Re: [PATCH v2] pps: Don't allow PPS_KC_BIND on removed devices

[Rodolfo Giometti <giometti@enneenne.com>]

On 03/06/2026 19:25, Calvin Owens wrote:
> If userspace holds its file descriptor open, it can call PPS_KC_BIND on
> a device which has been unplugged, leaving pps_kc_hardpps_dev as a
> dangling pointer after close().
> 
> After that sequence, PPS_KC_BIND is broken until the system is rebooted,
> because the pointer comparison in pps_kc_bind() can never be true.
> 
>      calling  pps_ktimer_init+0x0/0x1000 [pps_ktimer] @ 1081
>      initcall pps_ktimer_init+0x0/0x1000 [pps_ktimer] returned 0 after 811 usecs
>      pps pps0: bound kernel consumer: edge=0x1
>      pps pps0: unbound kernel consumer on device removal
>      pps pps0: bound kernel consumer: edge=0x1
>      calling  pps_ktimer_init+0x0/0x1000 [pps_ktimer] @ 1085
>      initcall pps_ktimer_init+0x0/0x1000 [pps_ktimer] returned 0 after 340 usecs
>      pps pps0: another kernel consumer is already bound
> 
> Here is a short reproducer, which uses rmmod of the pps-ktimer testcase
> to simulate a device being unplugged:
> 
>      #include <stdlib.h>
>      #include <stdio.h>
>      #include <unistd.h>
>      #include <fcntl.h>
>      #include <sys/ioctl.h>
>      #include <linux/pps.h>
>      #include <errno.h>
>      #include <err.h>
> 
>      int main(void)
>      {
>          while (1) {
>              int fd;
> 
>              if (system("insmod ./pps-ktimer.ko"))
>                  err(1, "insmod failed");
> 
>              fd = open("/dev/pps0", O_RDWR);
>              if (fd == -1)
>                  err(1, "open failed");
> 
>              struct pps_bind_args args = {
>                  .tsformat = PPS_TSFMT_TSPEC,
>                  .edge = PPS_CAPTUREASSERT,
>                  .consumer = PPS_KC_HARDPPS,
>              };
> 
>              if (ioctl(fd, PPS_KC_BIND, &args))
>                  err(1, "first PPS_KC_BIND failed");
> 
>              if (system("rmmod pps-ktimer"))
>                  err(1, "rmmod failed");
> 
>              if (ioctl(fd, PPS_KC_BIND, &args)) {
>                  if (errno != ENODEV)
>                      err(1, "second PPS_KC_BIND failed");
>                  else
>                      puts("Got ENODEV, kernel is patched");
>              }
> 
>              close(fd);
>          }
>      }
> 
> Fix this by setting a flag when the device is unplugged, returning
> -ENODEV from PPS_KC_BIND if the flag is set.
> 
> For userspace to encounter this new behavior, it must do something which
> breaks the interface today, so this fix shouldn't cause any observable
> behavior change for working programs.
> 
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/cover.1779733602.git.calvin%40wbinvd.org?part=1
> Signed-off-by: Calvin Owens <calvin@wbinvd.org>

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

> ---
> Changes in v2:
> * Drop guard() changes so irqs are unmasked for printk() calls [Rodolfo]
> 
> v1: https://lore.kernel.org/lkml/c9bfee54cbabbbfc8da7aa3d3893cf61d3ebf3b2.1780108479.git.calvin@wbinvd.org/
> 
>   drivers/pps/kc.c           | 10 ++++++++++
>   include/linux/pps_kernel.h |  1 +
>   2 files changed, 11 insertions(+)
> 
> diff --git a/drivers/pps/kc.c b/drivers/pps/kc.c
> index fbd23295afd7..4f8fffa7edd6 100644
> --- a/drivers/pps/kc.c
> +++ b/drivers/pps/kc.c
> @@ -38,6 +38,14 @@ int pps_kc_bind(struct pps_device *pps, struct pps_bind_args *bind_args)
>   	/* Check if another consumer is already bound */
>   	spin_lock_irq(&pps_kc_hardpps_lock);
>   
> +	/*
> +	 * Don't allow PPS_KC_BIND on a removed device.
> +	 */
> +	if (pps->kc_removed) {
> +		spin_unlock_irq(&pps_kc_hardpps_lock);
> +		return -ENODEV;
> +	}
> +
>   	if (bind_args->edge == 0)
>   		if (pps_kc_hardpps_dev == pps) {
>   			pps_kc_hardpps_mode = 0;
> @@ -79,6 +87,8 @@ int pps_kc_bind(struct pps_device *pps, struct pps_bind_args *bind_args)
>   void pps_kc_remove(struct pps_device *pps)
>   {
>   	spin_lock_irq(&pps_kc_hardpps_lock);
> +
> +	pps->kc_removed = true;
>   	if (pps == pps_kc_hardpps_dev) {
>   		pps_kc_hardpps_mode = 0;
>   		pps_kc_hardpps_dev = NULL;
> diff --git a/include/linux/pps_kernel.h b/include/linux/pps_kernel.h
> index aab0aebb529e..3c2979f8a5ac 100644
> --- a/include/linux/pps_kernel.h
> +++ b/include/linux/pps_kernel.h
> @@ -60,6 +60,7 @@ struct pps_device {
>   	struct device dev;
>   	struct fasync_struct *async_queue;	/* fasync method */
>   	spinlock_t lock;
> +	bool kc_removed;
>   };
>   
>   /*


-- 
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