Re: [PATCH v5 1/9] gpio: aggregator: protect driver attr handlers against module unload

[Bartosz Golaszewski <brgl@xxxxxxxx>]

On Mon, Feb 24, 2025 at 3:31 PM Koichiro Den <koichiro.den@xxxxxxxxxxxxx> wrote:
>
> Both new_device_store and delete_device_store touch module global
> resources (e.g. gpio_aggregator_lock). To prevent race conditions with
> module unload, a reference needs to be held.
>
> Add try_module_get() in these handlers.
>
> For new_device_store, this eliminates what appears to be the most dangerous
> scenario: if an id is allocated from gpio_aggregator_idr but
> platform_device_register has not yet been called or completed, a concurrent
> module unload could fail to unregister/delete the device, leaving behind a
> dangling platform device/GPIO forwarder. This can result in various issues.
> The following simple reproducer demonstrates these problems:
>
>   #!/bin/bash
>   while :; do
>     # note: whether 'gpiochip0 0' exists or not does not matter.
>     echo 'gpiochip0 0' > /sys/bus/platform/drivers/gpio-aggregator/new_device
>   done &
>   while :; do
>     modprobe gpio-aggregator
>     modprobe -r gpio-aggregator
>   done &
>   wait
>
>   Starting with the following warning, several kinds of warnings will appear
>   and the system may become unstable:
>
>   ------------[ cut here ]------------
>   list_del corruption, ffff888103e2e980->next is LIST_POISON1 (dead000000000100)
>   WARNING: CPU: 1 PID: 1327 at lib/list_debug.c:56 __list_del_entry_valid_or_report+0xa3/0x120
>   [...]
>   RIP: 0010:__list_del_entry_valid_or_report+0xa3/0x120
>   [...]
>   Call Trace:
>    <TASK>
>    ? __list_del_entry_valid_or_report+0xa3/0x120
>    ? __warn.cold+0x93/0xf2
>    ? __list_del_entry_valid_or_report+0xa3/0x120
>    ? report_bug+0xe6/0x170
>    ? __irq_work_queue_local+0x39/0xe0
>    ? handle_bug+0x58/0x90
>    ? exc_invalid_op+0x13/0x60
>    ? asm_exc_invalid_op+0x16/0x20
>    ? __list_del_entry_valid_or_report+0xa3/0x120
>    gpiod_remove_lookup_table+0x22/0x60
>    new_device_store+0x315/0x350 [gpio_aggregator]
>    kernfs_fop_write_iter+0x137/0x1f0
>    vfs_write+0x262/0x430
>    ksys_write+0x60/0xd0
>    do_syscall_64+0x6c/0x180
>    entry_SYSCALL_64_after_hwframe+0x76/0x7e
>    [...]
>    </TASK>
>   ---[ end trace 0000000000000000 ]---
>
> Fixes: 828546e24280 ("gpio: Add GPIO Aggregator")
> Signed-off-by: Koichiro Den <koichiro.den@xxxxxxxxxxxxx>
> ---

Geert, does this look good to you? I'd like to send this fix upstream
first and backport it to stable.

Bartosz