Re: [PATCH] Drivers: hv: vmbus: Fix kernel crash upon unbinding a device from uio_hv_generic driver

[Andrea Parri]

On Tue, Aug 31, 2021 at 04:39:16PM +0200, Vitaly Kuznetsov wrote:
> The following crash happens when a never-used device is unbound from
> uio_hv_generic driver:
> 
>  kernel BUG at mm/slub.c:321!
>  invalid opcode: 0000 [#1] SMP PTI
>  CPU: 0 PID: 4001 Comm: bash Kdump: loaded Tainted: G               X --------- ---  5.14.0-0.rc2.23.el9.x86_64 #1
>  Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090008  12/07/2018
>  RIP: 0010:__slab_free+0x1d5/0x3d0
> ...
>  Call Trace:
>   ? pick_next_task_fair+0x18e/0x3b0
>   ? __cond_resched+0x16/0x40
>   ? vunmap_pmd_range.isra.0+0x154/0x1c0
>   ? __vunmap+0x22d/0x290
>   ? hv_ringbuffer_cleanup+0x36/0x40 [hv_vmbus]
>   kfree+0x331/0x380
>   ? hv_uio_remove+0x43/0x60 [uio_hv_generic]
>   hv_ringbuffer_cleanup+0x36/0x40 [hv_vmbus]
>   vmbus_free_ring+0x21/0x60 [hv_vmbus]
>   hv_uio_remove+0x4f/0x60 [uio_hv_generic]
>   vmbus_remove+0x23/0x30 [hv_vmbus]
>   __device_release_driver+0x17a/0x230
>   device_driver_detach+0x3c/0xa0
>   unbind_store+0x113/0x130
> ...
> 
> The problem appears to be that we free 'ring_info->pkt_buffer' twice:
> first, when the device is unbound from in-kernel driver (netvsc in this
> case) and second from hv_uio_remove(). Normally, ring buffer is supposed
> to be re-initialized from hv_uio_open() but this happens when UIO device
> is being opened and this is not guaranteed to happen.
> 
> Generally, it is OK to call hv_ringbuffer_cleanup() twice for the same
> channel (which is being handed over between in-kernel drivers and UIO) even
> if we didn't call hv_ringbuffer_init() in between. We, however, need to
> avoid kfree() call for an already freed pointer.
> 
> Fixes: adae1e931acd ("Drivers: hv: vmbus: Copy packets sent by Hyper-V out of the ring buffer")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

LGTM.

Reviewed-by: Andrea Parri <parri.andrea@xxxxxxxxx>

ae6935ed7d424f appears to have anticipated this problem on ->ring_buffer
and adopted the solution proposed here by Vitaly.

  Andrea


> ---
>  drivers/hv/ring_buffer.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
> index 2aee356840a2..314015d9e912 100644
> --- a/drivers/hv/ring_buffer.c
> +++ b/drivers/hv/ring_buffer.c
> @@ -245,6 +245,7 @@ void hv_ringbuffer_cleanup(struct hv_ring_buffer_info *ring_info)
>  	mutex_unlock(&ring_info->ring_buffer_mutex);
>  
>  	kfree(ring_info->pkt_buffer);
> +	ring_info->pkt_buffer = NULL;
>  	ring_info->pkt_buffer_size = 0;
>  }
>  
> -- 
> 2.31.1
>