Re: [PATCH 1/1] Drivers: hv: vmbus: Don't release fb_mmio resource in vmbus_free_mmio()

[Saurabh Singh Sengar]

On Mon, Feb 10, 2025 at 09:01:14PM -0800, mhkelley58@xxxxxxxxx wrote:
> From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> 
> The VMBus driver manages the MMIO space it owns via the hyperv_mmio
> resource tree. Because the synthetic video framebuffer portion of the
> MMIO space is initially setup by the Hyper-V host for each guest, the
> VMBus driver does an early reserve of that portion of MMIO space in the
> hyperv_mmio resource tree. It saves a pointer to that resource in
> fb_mmio. When a VMBus driver requests MMIO space and passes "true"
> for the "fb_overlap_ok" argument, the reserved framebuffer space is
> used if possible. In that case it's not necessary to do another request
> against the "shadow" hyperv_mmio resource tree because that resource
> was already requested in the early reserve steps.
> 
> However, the vmbus_free_mmio() function currently does no special
> handling for the fb_mmio resource. When a framebuffer device is
> removed, or the driver is unbound, the current code for
> vmbus_free_mmio() releases the reserved resource, leaving fb_mmio
> pointing to memory that has been freed. If the same or another
> driver is subsequently bound to the device, vmbus_allocate_mmio()
> checks against fb_mmio, and potentially gets garbage. Furthermore
> a second unbind operation produces this "nonexistent resource" error
> because of the unbalanced behavior between vmbus_allocate_mmio() and
> vmbus_free_mmio():
> 
> [   55.499643] resource: Trying to free nonexistent
> 			resource <0x00000000f0000000-0x00000000f07fffff>
> 
> Fix this by adding logic to vmbus_free_mmio() to recognize when
> MMIO space in the fb_mmio reserved area would be released, and don't
> release it. This filtering ensures the fb_mmio resource always exists,
> and makes vmbus_free_mmio() more parallel with vmbus_allocate_mmio().
> 
> Fixes: be000f93e5d7 ("drivers:hv: Track allocations of children of hv_vmbus in private resource tree")
> Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> ---
>  drivers/hv/vmbus_drv.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 2892b8da20a5..7507b3641ebd 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -2262,12 +2262,25 @@ void vmbus_free_mmio(resource_size_t start, resource_size_t size)
>  	struct resource *iter;
>  
>  	mutex_lock(&hyperv_mmio_lock);
> +
> +	/*
> +	 * If all bytes of the MMIO range to be released are within the
> +	 * special case fb_mmio shadow region, skip releasing the shadow
> +	 * region since no corresponding __request_region() was done
> +	 * in vmbus_allocate_mmio().
> +	 */
> +	if (fb_mmio && (start >= fb_mmio->start) &&
> +				(start + size - 1 <= fb_mmio->end))
> +		goto skip_shadow_release;
> +
>  	for (iter = hyperv_mmio; iter; iter = iter->sibling) {
>  		if ((iter->start >= start + size) || (iter->end <= start))
>  			continue;
>  
>  		__release_region(iter, start, size);
>  	}
> +
> +skip_shadow_release:
>  	release_mem_region(start, size);
>  	mutex_unlock(&hyperv_mmio_lock);
>  
> -- 
> 2.25.1
> 

Thanks for the fix.
There are couple of checkpatch.pl --strict CHECK, post fixing that:

Tested-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>

Regards,
Saurabh