[dpdk-dev] [PATCH] vhost: fix segfault on bad descriptor address.

[Yuanhan Liu]

On Fri, May 20, 2016 at 03:50:04PM +0300, Ilya Maximets wrote:
> In current implementation guest application can reinitialize vrings
> by executing start after stop. In the same time host application
> can still poll virtqueue while device stopped in guest and it will
> crash with segmentation fault while vring reinitialization because
> of dereferencing of bad descriptor addresses.
> 
> OVS crash for example:
> <------------------------------------------------------------------------>
> [test-pmd inside guest VM]
> 
> 	testpmd> port stop all
> 	    Stopping ports...
> 	    Checking link statuses...
> 	    Port 0 Link Up - speed 10000 Mbps - full-duplex
> 	    Done
> 	testpmd> port config all rxq 2
> 	testpmd> port config all txq 2
> 	testpmd> port start all
> 	    Configuring Port 0 (socket 0)
> 	    Port 0: 52:54:00:CB:44:C8
> 	    Checking link statuses...
> 	    Port 0 Link Up - speed 10000 Mbps - full-duplex
> 	    Done

I actually didn't manage to reproduce it on my side, with the
vhost-example instead of OVS though. Is that all the commands
to reproduce it, and run them just after start test-pmd?

> [OVS on host]
> 	Program received signal SIGSEGV, Segmentation fault.
> 	rte_memcpy (n=2056, src=0xc, dst=0x7ff4d5247000) at rte_memcpy.h
> 
> 	(gdb) bt
> 	    #0  rte_memcpy (n=2056, src=0xc, dst=0x7ff4d5247000)
> 	    #1  copy_desc_to_mbuf
> 	    #2  rte_vhost_dequeue_burst
> 	    #3  netdev_dpdk_vhost_rxq_recv
> 	    ...
> 
> 	(gdb) bt full
> 	    #0  rte_memcpy
> 	        ...
> 	    #1  copy_desc_to_mbuf
> 	        desc_addr = 0
> 	        mbuf_offset = 0
> 	        desc_offset = 12
> 	        ...
> <------------------------------------------------------------------------>
> 
> Fix that by checking addresses of descriptors before using them.
> 
> Note: For mergeable buffers this patch checks only guest's address for
> zero, but in non-meargeable case host's address checked. This is done
> because checking of host's address in mergeable case requires additional
> refactoring to keep virtqueue in consistent state in case of error.
> 
> Signed-off-by: Ilya Maximets <i.maximets at samsung.com>
> ---
> 
> Actually, current virtio implementation looks broken for me. Because
> 'virtio_dev_start' breaks virtqueue while it still available from the vhost
> side.
> 
> There was 2 patches about this behaviour:
> 
> 	1. a85786dc816f ("virtio: fix states handling during initialization")
> 	2. 9a0615af7746 ("virtio: fix restart")
> 
> The second patch fixes somehow issue intoduced in the first patch, but actually
> also breaks vhost in the way described above.
> It's not pretty clear for me what to do in current situation with virtio,
> because it will be broken for guest application even if vhost will not crash.
> 
> May be it'll be better to forbid stopping of virtio device and force user to
> exit and start again (may be implemented in hidden from user way)?
> 
> This patch adds additional sane checks, so it should be applied anyway, IMHO.

Agreed.

	--yliu