Re: [PATCH 2/2] ntsync: honour caller's time namespace for absolute MONOTONIC timeouts

[Elizabeth Figura <zfigura@codeweavers.com>]

On Thursday, 28 May 2026 01:33:11 CDT Maoyi Xie wrote:
> ntsync_schedule() takes the absolute timeout from userspace and hands
> it to schedule_hrtimeout_range_clock() with HRTIMER_MODE_ABS. For the
> default CLOCK_MONOTONIC path, it does not call timens_ktime_to_host()
> first.
> 
> A process inside a CLOCK_MONOTONIC time namespace computes the
> absolute timeout in its own clock view. The kernel reads the same
> value against the host clock. The two differ by the namespace offset.
> The timeout then fires too early or too late.
> 
> Other consumers of absolute timeouts run the ktime through
> timens_ktime_to_host() before hrtimer. Examples are timerfd,
> posix-timers, alarmtimer, posix-stubs and futex. ntsync was added
> later and missed that step.
> 
> /dev/ntsync is mode 0666. Any user inside a time namespace that can
> open it is affected. The visible effect is wrong timeout behaviour
> for Wine in a container that sets a CLOCK_MONOTONIC offset.
> 
> Reproducer: unshare --user --time, set the monotonic offset to -10s,
> issue NTSYNC_IOC_WAIT_ANY with a 100 ms absolute MONOTONIC timeout.
> The baseline run elapses about 100 ms. The run inside the namespace
> elapses about 0 ms.
> 
> Apply timens_ktime_to_host() to the parsed timeout when the caller
> did not set NTSYNC_WAIT_REALTIME. The helper does nothing in the
> initial time namespace, so the fast path is unchanged.
> 
> Fixes: b4a7b5fe3f51 ("ntsync: Introduce NTSYNC_IOC_WAIT_ANY.")
> Cc: stable@vger.kernel.org # v6.14+
> Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
> ---
>  drivers/misc/ntsync.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/misc/ntsync.c b/drivers/misc/ntsync.c
> index 30af282262ef..02c9d1192812 100644
> --- a/drivers/misc/ntsync.c
> +++ b/drivers/misc/ntsync.c
> @@ -19,6 +19,7 @@
>  #include <linux/sched/signal.h>
>  #include <linux/slab.h>
>  #include <linux/spinlock.h>
> +#include <linux/time_namespace.h>
>  #include <uapi/linux/ntsync.h>
>  
>  #define NTSYNC_NAME	"ntsync"
> @@ -836,6 +837,8 @@ static int ntsync_schedule(const struct ntsync_q *q, const struct ntsync_wait_ar
>  
>  	if (args->flags & NTSYNC_WAIT_REALTIME)
>  		clock = CLOCK_REALTIME;
> +	else
> +		timeout = timens_ktime_to_host(clock, timeout);
>  
>  	do {
>  		if (signal_pending(current)) {
> 

Looks correct and passes tests here.

Reviewed-by: Elizabeth Figura <zfigura@codeweavers.com>