[PATCH v2 1/1] Drivers: hv: vmbus: Don't release fb_mmio resource in vmbus_free_mmio()

[mhkelley58@gmail.com]

From: Michael Kelley <mhklinux@outlook.com>

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@outlook.com>
Tested-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Reviewed-by: Saurabh Sengar <ssengar@linux.microsoft.com>
---
Changes in v2:
* Fixed minor formatting issues reported by checkpatch.pl --strict
  [Saurabh Sengar]

 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 22afebfc28ff..8d3cff42bdbb 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -2272,12 +2272,25 @@ 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