Re: [RFC] kprobes: Fix locking in recycle_rp_inst

From: Jiri Olsa
Date: Wed Feb 27 2019 - 08:33:15 EST

On Wed, Feb 27, 2019 at 05:38:46PM +0900, Masami Hiramatsu wrote:


> > 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@xxxxxxxxxx>
> 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")

ok, so I'll add:

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

> introduced assembly-based trampoline which didn't disable irq.
> Could you add Cc:stable to this patch too?

sure, attaching patch with updated changelog


We can call recycle_rp_inst from both task and irq contexts,
so we should 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:

@time[tid] = nsecs;
@symb[tid] = arg0;

/ @time[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 will
trigger another kprobe instance while already processing one
and lock 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.

Fixes: c9becf58d935 ("[PATCH] kretprobe: kretprobe-booster")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: David Valin <dvalin@xxxxxxxxxx>
Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
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,
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);