Re: [syzbot] possible deadlock in ktime_get_coarse_ts64

Dmitrii Banshchikov
2021-11-05T17:03:41Z

On Fri, Nov 05, 2021 at 08:53:06AM -0700, Alexei Starovoitov wrote:
> On Fri, Nov 5, 2021 at 6:10 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > >
> > > -> #0 (tk_core.seq.seqcount){----}-{0:0}:
> > > check_prev_add kernel/locking/lockdep.c:3051 [inline]
> > > check_prevs_add kernel/locking/lockdep.c:3174 [inline]
> > > validate_chain+0x1dfb/0x8240 kernel/locking/lockdep.c:3789
> > > __lock_acquire+0x1382/0x2b00 kernel/locking/lockdep.c:5015
> > > lock_acquire+0x19f/0x4d0 kernel/locking/lockdep.c:5625
> > > seqcount_lockdep_reader_access+0xfe/0x230 include/linux/seqlock.h:103
> > > ktime_get_coarse_ts64+0x25/0x110 kernel/time/timekeeping.c:2255
> > > ktime_get_coarse include/linux/timekeeping.h:120 [inline]
> > > ktime_get_coarse_ns include/linux/timekeeping.h:126 [inline]
> >
> > --> this call is invalid
> >
> > > ____bpf_ktime_get_coarse_ns kernel/bpf/helpers.c:173 [inline]
> > > bpf_ktime_get_coarse_ns+0x7e/0x130 kernel/bpf/helpers.c:171
> > > bpf_prog_a99735ebafdda2f1+0x10/0xb50
> > > bpf_dispatcher_nop_func include/linux/bpf.h:721 [inline]
> > > __bpf_prog_run include/linux/filter.h:626 [inline]
> > > bpf_prog_run include/linux/filter.h:633 [inline]
> > > BPF_PROG_RUN_ARRAY include/linux/bpf.h:1294 [inline]
> > > trace_call_bpf+0x2cf/0x5d0 kernel/trace/bpf_trace.c:127
> > > perf_trace_run_bpf_submit+0x7b/0x1d0 kernel/events/core.c:9708
> > > perf_trace_lock+0x37c/0x440 include/trace/events/lock.h:39
> > > trace_lock_release+0x128/0x150 include/trace/events/lock.h:58
> >
> > 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.

--

Dmitrii Banshchikov