0x0000555555cb5dd7 in aio_dispatch_handlers (ctx=ctx@entry=0x55555671a420) at util/aio-posix.c:429
> >   #13 0x0000555555cb67a8 in aio_dispatch (ctx=0x55555671a420) at util/aio-posix.c:460
> >   #14 0x0000555555cb307e in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at util/async.c:260
> >   #15 0x00007ffff7bbc510 in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
> >   #16 0x0000555555cb5848 in glib_pollfds_poll () at util/main-loop.c:219
> >   #17 os_host_main_loop_wait (timeout=<optimized out>) at util/main-loop.c:242
> >   #18 main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:518
> >   #19 0x00005555559b20c9 in main_loop () at vl.c:1683
> >   #20 0x0000555555838115 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4441
> > 
> > Reported-by: Alexander Bulekov <alxndr@bu.edu>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  hw/virtio/virtio.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 2c5410e981..5d7f619a1e 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -2163,6 +2163,11 @@ void virtio_queue_set_rings(VirtIODevice *vdev, int n, hwaddr desc,
> >      vdev->vq[n].vring.avail = avail;
> >      vdev->vq[n].vring.used = used;
> >      virtio_init_region_cache(vdev, n);
> > +    if (vdev->broken) {
> > +        vdev->vq[n].vring.desc = 0;
> > +        vdev->vq[n].vring.avail = 0;
> > +        vdev->vq[n].vring.used = 0;
> > +    }
> >  }
> 
> OK but now that you mention it, guest can cause us to call
> virtio_queue_set_rings at any time.
> If device was running there's still a window when desc is set
> but cache isn't.
> 
> Is this an issue?

Yes, this is an issue for devices that support iothreads because the
virtqueue could be processed in the iothread while the vcpu thread
enters the virtio.c code.

> I also worry desc is tested in a way that isn't safe for RCU ...
> 
> Should we convert most/all users to check the presence of region cache?

Yes.

I suspect there are additional issues because virtio.c doesn't use
locking, except for vring.caches RCU. :(

Stefan