[RFC] kprobes: Fix locking in recycle_rp_inst

[Jiri Olsa <jolsa@kernel.org>]

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.

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.

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