Re: [PATCH AUTOSEL 6.1 31/51] pps: clients: gpio: fix interrupt handling order in remove path

[Rodolfo Giometti <giometti@enneenne.com>]

On 04/08/25 02:36, Sasha Levin wrote:
> From: Eliav Farber <farbere@amazon.com>
> 
> [ Upstream commit 6bca1e955830808dc90e0506b2951b4256b81bbb ]
> 
> The interrupt handler in pps_gpio_probe() is registered after calling
> pps_register_source() using devm_request_irq(). However, in the
> corresponding remove function, pps_unregister_source() is called before
> the IRQ is freed, since devm-managed resources are released after the
> remove function completes.
> 
> This creates a potential race condition where an interrupt may occur
> after the PPS source is unregistered but before the handler is removed,
> possibly leading to a kernel panic.
> 
> To prevent this, switch from devm-managed IRQ registration to manual
> management by using request_irq() and calling free_irq() explicitly in
> the remove path before unregistering the PPS source. This ensures the
> interrupt handler is safely removed before deactivating the PPS source.
> 
> Signed-off-by: Eliav Farber <farbere@amazon.com>
> Link: https://lore.kernel.org/r/20250527053355.37185-1-farbere@amazon.com
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Sasha Levin <sashal@kernel.org>

Signed-off-by: Rodolfo Giometti <giometti@enneenne.com>

> ---
> 
> LLM Generated explanations, may be completely bogus:
> 
> Based on my analysis of the commit and the surrounding context, here is
> my assessment:
> 
> **Backport Status: YES**
> 
> This commit should be backported to stable kernel trees for the
> following reasons:
> 
> 1. **Fixes a Real Bug**: The commit addresses a genuine race condition
>     where an interrupt can occur after the PPS source is unregistered but
>     before the IRQ handler is freed. This can lead to a kernel panic - a
>     serious stability issue that affects users.
> 
> 2. **Clear Race Condition Pattern**: The code shows the problematic
>     ordering:
>     - In probe: `pps_register_source()` → `devm_request_irq()`
>     - In remove: `pps_unregister_source()` → (implicit devm cleanup frees
>       IRQ)
> 
>     This creates a window where the hardware can generate an interrupt
> after the PPS source is gone but before the handler is removed, causing
> the handler to access freed memory.
> 
> 3. **Minimal and Contained Fix**: The change is small and focused:
>     - Changes `devm_request_irq()` to `request_irq()`
>     - Adds explicit `free_irq()` call before `pps_unregister_source()`
>     - Only 3 lines of functional code changes
>     - No new features or architectural changes
> 
> 4. **Similar to Other Stable Fixes**: This follows a common pattern seen
>     in other drivers where devm resource ordering causes issues in
>     removal paths. The kernel has many similar fixes for CAN drivers,
>     network drivers, and other subsystems that have been backported to
>     stable.
> 
> 5. **Low Risk of Regression**: The change is straightforward and doesn't
>     modify the driver's functionality - it only fixes the resource
>     cleanup ordering. The manual IRQ management is a well-established
>     pattern.
> 
> 6. **Affects User-Visible Stability**: A kernel panic during device
>     removal/module unload is a serious issue that can affect system
>     stability, especially in environments where PPS devices might be
>     dynamically added/removed or during system shutdown.
> 
> The commit message clearly describes the problem, the root cause, and
> the solution. The fix is conservative and follows established kernel
> patterns for fixing devm ordering issues. This is exactly the type of
> bug fix that stable kernel rules recommend for backporting.
> 
>   drivers/pps/clients/pps-gpio.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pps/clients/pps-gpio.c b/drivers/pps/clients/pps-gpio.c
> index bf3b6f1aa984..41e1fdbcda16 100644
> --- a/drivers/pps/clients/pps-gpio.c
> +++ b/drivers/pps/clients/pps-gpio.c
> @@ -206,8 +206,8 @@ static int pps_gpio_probe(struct platform_device *pdev)
>   	}
>   
>   	/* register IRQ interrupt handler */
> -	ret = devm_request_irq(dev, data->irq, pps_gpio_irq_handler,
> -			get_irqf_trigger_flags(data), data->info.name, data);
> +	ret = request_irq(data->irq, pps_gpio_irq_handler,
> +			  get_irqf_trigger_flags(data), data->info.name, data);
>   	if (ret) {
>   		pps_unregister_source(data->pps);
>   		dev_err(dev, "failed to acquire IRQ %d\n", data->irq);
> @@ -224,6 +224,7 @@ static int pps_gpio_remove(struct platform_device *pdev)
>   {
>   	struct pps_gpio_device_data *data = platform_get_drvdata(pdev);
>   
> +	free_irq(data->irq, data);
>   	pps_unregister_source(data->pps);
>   	del_timer_sync(&data->echo_timer);
>   	/* reset echo pin in any case */


-- 
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