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kretprobe: Prevent triggering kretprobe from within kprobe_flush_task
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Ziqian reported lockup when adding retprobe on _raw_spin_lock_irqsave.
My test was also able to trigger lockdep output:

 ============================================
 WARNING: possible recursive locking detected
 5.6.0-rc6+ #6 Not tainted
 --------------------------------------------
 sched-messaging/2767 is trying to acquire lock:
 ffffffff9a492798 (&(kretprobe_table_locks[i].lock)){-.-.}, at: kretprobe_hash_lock+0x52/0xa0

 but task is already holding lock:
 ffffffff9a491a18 (&(kretprobe_table_locks[i].lock)){-.-.}, at: kretprobe_trampoline+0x0/0x50

 other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(&(kretprobe_table_locks[i].lock));
   lock(&(kretprobe_table_locks[i].lock));

  *** DEADLOCK ***

  May be due to missing lock nesting notation

 1 lock held by sched-messaging/2767:
  #0: ffffffff9a491a18 (&(kretprobe_table_locks[i].lock)){-.-.}, at: kretprobe_trampoline+0x0/0x50

 stack backtrace:
 CPU: 3 PID: 2767 Comm: sched-messaging Not tainted 5.6.0-rc6+ #6
 Call Trace:
  dump_stack+0x96/0xe0
  __lock_acquire.cold.57+0x173/0x2b7
  ? native_queued_spin_lock_slowpath+0x42b/0x9e0
  ? lockdep_hardirqs_on+0x590/0x590
  ? __lock_acquire+0xf63/0x4030
  lock_acquire+0x15a/0x3d0
  ? kretprobe_hash_lock+0x52/0xa0
  _raw_spin_lock_irqsave+0x36/0x70
  ? kretprobe_hash_lock+0x52/0xa0
  kretprobe_hash_lock+0x52/0xa0
  trampoline_handler+0xf8/0x940
  ? kprobe_fault_handler+0x380/0x380
  ? find_held_lock+0x3a/0x1c0
  kretprobe_trampoline+0x25/0x50
  ? lock_acquired+0x392/0xbc0
  ? _raw_spin_lock_irqsave+0x50/0x70
  ? __get_valid_kprobe+0x1f0/0x1f0
  ? _raw_spin_unlock_irqrestore+0x3b/0x40
  ? finish_task_switch+0x4b9/0x6d0
  ? __switch_to_asm+0x34/0x70
  ? __switch_to_asm+0x40/0x70

The code within the kretprobe handler checks for probe reentrancy,
so we won't trigger any _raw_spin_lock_irqsave probe in there.

The problem is in outside kprobe_flush_task, where we call:

  kprobe_flush_task
    kretprobe_table_lock
      raw_spin_lock_irqsave
        _raw_spin_lock_irqsave

where _raw_spin_lock_irqsave triggers the kretprobe and installs
kretprobe_trampoline handler on _raw_spin_lock_irqsave return.

The kretprobe_trampoline handler is then executed with already
locked kretprobe_table_locks, and first thing it does is to
lock kretprobe_table_locks ;-) the whole lockup path like:

  kprobe_flush_task
    kretprobe_table_lock
      raw_spin_lock_irqsave
        _raw_spin_lock_irqsave ---> probe triggered, kretprobe_trampoline installed

        ---> kretprobe_table_locks locked

        kretprobe_trampoline
          trampoline_handler
            kretprobe_hash_lock(current, &head, &flags);  <--- deadlock

Adding kprobe_busy_begin/end helpers that mark code with fake
probe installed to prevent triggering of another kprobe within
this code.

Using these helpers in kprobe_flush_task, so the probe recursion
protection check is hit and the probe is never set to prevent
above lockup.

Link: http://lkml.kernel.org/r/158927059835.27680.7011202830041561604.stgit@devnote2

Fixes: ef53d9c ("kprobes: improve kretprobe scalability with hashed locking")
Cc: Ingo Molnar <[email protected]>
Cc: "Gustavo A . R . Silva" <[email protected]>
Cc: Anders Roxell <[email protected]>
Cc: "Naveen N . Rao" <[email protected]>
Cc: Anil S Keshavamurthy <[email protected]>
Cc: David Miller <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Reported-by: "Ziqian SUN (Zamir)" <[email protected]>
Acked-by: Masami Hiramatsu <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>
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@rostedt
Jiri Olsa authored and rostedt committed Jun 17, 2020
1 parent 75ddf64 commit 9b38cc7
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Showing 3 changed files with 31 additions and 13 deletions.
16 changes: 3 additions & 13 deletions arch/x86/kernel/kprobes/core.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -753,16 +753,11 @@ asm(
NOKPROBE_SYMBOL(kretprobe_trampoline);
STACK_FRAME_NON_STANDARD(kretprobe_trampoline);

static struct kprobe kretprobe_kprobe = {
.addr = (void *)kretprobe_trampoline,
};

/*
* Called from kretprobe_trampoline
*/
__used __visible void *trampoline_handler(struct pt_regs *regs)
{
struct kprobe_ctlblk *kcb;
struct kretprobe_instance *ri = NULL;
struct hlist_head *head, empty_rp;
struct hlist_node *tmp;
Expand All @@ -772,16 +767,12 @@ __used __visible void *trampoline_handler(struct pt_regs *regs)
void *frame_pointer;
bool skipped = false;

preempt_disable();

/*
* Set a dummy kprobe for avoiding kretprobe recursion.
* Since kretprobe never run in kprobe handler, kprobe must not
* be running at this point.
*/
kcb = get_kprobe_ctlblk();
__this_cpu_write(current_kprobe, &kretprobe_kprobe);
kcb->kprobe_status = KPROBE_HIT_ACTIVE;
kprobe_busy_begin();

INIT_HLIST_HEAD(&empty_rp);
kretprobe_hash_lock(current, &head, &flags);
Expand Down Expand Up @@ -857,7 +848,7 @@ __used __visible void *trampoline_handler(struct pt_regs *regs)
__this_cpu_write(current_kprobe, &ri->rp->kp);
ri->ret_addr = correct_ret_addr;
ri->rp->handler(ri, regs);
__this_cpu_write(current_kprobe, &kretprobe_kprobe);
__this_cpu_write(current_kprobe, &kprobe_busy);
}

recycle_rp_inst(ri, &empty_rp);
Expand All @@ -873,8 +864,7 @@ __used __visible void *trampoline_handler(struct pt_regs *regs)

kretprobe_hash_unlock(current, &flags);

__this_cpu_write(current_kprobe, NULL);
preempt_enable();
kprobe_busy_end();

hlist_for_each_entry_safe(ri, tmp, &empty_rp, hlist) {
hlist_del(&ri->hlist);
Expand Down
4 changes: 4 additions & 0 deletions include/linux/kprobes.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -350,6 +350,10 @@ static inline struct kprobe_ctlblk *get_kprobe_ctlblk(void)
return this_cpu_ptr(&kprobe_ctlblk);
}

extern struct kprobe kprobe_busy;
void kprobe_busy_begin(void);
void kprobe_busy_end(void);

kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset);
int register_kprobe(struct kprobe *p);
void unregister_kprobe(struct kprobe *p);
Expand Down
24 changes: 24 additions & 0 deletions kernel/kprobes.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1241,6 +1241,26 @@ __releases(hlist_lock)
}
NOKPROBE_SYMBOL(kretprobe_table_unlock);

struct kprobe kprobe_busy = {
.addr = (void *) get_kprobe,
};

void kprobe_busy_begin(void)
{
struct kprobe_ctlblk *kcb;

preempt_disable();
__this_cpu_write(current_kprobe, &kprobe_busy);
kcb = get_kprobe_ctlblk();
kcb->kprobe_status = KPROBE_HIT_ACTIVE;
}

void kprobe_busy_end(void)
{
__this_cpu_write(current_kprobe, NULL);
preempt_enable();
}

/*
* This function is called from finish_task_switch when task tk becomes dead,
* so that we can recycle any function-return probe instances associated
Expand All @@ -1258,6 +1278,8 @@ void kprobe_flush_task(struct task_struct *tk)
/* Early boot. kretprobe_table_locks not yet initialized. */
return;

kprobe_busy_begin();

INIT_HLIST_HEAD(&empty_rp);
hash = hash_ptr(tk, KPROBE_HASH_BITS);
head = &kretprobe_inst_table[hash];
Expand All @@ -1271,6 +1293,8 @@ void kprobe_flush_task(struct task_struct *tk)
hlist_del(&ri->hlist);
kfree(ri);
}

kprobe_busy_end();
}
NOKPROBE_SYMBOL(kprobe_flush_task);

Expand Down

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