linux-kernel - Re: [syzbot] possible deadlock in ktime_get_coarse

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Date:   Sun, 7 Nov 2021 00:07:33 +0400
From:   Dmitrii Banshchikov <me@...que.spb.ru>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        syzbot <syzbot+43fd005b5a1b4d10781e@...kaller.appspotmail.com>,
        John Stultz <john.stultz@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>, sboyd@...nel.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Steven Rostedt <rosted@...dmis.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [syzbot] possible deadlock in ktime_get_coarse_ts64

On Fri, Nov 05, 2021 at 06:24:30PM +0100, Thomas Gleixner wrote:
> On Fri, Nov 05 2021 at 21:03, Dmitrii Banshchikov wrote:
> > On Fri, Nov 05, 2021 at 08:53:06AM -0700, Alexei Starovoitov wrote:
> >> > Timestamps from within a tracepoint can only be taken with:
> >> >
> >> >          1) jiffies
> >> >          2) sched_clock()
> >> >          3) ktime_get_*_fast_ns()
> >> >
> >> > Those are NMI safe and can be invoked from anywhere.
> >> >
> >> > All other time getters which have to use the timekeeping seqcount
> >> > protection are prone to live locks and _cannot_ be used from
> >> > tracepoints ever.
> >> 
> >> Obviously.
> >> That helper was added for networking use cases and accidentally
> >> enabled for tracing.
> >
> > Sorry for that.
> > I'm preparing a patch that will forbid using bpf_ktime_get_coarse_ns()
> > helper in BPF_LINK_TYPE_RAW_TRACEPOINT.
> 
> It cannot be used in TRACING and PERF_EVENT either. But those contexts
> have to exclude other functions as well:
> 
>      bpf_ktime_get_ns
>      bpf_ktime_get_boot_ns
> 
> along with
> 
>     bpf_spin_lock/unlock
>     bpf_timer_*

1) bpf_ktime_get_ns and bpf_ktime_get_boot_ns use
ktime_get_{mono,boot}_fast_ns.
2) bpf_spin_lock/unlock have notrace attribute set.
3) bpf_timer_* helpers fail early if they are in NMI.

Why they have to be excluded?



-- 

Dmitrii Banshchikov