dax: Fix potential uaf in __dax_pmem_probe()

__dax_pmem_probe() allocates a memory chunk from dax_region with
alloc_dax_region(). alloc_dax_region() increases the refcount for
dax_region and uses devm_add_action_or_reset() to make the parent
dev manage the dax_region. The dax_region will be used if the parent
dev is destroyed.

Then the function calls devm_create_dev_dax() to make child dev_dax
instances own the lifetime of the dax_region. devm_create_dev_dax()
calls devm_add_action_or_reset(dax_region->dev, unregister_dev_dax, dev);
to make the child dev_dax manage the dax_region and register the destroy
function "unregister_dev_dax".The devm_create_dev_dax() increases the
refcount for dax_region when the function is successfully executed. But
when some error occurs, devm_create_dev_dax() may return ERR_PTR before
increasing the refcount for dax_region. In these cases, the call for
dax_region_put() will decrease the ref count for dax_region and trigger
dax_region_free(), which will execute kfree(dax_region).

When the parent dev is destroyed, the registered destroy function
"unregister_dev_dax" will be triggered and calls dax_region_free(), which
will use the freed dax_region, leading to a use after free bug.

We should check the return value of devm_create_dev_dax(). If it returns
ERR_PTR, we should return this function with ERR_PTR.

Signed-off-by: Jianglei Nie <niejianglei2021@163.com>

drivers/dax/pmem.c

@@ -66,6 +66,8 @@ static struct dev_dax *__dax_pmem_probe(struct device *dev)
 		.size = range_len(&range),
 	};
 	dev_dax = devm_create_dev_dax(&data);
+	if (IS_ERR(dev_dax))
+		return ERR_PTR((dev_dax);
 
 	/* child dev_dax instances now own the lifetime of the dax_region */
 	dax_region_put(dax_region);