linux-kernel - Re: [PATCH v2 2/4] time: make getboottime64 aware of time namespace

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Message-ID: <20201009132815.5afulu5poh5ti57m@wittgenstein>
Date:   Fri, 9 Oct 2020 15:28:15 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Michael Weiß <michael.weiss@...ec.fraunhofer.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andrei Vagin <avagin@...il.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        linux-kernel@...r.kernel.org,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>
Subject: Re: [PATCH v2 2/4] time: make getboottime64 aware of time namespace

On Thu, Oct 08, 2020 at 07:39:42AM +0200, Michael Weiß wrote:
> getboottime64() provides the time stamp of system boot. In case of
> time namespaces, the offset to the boot time stamp was not applied
> earlier. However, getboottime64 is used e.g., in /proc/stat to print
> the system boot time to userspace. In container runtimes which utilize
> time namespaces to virtualize boottime of a container, this leaks
> information about the host system boot time.
> 
> Therefore, we make getboottime64() to respect the time namespace offset
> for boottime by subtracting the boottime offset.
> 
> Signed-off-by: Michael Weiß <michael.weiss@...ec.fraunhofer.de>
> ---
>  kernel/time/timekeeping.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 4c47f388a83f..67530cdb389e 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -17,6 +17,7 @@
>  #include <linux/clocksource.h>
>  #include <linux/jiffies.h>
>  #include <linux/time.h>
> +#include <linux/time_namespace.h>
>  #include <linux/tick.h>
>  #include <linux/stop_machine.h>
>  #include <linux/pvclock_gtod.h>
> @@ -2154,6 +2155,8 @@ void getboottime64(struct timespec64 *ts)
>  {
>  	struct timekeeper *tk = &tk_core.timekeeper;
>  	ktime_t t = ktime_sub(tk->offs_real, tk->offs_boot);
> +	/* shift boot time stamp according to the timens offset */
> +	t = timens_ktime_to_host(CLOCK_BOOTTIME, t);

Note that getbootime64() is mostly used in net/sunrpc and I don't know
if this change has any security implications for them.

Hey, Trond, Anna, Bruce, and Chuck this virtualizes boottime according
to the time namespace of the caller, i.e. a container can e.g. reset
it's boottime when started. This is already possible. The series here
fixes a bug where /proc/stat's btime field is not virtualized but since
this changes getboottime64() this would also apply to sunrpc's
timekeeping. Is that ok or does sunrpc rely on the hosts's boot time,
i.e. the time in the initial time namespace?

Christian