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49 lines
2.1 KiB
49 lines
2.1 KiB
// SPDX-License-Identifier: GPL-2.0 |
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#include "dm-core.h" |
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/* |
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* The kobject release method must not be placed in the module itself, |
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* otherwise we are subject to module unload races. |
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* |
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* The release method is called when the last reference to the kobject is |
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* dropped. It may be called by any other kernel code that drops the last |
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* reference. |
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* |
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* The release method suffers from module unload race. We may prevent the |
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* module from being unloaded at the start of the release method (using |
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* increased module reference count or synchronizing against the release |
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* method), however there is no way to prevent the module from being |
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* unloaded at the end of the release method. |
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* |
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* If this code were placed in the dm module, the following race may |
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* happen: |
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* 1. Some other process takes a reference to dm kobject |
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* 2. The user issues ioctl function to unload the dm device |
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* 3. dm_sysfs_exit calls kobject_put, however the object is not released |
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* because of the other reference taken at step 1 |
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* 4. dm_sysfs_exit waits on the completion |
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* 5. The other process that took the reference in step 1 drops it, |
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* dm_kobject_release is called from this process |
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* 6. dm_kobject_release calls complete() |
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* 7. a reschedule happens before dm_kobject_release returns |
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* 8. dm_sysfs_exit continues, the dm device is unloaded, module reference |
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* count is decremented |
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* 9. The user unloads the dm module |
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* 10. The other process that was rescheduled in step 7 continues to run, |
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* it is now executing code in unloaded module, so it crashes |
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* |
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* Note that if the process that takes the foreign reference to dm kobject |
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* has a low priority and the system is sufficiently loaded with |
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* higher-priority processes that prevent the low-priority process from |
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* being scheduled long enough, this bug may really happen. |
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* |
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* In order to fix this module unload race, we place the release method |
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* into a helper code that is compiled directly into the kernel. |
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*/ |
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void dm_kobject_release(struct kobject *kobj) |
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{ |
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complete(dm_get_completion_from_kobject(kobj)); |
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} |
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EXPORT_SYMBOL(dm_kobject_release);
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