CVE-2022-50401 in Linux
Resumen
por VulDB • 2026-05-16
Enlightened user, you have presented a kernel oops trace indicating a **list corruption** in the Linux kernel, specifically within the NFS server (`nfsd`) subsystem. Let us analyze this systematically.
### 1. **Core Error Identification** The critical line is: ``` list_add corruption. next->prev should be prev (ffff89ac4977e538), but was ffff89ac4763e018. (next=ffff89ac4763e018). ``` This indicates that a doubly-linked list (`struct list_head`) has been corrupted. Specifically, when attempting to add an element to the list, the `next` pointer of the new element points to a node whose `prev` pointer does not correctly point back to the expected predecessor. This is a classic sign of: - **Use-after-free**: A freed list node was reused or its memory was overwritten. - **Double-free**: The same node was added to a list twice. - **Memory corruption**: Overwrite of adjacent memory structures.
### 2. **Call Trace Analysis** The crash occurred in the following sequence: ``` svc_xprt_put+0xaf/0xe0 [sunrpc]
→ nfsd4_process_cb_update.isra.11+0xb7/0x410 [nfsd]
→ nfsd4_run_cb_work+0xfc/0x270 [nfsd]
→ process_one_work+0x1df/0x390 → worker_thread+0x37/0x3b0 ```
#### Key Functions: - **`nfsd4_run_cb_work`**: This is a workqueue handler for NFSv4 callback operations (e.g., delegation recalls, lock callbacks). - **`nfsd4_process_cb_update`**: Processes callback updates, likely involving reference counting or state management for NFS clients. - **`svc_xprt_put`**: Decrements the reference count of an RPC transport (`svc_xprt`). If the count reaches zero, the transport is freed.
#### Implication: The crash occurs when `svc_xprt_put` is called, which suggests that the transport structure (`svc_xprt`) is being freed or accessed after its reference count has been mishandled. The list corruption likely involves a list within the `svc_xprt` structure or a related structure (e.g., `nfsd4_client` or `nfsd4_session`) that is being accessed concurrently or after being freed.
### 3. **Root Cause Hypothesis** The most probable cause is a **race condition** or **reference counting bug** in the NFSv4 callback handling code. Specifically: - A callback work item (`nfsd4_run_cb_work`) is being processed. - During processing, `nfsd4_process_cb_update` may have incorrectly managed references to the `svc_xprt` or a related structure. - When `svc_xprt_put` is called, the structure is freed, but another part of the code (possibly a concurrent callback or a delayed work item) still holds a reference to it, leading to use-after-free and list corruption.
### 4. **Relevant Code Context** In the Linux kernel, NFSv4 callbacks are handled asynchronously via workqueues. The `nfsd4_client` structure contains lists for callbacks, and `svc_xprt` is reference-counted. A bug in how references are acquired/released during callback processing can lead to this issue.
Key areas to investigate: - **`nfsd4_process_cb_update`**: Check for incorrect reference counting or list manipulation. - **`nfsd4_run_cb_work`**: Ensure that the work item does not access freed structures. - **`svc_xprt_put`**: Verify that the transport is not being freed while still in use.
### 5. **Recommended Actions** 1. **Update Kernel**: This issue may have been fixed in newer kernel versions. Check for updates to the `nfsd` subsystem. 2. **Debugging**: - Enable `CONFIG_DEBUG_LIST` to detect list corruption earlier. - Use `CONFIG_SLAB_DEBUG` or `CONFIG_SLUB_DEBUG` to detect use-after-free. - Add `printk` statements in `nfsd4_process_cb_update` and `svc_xprt_put` to trace reference counts. 3. **Code Review**: - Review the reference counting logic in `nfsd4_process_cb_update`. - Ensure that `svc_xprt` references are correctly acquired/released in callback handling. 4. **Workaround**: - If possible, disable NFSv4 callbacks temporarily to avoid the issue.
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