From: Jakub Lewalski jakub.lewalski@nokia.com
[ Upstream commit 6bd697b5fc39fd24e2aa418c7b7d14469f550a93 ]
When two instances of uart devices are probing, a concurrency race can occur. If one thread calls uart_register_driver function, which first allocates and assigns memory to 'uart_state' member of uart_driver structure, the other instance can bypass uart driver registration and call ulite_assign. This calls uart_add_one_port, which expects the uart driver to be fully initialized. This leads to a kernel panic due to a null pointer dereference:
[ 8.143581] BUG: kernel NULL pointer dereference, address: 00000000000002b8 [ 8.156982] #PF: supervisor write access in kernel mode [ 8.156984] #PF: error_code(0x0002) - not-present page [ 8.156986] PGD 0 P4D 0 ... [ 8.180668] RIP: 0010:mutex_lock+0x19/0x30 [ 8.188624] Call Trace: [ 8.188629] ? __die_body.cold+0x1a/0x1f [ 8.195260] ? page_fault_oops+0x15c/0x290 [ 8.209183] ? __irq_resolve_mapping+0x47/0x80 [ 8.209187] ? exc_page_fault+0x64/0x140 [ 8.209190] ? asm_exc_page_fault+0x22/0x30 [ 8.209196] ? mutex_lock+0x19/0x30 [ 8.223116] uart_add_one_port+0x60/0x440 [ 8.223122] ? proc_tty_register_driver+0x43/0x50 [ 8.223126] ? tty_register_driver+0x1ca/0x1e0 [ 8.246250] ulite_probe+0x357/0x4b0 [uartlite]
To prevent it, move uart driver registration in to init function. This will ensure that uart_driver is always registered when probe function is called.
Signed-off-by: Jakub Lewalski jakub.lewalski@nokia.com Signed-off-by: Elodie Decerle elodie.decerle@nokia.com Link: https://lore.kernel.org/r/20250331160732.2042-1-elodie.decerle@nokia.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org ---
Based on my comprehensive analysis, I can now provide a definitive assessment:
**YES**
## Extensive Analysis
This commit **should be backported** to stable kernel trees. Here's my detailed analysis:
### **1. Critical Bug Fix Nature**
The commit fixes a **serious race condition** that causes kernel panics with NULL pointer dereferences. The stack trace in the commit message shows:
``` [ 8.143581] BUG: kernel NULL pointer dereference, address: 00000000000002b8 [ 8.180668] RIP: 0010:mutex_lock+0x19/0x30 [ 8.223116] uart_add_one_port+0x60/0x440 ```
This is a **critical system stability issue** that can crash the kernel during device initialization.
### **2. Race Condition Analysis**
**The Problem:** - **Before the fix:** `uart_register_driver()` was called in `ulite_probe()` (lines 883-891 in current code) - **Race scenario:** When two UART devices probe simultaneously: 1. Thread 1 calls `uart_register_driver()` and starts allocating `uart_driver.state` 2. Thread 2 sees `!ulite_uart_driver.state` as false, bypasses registration, and calls `ulite_assign()` 3. Thread 2's `ulite_assign()` calls `uart_add_one_port()` (line 678) expecting a fully initialized driver 4. Thread 1's registration hasn't completed, causing NULL pointer dereference in `uart_add_one_port()`
**The Fix:** - **After the fix:** `uart_register_driver()` is moved to `ulite_init()` ensuring: 1. Driver registration completes **before** any platform driver registration 2. **No race window** exists between driver registration and device probing 3. All probe calls find a **fully initialized** uart driver
### **3. Code Changes Analysis**
**Changes in `ulite_init()`:** ```c static int __init ulite_init(void) { + int ret; + + pr_debug("uartlite: calling uart_register_driver()\n"); + ret = uart_register_driver(&ulite_uart_driver); + if (ret) + return ret;
pr_debug("uartlite: calling platform_driver_register()\n"); - return platform_driver_register(&ulite_platform_driver); + ret = platform_driver_register(&ulite_platform_driver); + if (ret) + uart_unregister_driver(&ulite_uart_driver); + + return ret; } ```
**Changes in `ulite_probe()`:** ```c // REMOVED the problematic race-prone code: - if (!ulite_uart_driver.state) { - dev_dbg(&pdev->dev, "uartlite: calling uart_register_driver()\n"); - ret = uart_register_driver(&ulite_uart_driver); - if (ret < 0) { - dev_err(&pdev->dev, "Failed to register driver\n"); - clk_disable_unprepare(pdata->clk); - return ret; - } - } ```
**Changes in `ulite_exit()`:** ```c static void __exit ulite_exit(void) { platform_driver_unregister(&ulite_platform_driver); - if (ulite_uart_driver.state) - uart_unregister_driver(&ulite_uart_driver); + uart_unregister_driver(&ulite_uart_driver); } ```
### **4. Follows Kernel Best Practices**
The fix aligns with **established kernel patterns**. My analysis of other UART drivers shows that well-architected drivers like 8250, altera, atmel, and liteuart all register their uart drivers in `__init` functions, not probe functions. This architectural pattern:
- **Separates concerns:** Driver registration (what) vs device binding (when) - **Eliminates races:** Driver available before any device probing - **Improves reliability:** Predictable initialization order
### **5. Stable Tree Suitability Criteria**
✅ **Fixes important bug:** Kernel panic with NULL pointer dereference ✅ **Small and contained:** Moves existing code, no new functionality ✅ **Minimal risk:** Follows established kernel patterns ✅ **Clear side effects:** Better initialization order, no regressions expected ✅ **Confined to subsystem:** Only affects uartlite driver ✅ **Important for users:** Prevents boot crashes on Xilinx platforms
### **6. Similar Historical Pattern**
This fix is very similar to **Similar Commit #5** (Status: YES) which also fixed race conditions in TTY/serial code by ensuring proper initialization order. Both commits: - Fix race conditions in serial/TTY subsystem - Make architectural improvements to initialization order - Prevent NULL pointer dereferences - Have clear stable tree value
### **7. Low Regression Risk**
The change is **architecturally sound** because: - It ensures `uart_register_driver()` completes before any `uart_add_one_port()` calls - Proper error handling ensures cleanup if platform driver registration fails - The exit sequence properly reverses the initialization sequence - No functional behavior changes, only timing/ordering improvements
### **Conclusion**
This commit fixes a **critical kernel panic** caused by a race condition in the uartlite driver. The fix is **small, contained, and follows established kernel patterns**. It has **minimal regression risk** and provides **significant stability improvements** for users of Xilinx UART devices. The architectural change from probe-time to init-time driver registration is a **best practice** that eliminates the race condition entirely.
drivers/tty/serial/uartlite.c | 25 ++++++++++++------------- 1 file changed, 12 insertions(+), 13 deletions(-)
diff --git a/drivers/tty/serial/uartlite.c b/drivers/tty/serial/uartlite.c index a41e7fc373b7c..39c1fd1ff9ced 100644 --- a/drivers/tty/serial/uartlite.c +++ b/drivers/tty/serial/uartlite.c @@ -880,16 +880,6 @@ static int ulite_probe(struct platform_device *pdev) pm_runtime_set_active(&pdev->dev); pm_runtime_enable(&pdev->dev);
- if (!ulite_uart_driver.state) { - dev_dbg(&pdev->dev, "uartlite: calling uart_register_driver()\n"); - ret = uart_register_driver(&ulite_uart_driver); - if (ret < 0) { - dev_err(&pdev->dev, "Failed to register driver\n"); - clk_disable_unprepare(pdata->clk); - return ret; - } - } - ret = ulite_assign(&pdev->dev, id, res->start, irq, pdata);
pm_runtime_mark_last_busy(&pdev->dev); @@ -929,16 +919,25 @@ static struct platform_driver ulite_platform_driver = {
static int __init ulite_init(void) { + int ret; + + pr_debug("uartlite: calling uart_register_driver()\n"); + ret = uart_register_driver(&ulite_uart_driver); + if (ret) + return ret;
pr_debug("uartlite: calling platform_driver_register()\n"); - return platform_driver_register(&ulite_platform_driver); + ret = platform_driver_register(&ulite_platform_driver); + if (ret) + uart_unregister_driver(&ulite_uart_driver); + + return ret; }
static void __exit ulite_exit(void) { platform_driver_unregister(&ulite_platform_driver); - if (ulite_uart_driver.state) - uart_unregister_driver(&ulite_uart_driver); + uart_unregister_driver(&ulite_uart_driver); }
module_init(ulite_init);