Re: [PATCH] pps: Don't try to wait for negative timeouts in PPS_FETCH

[Rodolfo Giometti <giometti@enneenne.com>]

On 29/05/2026 18:21, Calvin Owens wrote:
> If userspace passes a negative timeout to PPS_FETCH, it triggers a
> kernel splat from schedule_timeout():
> 
>      schedule_timeout: wrong timeout value fffffffffff0bfb4
>      CPU: 17 UID: 0 PID: 4720 Comm: a.out Not tainted 7.1.0-rc5-x86-kvm-00150-g331d97e36b37 #1 PREEMPT_RT
>      Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-20240910_120124-localhost 04/01/2014
>      Call Trace:
>       <TASK>
>       dump_stack_lvl+0x4b/0x70
>       schedule_timeout+0xb7/0xe0
>       pps_cdev_pps_fetch.isra.0+0x93/0x150
>       pps_cdev_ioctl+0x70/0x310
>       __x64_sys_ioctl+0x7b/0xc0
>       do_syscall_64+0xb6/0xfc0
>       entry_SYSCALL_64_after_hwframe+0x4b/0x53
> 
> Sashiko imagines this to be some sort of security problem, which is
> obviously really silly. But I think it is still worth fixing, so buggy
> userspace code can't trigger the splat.
> 
> Silence the splat by skipping the wait if the ticks count is negative.
> The current behavior is to return -ETIMEDOUT in that case, so keep that
> return value in case any userspace code might rely on it.
> 
> Fixes: eae9d2ba0cfc ("LinuxPPS: core support")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/cover.1779733602.git.calvin%40wbinvd.org?part=3
> Signed-off-by: Calvin Owens <calvin@wbinvd.org>
> ---
>   drivers/pps/pps.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
> index de1122bb69ea..6755901fbdae 100644
> --- a/drivers/pps/pps.c
> +++ b/drivers/pps/pps.c
> @@ -65,17 +65,19 @@ static int pps_cdev_pps_fetch(struct pps_device *pps, struct pps_fdata *fdata)
>   	if (fdata->timeout.flags & PPS_TIME_INVALID)
>   		err = wait_event_interruptible(pps->queue,
>   				ev != pps->last_ev);
>   	else {
> -		unsigned long ticks;
> +		long ticks;
>   
>   		dev_dbg(&pps->dev, "timeout %lld.%09d\n",
>   				(long long) fdata->timeout.sec,
>   				fdata->timeout.nsec);
>   		ticks = fdata->timeout.sec * HZ;
>   		ticks += fdata->timeout.nsec / (NSEC_PER_SEC / HZ);
>   
> -		if (ticks != 0) {
> +		if (ticks < 0) {
> +			return -ETIMEDOUT;
> +		} else if (ticks > 0) {
>   			err = wait_event_interruptible_timeout(
>   					pps->queue,
>   					ev != pps->last_ev,
>   					ticks);

Should the problem originate from user-space data, I deem it more appropriate to 
verify them directly, rather than relying on computed data.

	unsigned long ticks;

	if (fdata->timeout.sec < 0 || fdata->timeout.nsec < 0)
		return -ETIMEDOUT;

	dev_dbg(&pps->dev, "timeout %lld.%09d\n",
			(long long) fdata->timeout.sec,
			fdata->timeout.nsec);
		
	ticks = fdata->timeout.sec * HZ;
	ticks += fdata->timeout.nsec / (NSEC_PER_SEC / HZ);

	if (ticks > 0) {

Ciao,

Rodolfo

-- 
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