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SubjectRe: [RFC] kprobes: Fix locking in recycle_rp_inst
Hi Jiri,

On Tue, 26 Feb 2019 17:10:36 +0100
Jiri Olsa <jolsa@kernel.org> wrote:

> hi,
> David reported a crash related to return kprobes.
> The change below tries to explain it and fix it,
> but I'm sending it as RFC because I don't follow
> kprobes code that much, so I might have missed
> something.

Thank you for the report and patch!

>
> thanks,
> jirka
>
>
> ---
> We can call recycle_rp_inst from both task and irq contexts,
> so we should use irqsave/irqrestore locking functions.
>
> I wasn't able to hit this particular lockup, but I found it
> while checking on why return probe on _raw_spin_lock locks
> the system, reported by David by using bpftrace on simple
> script, like:
>
> kprobe:_raw_spin_lock
> {
> @time[tid] = nsecs;
> @symb[tid] = arg0;
> }
>
> kretprobe:_raw_spin_lock
> / @time[tid] /
> {
> delete(@time[tid]);
> delete(@symb[tid]);
> }
>
> or by perf tool:
>
> # perf probe -a _raw_spin_lock:%return
> # perf record -e probe:_raw_spin_lock__return -a
>
> The thing is that the _raw_spin_lock call in recycle_rp_inst,
> is the only one that return-probe-code-paths call, and it
> triggers another kprobe instance while already processing one
> and locks up on kretprobe_table_lock lock:
>
> #12 [ffff99c337403d28] queued_spin_lock_slowpath at ffffffff9712693b
> #13 [ffff99c337403d28] _raw_spin_lock_irqsave at ffffffff9794c100
> #14 [ffff99c337403d38] pre_handler_kretprobe at ffffffff9719435c
> #15 [ffff99c337403d68] kprobe_ftrace_handler at ffffffff97059f12
> #16 [ffff99c337403d98] ftrace_ops_assist_func at ffffffff971a0421
> #17 [ffff99c337403dd8] handle_edge_irq at ffffffff97139f55
> #18 [ffff99c337403df0] handle_edge_irq at ffffffff97139f55
> #19 [ffff99c337403e58] _raw_spin_lock at ffffffff9794c111
> #20 [ffff99c337403e88] _raw_spin_lock at ffffffff9794c115
> #21 [ffff99c337403ea8] trampoline_handler at ffffffff97058a8f
> #22 [ffff99c337403f00] kretprobe_trampoline at ffffffff970586d5
> #23 [ffff99c337403fb0] handle_irq at ffffffff97027b1f
> #24 [ffff99c337403fc0] do_IRQ at ffffffff97a01bc9
> --- <IRQ stack> ---
> #25 [ffffa5c3c1f9fb38] ret_from_intr at ffffffff97a0098f
> [exception RIP: smp_call_function_many+460]
> RIP: ffffffff9716685c RSP: ffffa5c3c1f9fbe0 RFLAGS: 00000202
> RAX: 0000000000000005 RBX: ffff99c337421c80 RCX: ffff99c337566260
> RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff99c337421c88
> RBP: ffff99c337421c88 R8: 0000000000000001 R9: ffffffff98352940
> R10: ffff99c33703c910 R11: ffffffff9794c110 R12: ffffffff97055680
> R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000000040
> ORIG_RAX: ffffffffffffffde CS: 0010 SS: 0018
> #26 [ffffa5c3c1f9fc20] on_each_cpu at ffffffff97166918
> #27 [ffffa5c3c1f9fc40] ftrace_replace_code at ffffffff97055a34
> #28 [ffffa5c3c1f9fc88] ftrace_modify_all_code at ffffffff971a3552
> #29 [ffffa5c3c1f9fca8] arch_ftrace_update_code at ffffffff97055a6c
> #30 [ffffa5c3c1f9fcb0] ftrace_run_update_code at ffffffff971a3683
> #31 [ffffa5c3c1f9fcc0] ftrace_startup at ffffffff971a6638
> #32 [ffffa5c3c1f9fce8] register_ftrace_function at ffffffff971a66a0
>
> When we switch it to raw_spin_lock_irqsave the return probe
> on _raw_spin_lock starts working.

Yes, there can be a race between probes and probe on irq handler.

kretprobe_hash_lock()/kretprobe_hash_unlock() are safe because
those disables irqs. Only recycle_rp_inst() has this problem.

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>

And this is one of the oldest bug in kprobe.

commit ef53d9c5e4da ("kprobes: improve kretprobe scalability with hashed locking")

introduced the spin_lock(&rp->lock) in recycle_rp_inst() but forgot to disable irqs.
And

commit c9becf58d935 ("[PATCH] kretprobe: kretprobe-booster")

introduced assembly-based trampoline which didn't disable irq.

Could you add Cc:stable to this patch too?

Thank you!!

>
> Reported-by: David Valin <dvalin@redhat.com>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> kernel/kprobes.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index c83e54727131..c82056b354cc 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1154,9 +1154,11 @@ void recycle_rp_inst(struct kretprobe_instance *ri,
> hlist_del(&ri->hlist);
> INIT_HLIST_NODE(&ri->hlist);
> if (likely(rp)) {
> - raw_spin_lock(&rp->lock);
> + unsigned long flags;
> +
> + raw_spin_lock_irqsave(&rp->lock, flags);
> hlist_add_head(&ri->hlist, &rp->free_instances);
> - raw_spin_unlock(&rp->lock);
> + raw_spin_unlock_irqrestore(&rp->lock, flags);
> } else
> /* Unregistering */
> hlist_add_head(&ri->hlist, head);
> --
> 2.17.2
>


--
Masami Hiramatsu <mhiramat@kernel.org>

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